mass and to prevent the formation of laitance in the interior.
In case the flow is interrupted it is important that all laitance
be removed before proceeding with the work.
In large structures it may be necessary to divide the mass of
concrete into several small compartments or units to permit the
continuous filling of each one. With proper care it is possible in
this manner to obtain as good results under water as in the air.
A less desirable method is the use of the drop bottom bucket.
Where this method is used the bottom of the bucket should be
released when in contact with the surface of the place of deposit.
Concrete sewers should be constructed in longitudinal sections in a
continuous operation without interruption for the entire invert, side
walls, or arch. In pouring the concrete it should be kept level in the
forms and should rise evenly on each side of the sewer. All rough places
in the concrete should be finished smooth by brushing with a grout of
neat cement and water and honeycombs should be filled with neat cement
or a one-to-one mortar.
=96. Sewer Brick.=—The quality of brick used in sewers is seldom
specified with the minute care that is taken in the specifications for
concrete, iron, and certain other materials of construction, as inferior
materials in brick are more easily detected. The specifications of the
Baltimore Sewerage Commission for sewer brick are:
Sewer brick shall be whole, new bricks of the best quality, of
uniform standard size, with straight and parallel edges and square
corners: they shall be of compact texture, burned hard and
entirely through, free from injurious cracks and flaws, tough and
strong, and shall have a clear ring when struck together. The
sides, ends and faces of all bricks shall be plane surfaces at
right angles and parallel to each other. Bricks of any one make
shall not vary more than 1/16th of an inch in thickness, nor more
than 1⅛th of an inch in width or length, from the average of the
samples submitted for approval.
The truest bricks shall be used in the face of the masonry and the
exposed surfaces shall be true and smooth planes.
All bricks delivered for use shall be culled by the Contractor
when required. No brick thrown out in the culling shall be used in
any work done under any contract of the Sewerage Commission,
except that the best of the culls may be used in manholes, above
the level of the top of the sewer, if permitted by the Engineer.
The average amount of water absorbed by the bricks, after being
thoroughly dried and then immersed for 24 hours, shall not exceed
6 per cent. All bricks shall be uniform in quality and percentage
of absorption.
Whenever vitrified bricks are required in the invert of the sewer,
they shall be smooth, hard, tough, and of such durability as will
fit them for this use. They shall be of standard size, well and
uniformly burned, thoroughly vitrified throughout, and free from
warps, cracks, and other defects. The surfaces and edges shall be
true and straight and the corners sharp and square. They shall be
in every respect satisfactory to the Engineer, and in all respects
equal to the sample in the office of the Engineer.
The remaining paragraphs of the specifications deal with the manner in
which samples shall be submitted and the necessity for conformity
between the samples submitted and the bricks used.
A common size of brick in use for sewers is 2¼ × 4 × 8¼ inches, but the
variations in size are many. The bricks in use on any one job should be
as near the same size as possible as the extra mortar filling necessary
to make up for small brick detracts from the strength of the sewer.
Small brick are undesirable as the cost of laying small and large bricks
is the same, but the thickness of the finished sewer is less. Sewer
brick should not absorb more than 10 to 20 per cent moisture by volume,
in 24 hours; except the special paving brick used to prevent erosion at
the invert which should absorb less than 5 per cent moisture.
=97. Vitrified Sewer Block.=—Blocks and bricks are manufactured in a
manner similar to the manufacture of vitrified sewer pipe described in
Art. 91. J. M. Egan describes two types of sewer blocks[62] as follows:
There are on the market two designs of blocks, one being a
single-ring block and the other a double-ring block. The former
has a ship-lap joint on the ends and a tongue-and-groove joint on
the sides. In the double block the laps and joints are made in the
construction of the sewer and the blocks are placed one on top of
the other as in a two ring brick sewer. The blocks are hollow
longitudinally with web braces. They are made for sewers from 30
inches to 108 inches in diameter and weigh from 40 to 120 pounds.
They are 18 inches to 24 inches long, 9 to 15 inches wide, and 5
to 10 inches thick. Short lengths are made for convenience in
construction and for use on sharp curves. Special blocks are made
for connections and junctions.
A special block is also made for inverts, which has occasionally been
used with brick sewers to avoid the difficulty of constructing with
brick at this point. Such blocks are objectionable, as they leave a line
of weakness along the longitudinal joint so formed. They are not used
frequently in present day practice.
Vitrified blocks are generally cheaper than bricks, but they do not make
so strong a structure. In some cases it is possible to lay vitrified
block without the expense of high-priced bricklayers, thus saving on the
cost of the sewer and obtaining a conduit with a smoother interior
finish.
=98. Cast Iron, Steel, and Wood.=—Cast iron, steel, and wood pipe belong
more to the field of waterworks than of sewerage, as they are not
extensively used in the construction of sewers. There are, however, some
special conditions under which these materials may be serviceable.
The iron used in cast-iron pipe for sewers, and in castings for manhole
covers, inlet frames, etc., is seldom carefully or definitely specified.
The standard specifications of the American Water Works Association with
regard to the quality of iron for water pipe are:
All pipe and special castings shall be made of cast iron of good
quality and of such character as shall make the metal of the
castings strong, tough, and of even grain and soft enough to
satisfactorily admit of drilling and cutting. The metal shall be
made without the admixture of cinder iron or other inferior metal,
and shall be remelted in a cupola or air furnace.
The specifications of the Sanitary District of Chicago for the quality
of iron to be used in manhole covers, etc., are given on page 101.
Although sewer pipes are not ordinarily subjected to internal pressure,
cast-iron pipe for sewers should be as heavy or heavier than water pipe
to resist the corrosive action of the sewage and the external stresses
that are to be imposed upon it. The sizes and details of standard
cast-iron pipe used for both water works and sewerage can be found in
specification of the American and New England Water Works Associations.
The quality of steel used for reinforcing concrete should be carefully
specified because of the possibility of the substitution of inferior
material. The specifications for “Billet Steel Concrete Reinforcement
Bars,” of the American Society for Testing Materials[63] are the
standard for engineering practice, or the following specifications may
be used:
All reinforcement shall be free from excessive rust, scale, paint,
or coatings of any character which will tend to destroy the bond.
The bars shall be rolled from new billets. No rerolled material
will be accepted. All reinforcement bars shall develop an ultimate
tensile strength of not less than 70,000 pounds per square inch.
The test specimen shall bend cold around a pin, whose diameter is
two times the thickness of the bar, 180 degrees without cracking
on the outside portion. The reinforcing bars shall in all respects
fulfill the requirements of the standard specifications of the
American Society for Testing Materials for Billet Steel Concrete
Reinforcing Bars serial designation A 15–14.
The steel used in pipe should be a soft, open-hearth steel with an
ultimate tensile strength of 60,000 pounds per square inch, an elastic
limit of 30,000 pounds per square inch, an elongation in 8 inches before
fracture between 22 and 25 per cent, and a reduction in area before
fracture of 50 per cent. The working strength of the steel is taken at
16,000 to 20,000 pounds per square inch in tension, 10,000 to 12,000
pounds per square inch in shear, and 20,000 to 24,000 pounds per square
inch in bearing. A liberal allowance should be made for corrosion. The
standard specifications for Open-Hearth Boiler Plate and Rivet Steel of
the American Society for Testing Materials, Aug. 16, 1919, include
“flange steel,” which is suitable for the manufacture of plates, and
extra soft steel which is suitable for rivets.
Steel pipe should be coated both inside and out to protect it against
corrosion. The various proprietary coatings are mainly coal tar pitches,
or mixtures of coal tar pitch and asphalt. A coal tar pitch is a
distillate of coal tar from which the naphtha has been removed and to
which about one per cent of heavy linseed oil has been added. The
coating is applied to the pipe at a temperature of about 300 degrees
Fahrenheit, by dipping hot pipe in the heated coating material. The pipe
should be carefully cleaned and all rust and scale removed before it is
dipped. In some cases the steel is pickled before dipping. This consists
in rolling the cold plates to a short radius to loosen the scale,
heating them to about 125 degrees, and dipping them in a warm 5 per cent
acid solution for about 3 minutes, and finally rinsing in a weakly basic
wash water.
The woods commonly used for the manufacture of wood pipe are spruce,
Oregon fir, Douglas fir, and California redwood. Wood pipe lines have
been constructed of other kinds of lumber but only in more or less
unusual conditions. The following has been abstracted from the
specifications for California redwood given by J. F. Partridge.[64]
The staves shall be of clear, air-dried, California redwood,
seasoned at least one year in the open air, and shall be free from
knots (except small knots appearing on one face only), sap, dry
rot, wind shakes, pitch, pitch seams, pitch pockets, or other
defects which would materially impair their strength or
durability. The sides of the staves shall be milled to conform to
the inside and outside radii of the pipe; and the edges shall be
beveled to true radial planes. The staves shall be milled from
stock sizes of lumber, the net finished thickness of the stave,
for the various diameters of pipe, shall be as given in Table 40.
The ends shall be cut square and slotted to receive the metallic
tongues which form the butt joints. The slots shall appear in the
same position on each stave, and shall be cut to make a tight fit
with the tongues in all directions. The staves shall have an
average length of at least 15 ft. 6 in. and not more than one per
cent shall have a length of less than 9 ft. 6 in. Staves shorter
than 8 ft. will not be accepted.
The bands shall be spaced on the pipe with a factor of safety of
at least four, and shall consist of round, mild steel rods,
connected with malleable iron shoes. Either open-hearth or
Bessemer steel may be used.... The ultimate strength shall be from
55,000 to 65,000 lb. per sq. in.
The original reference should be consulted for complete details and for
specifications for various kinds of wood and classes of pipe. The
discussion following the specifications is of value.
Machine-made wood pipe is superior to stave pipe put together in the
field. It is seldom manufactured in sizes large enough for use in
sewers, which results in the almost exclusive use of field constructed
stave pipe. The steel bands used to hold the staves together should be
coated similarly to steel plates. All lumber, except California redwood
should receive a preservative coating of creosote[65] or other material.
One of the best methods of preserving the wood is to keep it submerged
and to maintain the pipe under internal pressure.
TABLE 40
DETAILS OF DESIGN FOR CONTINUOUS STAVE WOOD PIPE
CLASSES A, B, AND C
(By J. F. Partridge, Trans. A. S. C. E., Vol. 82, page 461)
───────────┬───────────┬───────────┬───────────┬───────────┬───────────
│ Stave │Stock Size │ Size of │ Top Width │Spacing of
│Thickness, │of Lumber, │ Band, │of Staves, │ Bands for
Diameter, │ Standard, │ Inches │ Inches │ Standard, │ 100 Feet
Inches │ Inches │ │ │ Inches │ Head
───────────┼───────────┼───────────┼───────────┼───────────┼───────────
12│ 1⅜ │ 2 × 4 │ ⅜ │ 3.56 │ 6.38
18│ 1–7/16 │ 2 × 4 │ 7/16 │ 3.66 │ 5.76
24│ 1–7/16 │ 2 × 4 │ 7/16 │ 3.70 │ 4.34
30│ 1½ │ 2 × 6 │ ½ │ 5.48 │ 4.53
36│ 1–9/16 │ 2 × 6 │ ½ │ 5.62 │ 3.77
42│ 1⅝ │ 2 × 6 │ ½ │ 5.51 │ 3.23
│ 1⅝ │ 2 × 6 │ ½ or ⅝ │ 5.60 │ 2.84 or
48│ │ │ │ │ 4.41
60│ 2½ │ 3 × 6 │ ⅝ │ 5.56 │ 3.54
│ 3½ │ 4 × 6 │ ⅝ or ¾ │ 5.69 │ 2.95 or
72│ │ │ │ │ 4.24
84│ 3½ │ 4 × 6 │ ¾ │ 5.65 │ 3.63
120│ 3⅝ │ 4 × 6 │ ¾ │ 5.68 │ 2.54
│ 3⅝ │ 4 × 6 │ ¾ or ⅞ │ 5.64 │ 2.12 or
144│ │ │ │ │ 2.89
───────────┴───────────┴───────────┴───────────┴───────────┴───────────
CHAPTER IX
DESIGN OF THE SEWER RING
=99. Stresses in Buried Pipe.=—The stresses which sewer pipe should be
designed to resist are: internal bursting pressure, for sewers flowing
under pressure; stresses due to handling, for precast pipe; temperature
stresses; and external loads. The latter is by far the most important
and frequently is the only stress considered in design.
The thickness of a pipe to resist internal stress should be
(_PR_)⁄_f__{_t_},
in which _P_ = the intensity of internal pressure;
_R_ = the radius of the inside of the pipe, and
_f__{_t_} = the unit-strength of the material in tension
The derivation of this expression is simple. The stresses due to
handling cannot be computed and are cared for by a thickness of material
dictated by experience. These thicknesses are given for vitrified clay
and cement pipe in the specifications in the preceding chapter.
Temperature stresses are not allowed for in the design of the pipe ring,
but allowance must be made for them in long rigid pipe lines exposed to
wide variations in temperature. Such a condition seldom exists in
sewerage works.
The external forces are ordinarily the controlling features in the
design of sewer rings. The simplest problems arise in the design of a
circular pipe. If the external loading is uniform about the
circumference of the pipe the internal stresses will all be compression.
Almost all other forms of loading will cause bending moments resulting
in tension and compression in different parts of the pipe. The maximum
bending is caused by two concentrated loads diametrically opposed. As
such a condition is extreme it is not cared for in ordinary design, but
a loading between this condition and perfect distribution is assumed, as
explained in Art. 103.
=100. Design of Steel Pipe.=—The stresses which may occur in steel sewer
pipes are commonly caused by the internal or bursting pressure of the
contained liquid. Occasionally a steel pipe may be used as a bridge or
as a stressed member of a bridge, but steel pipes should not be used to
withstand compression normal to the axis. In order to avoid such
stresses the bursting tensile stresses should exceed the external
compressive stresses. Such a condition in design requires that buried
pipes shall never be emptied, a condition that cannot always be
fulfilled. Precaution should be taken, by the installation of proper
valves, to prevent the emptying of the pipe at so rapid a rate that a
vacuum is created resulting in the collapse of the pipe.
Steel pipes are ordinarily made of plates curved to the proper diameter,
the edges being held together by rivets. The design of the pipe consists
in the determination of the thickness of the plate and the design of the
riveted joint. The longitudinal joint and the thickness of the plate are
first designed. The design of the joint consists in determining the
diameter and pitch of the rivets and the thickness of the plate so that
the full strength of the uncut metal shall be developed as nearly as
possible under bearing, tearing, and shearing. This is done by making
the efficiency of the joint the same under all stresses. The efficiency
of the joint is the ratio of the strength of the joint under any kind of
stress to the strength in tension of the unpunched plate. Properties of
riveted joints are given in Table 41.
The diameter of the rivet holes should be computed as 1/16 of an inch
larger than the diameter of the rivets. Rivets and plates should be
designed for the nearest or next largest commercial size, and a generous
allowance for corrosion should be made in determining the thickness of
the plate. The distance from the edge of the plate to the side of the
rivet should not be less than 1½ times the diameter of the rivet. The
unit-strengths of the metal are given in the preceding chapter.
The transverse joint must be designed empirically as the stresses in it
are indeterminate. The common form of joint for pipes less than 48
inches in diameter is a single-riveted lap joint, and for larger pipes
or for pipes exposed to unusual stresses, a double riveted lap joint is
used. The same size rivets are used as in the longitudinal joint. The
maximum permissible distance between rivets should be used in the
transverse joint.
TABLE 41
PROPERTIES OF RIVETED JOINTS
(Chicago Bridge and Iron Works)
──────────────────────┬─────────┬────────┬────────┬──────────┬─────────
Type of Joint │Thickness│Diameter│ Pitch, │Efficiency│Thickness
│ Plate, │ of │ Inches │of Joint, │ Butt
│ Inch │ Rivet, │ │ Per Cent │ Plate,
│ │ Inch │ │ │ Inches
──────────────────────┼─────────┼────────┼────────┼──────────┼─────────
Single-riveted lap │ ¼ │ ⅝ │ 1.88 │ 49 │
│ ¼ │ ¾ │ 2.25 │ 50 │
│ 5/16 │ ⅞ │ 2.63 │ 50 │
──────────────────────┼─────────┼────────┼────────┼──────────┼─────────
Double riveted lap │ ¼ │ ⅝ │ 2.50 │ 70 │
│ 5/16 │ ¾ │ 3.00 │ 71 │
│ ⅜ │ ⅞ │ 3.40 │ 71 │
──────────────────────┼─────────┼────────┼────────┼──────────┼─────────
Triple riveted lap │ ¼ │ ½ │ 2.39 │ 74 │
│ 5/16 │ ⅝ │ 2.96 │ 74 │
│ ⅜ │ ¾ │ 3.53 │ 75 │
│ 7/16 │ ⅞ │ 4.09 │ 76 │
──────────────────────┼─────────┼────────┼────────┼──────────┼─────────
Quadruple riveted lap │ ⅜ │ ⅝ │ 3.20 │ 77 │
│ 7/16 │ ¾ │ 3.90 │ 78 │
──────────────────────┼─────────┼────────┼────────┼──────────┼─────────
Double riveted butt │ ½ │ ⅞ │ 3.62 │ 72 │ ⅜
│ 9/16 │ ⅞ │ 3.62 │ 72 │ ⅜
│ ⅝ │ ⅞ │ 3.62 │ 72 │ ⅜
│ 11/16 │ ⅞ │ 3.62 │ 72 │ 7/16
│ ¾ │ 1 │ 4.12 │ 73 │ 7/16
│ ⅞ │ 1 │ 3.82 │ 71 │ ½
│ 1 │ 1 │ 3.48 │ 68 │ 9/16
──────────────────────┼─────────┼────────┼────────┼──────────┼─────────
Triple riveted butt │ ⅝ │ ⅞ │ 4.94 │ 80 │ ½
│ ¾ │ 1 │ 5.62 │ 80 │ 9/16
│ ⅞ │ 1 │ 5.16 │ 78 │ 9/16
│ 1 │ 1 │ 4.66 │ 76 │ 9/16
──────────────────────┼─────────┼────────┼────────┼──────────┼─────────
Quadruple riveted butt│ ¾ │ 1 │ 7.13 │ 84 │ ¾
│ ⅞ │ 1 │ 6.51 │ 83 │ 11/16
│ 1 │ 1 │ 5.84 │ 81 │ ⅝
──────────────────────┴─────────┴────────┴────────┴──────────┴─────────
Pipes used as compression members of a bridge are stiffened by riveting
standard rolled steel sections longitudinally on the pipe.
[Illustration:
FIG. 78.—Lock Bar Pipe.
]
Lock Bar Pipe is a steel pipe with a special form of joint made by the
East Jersey Pipe Corporation. It is arranged as shown in Fig. 78 and has
the advantage of developing the full strength of the plate. It is
equivalent to a joint with 100 per cent efficiency, which permits the
use of thinner plates.
=101. Design of Wood Stave Pipe.=—In the design of wood stave pipe[66]
the entire bursting pressure is taken up by steel bands wrapped around
the outside of wood staves which make up the shell of the pipe. The pipe
is not designed to resist external loads except those which may be
overcome by the internal pressure in the pipe. The thickness of the
staves is fixed by experience. The sizes of staves and bands recommended
by J. F. Partridge[67] are given in Table 40. The size of the steel
bands can be determined from the expression;
_S_ = _Cr_(_R_ + _t_)
in which _S_ = the total stress in the band;
_R_ = the radius of the inside of the pipe;
_t_ = the thickness of the stave;
_r_ = the area of bearing per unit length of the band on the
wood. For circular bands it is assumed as the radius
of the band;
_C_ = the crushing strength of wood, usually taken at 650
pounds per sq. in.
The preceding expression can be derived easily by the application of the
laws of mechanics, and from it the expression for the distance between
bands follows logically. It is,
_p_ = _S_⁄(_PR_ + _kt_)
in which _S_ = the strength of the band;
_p_ = the distance between bands;
_P_ = the intensity of bursting pressure in the pipe;
_R_ = the radius of the inside of the pipe;
_t_ = the thickness of the staves;
_k_ = the swelling strength of wood, usually taken at 100
pounds per sq. in.
[Illustration:
FIG. 79.—Shoe for Wood Stave Pipe.
]
Transverse joints between staves are closed by inserting metal strips
between them, or by shaping the edges irregularly so that they fit
closely together with an irregular joint. Transverse joints between all
staves at any one point are avoided by splitting the joints between
staves. Longitudinal joints between staves are usually made smooth and
are closed by steel bands which are drawn tight about the pipe by
inserting the ends in coupling shoes as shown in Fig. 79.
[Illustration:
FIG. 80.—_B_ in Formula _W_ = _CwB_^2
]
=102. External Loads on Buried Pipe.=—Prof. Anston Marston and H. C.
Anderson published[68] the results of a series of experiments on the
loads on buried pipes which are of extreme value in the design of sewer
pipe. The load on the pipe is given by the empirical expression _W_ =
_CwB_^2, in which _w_ is the weight of the backfilling material in
pounds per cubic foot, _B_ is the width of the trench in feet at the
elevation of the end of a radius making an angle of 45 degrees upwards
with the horizontal diameter of the pipe as illustrated in Fig. 80, and
_C_ is a coefficient dependent on the character of the backfill and the
ratio of the width to the depth of the trench. Values of _C_ are given
in Table 42. The weights of various classes of backfilling are given in
Table 43.
TABLE 42
APPROXIMATE SAFE WORKING VALUES OF _C_ IN THE EXPRESSION _W_ = _CwB_^2
From Bulletin No. 31 of the Engineering Experiment Station, Iowa State
College of Agriculture.
──────────────┬────────────────────────────────────────────────────────
Ratio of Depth│ Approximate Values of _C_
to Width │
──────────────┼──────────────┬─────────────┬─────────────┬─────────────
│Damp Top Soil │Saturated Top│ Damp Yellow │ Saturated
│ and Dry and │ Soil │ Clay │ Yellow Clay
│ Wet Sand │ │ │
──────────────┼──────────────┼─────────────┼─────────────┼─────────────
0.5 │ 0.46 │ 0.47 │ 0.47 │ 0.48
1.0 │ 0.35 │ 0.86 │ 0.88 │ 0.90
1.6 │ 1.16 │ 1.21 │ 1.25 │ 1.27
3.0 │ 1.47 │ 1.51 │ 1.56 │ 1.62
2.6 │ 1.70 │ 1.77 │ 1.83 │ 1.91
3.0 │ 1.90 │ 1.99 │ 2.08 │ 2.19
3.6 │ 2.08 │ 2.18 │ 2.28 │ 2.43
4.0 │ 2.22 │ 2.35 │ 2.47 │ 2.65
4.6 │ 2.34 │ 2.49 │ 2.63 │ 2.85
6.0 │ 2.45 │ 2.61 │ 2.78 │ 3.02
6.5 │ 2.54 │ 2.72 │ 2.90 │ 3.18
6.0 │ 2.61 │ 2.81 │ 3.01 │ 3.32
6.6 │ 2.68 │ 2.89 │ 3.11 │ 3.44
7.0 │ 2.73 │ 2.95 │ 3.19 │ 3.55
7.5 │ 2.78 │ 3.01 │ 3.27 │ 3.66
8.0 │ 2.82 │ 3.06 │ 3.33 │ 3.74
8.5 │ 2.85 │ 3.10 │ 3.39 │ 3.82
9.0 │ 2.88 │ 3.14 │ 3.44 │ 3.89
9.5 │ 2.90 │ 3.18 │ 3.48 │ 3.96
10.0 │ 2.92 │ 3.20 │ 3.52 │ 4.01
11.0 │ 2.95 │ 3.25 │ 3.58 │ 4.11
12.0 │ 2.97 │ 3.28 │ 3.63 │ 4.19
13.0 │ 2.99 │ 3.31 │ 3.67 │ 4.25
14.0 │ 3.00 │ 3.33 │ 3.70 │ 4.30
15.0 │ 3.01 │ 3.34 │ 3.72 │ 4.34
∞ │ 3.03 │ 3.38 │ 3.79 │ 4.50
──────────────┴──────────────┴─────────────┴─────────────┴─────────────
TABLE 43
APPROXIMATE WEIGHTS OF DITCH FILLING MATERIAL TO BE USED IN THE
EXPRESSION _W_ = _CwB_^2[69]
───────────────────────────────────┬───────────────────────────────────
Ditch Filling │ Pounds per Cubic Foot
───────────────────────────────────┼───────────────────────────────────
Partly compacted top soil (damp) │ 90
Saturated top soil │ 110
Partly compacted damp yellow clay │ 100
Saturated yellow clay │ 130
Dry sand │ 100
Wet sand │ 120
───────────────────────────────────┴───────────────────────────────────
Where surface loads are to be carried on the sewer trench the proper
proportion of the load to be carried by the sewer is determined by the
expression _L__{_p_} = _CL_, in which _L__{_p_} is the equivalent
backfill load per unit length of the trench, _L_ is the surface load per
unit length of the trench, and _C_ is a coefficient in which allowance
is made for the character of the backfilling, the ratio of depth to
width of trench, and the character of the load, whether long or short. A
long load is a load extending along the length of the trench such as a
pile of building material. A short load is one extending across the
trench and for only a short distance along it, such as that caused by a
street car or road roller crossing the trench. Values of _C_ are given
in Table 44 for long loads, and in Table 45 for short loads. Values of
long and short loads occasionally met in practice are given in Tables 46
and 47 respectively.
TABLE 44
RATIO OF LOAD ON PIPE TO LONG LOAD ON TRENCH[70]
──────────────┬──────────────┬──────────────┬──────────────┬──────────────
Ratio of Depth│Sand and Damp │Saturated Top │ Damp Yellow │ Saturated
to Width │ Top Soil │ Soil │ Clay │ Yellow Clay
──────────────┼──────────────┼──────────────┼──────────────┼──────────────
0.0│ 1.00│ 1.00│ 1.00│ 1.00
0.5│ 0.85│ 0.86│ 0.88│ 0.89
1.0│ 0.72│ 0.75│ 0.77│ 0.80
1.5│ 0.61│ 0.64│ 0.67│ 0.72
2.0│ 0.52│ 0.53│ 0.59│ 0.64
2.5│ 0.44│ 0.48│ 0.52│ 0.57
3.0│ 0.37│ 0.41│ 0.45│ 0.51
4.0│ 0.27│ 0.31│ 0.35│ 0.41
5.0│ 0.19│ 0.23│ 0.27│ 0.33
6.0│ 0.14│ 0.17│ 0.20│ 0.26
8.0│ 0.07│ 0.09│ 0.12│ 0.17
10.0│ 0.04│ 0.05│ 0.07│ 0.11
──────────────┴──────────────┴──────────────┴──────────────┴──────────────
For example, let it be desired to determine the load on a 72–inch
concrete sewer with a 9–inch shell under the following conditions:
depth of backfill over the top of the pipe, 15 feet; character of
backfill, saturated yellow clay; superimposed load, pile of
building brick 6 feet high. The ratio of the depth of backfill to
the width of the trench is 15 ÷ 9 or 1.67. The coefficient in the
expression _CwB_^2 is 1.39, from Table 42. The weight of saturated
yellow clay is 130 pounds per cubic foot, from Table 43. Therefore
the load per foot length of the sewer due to the backfill is:
_W_ = _CwB_^2 = 1.39 × 130 × 81 = 14,600 pounds.
TABLE 45
RATIO OF LOAD ON PIPE TO SHORT LOAD ON TRENCH[71]
───────┬───────────────┬───────────────┬───────────────┬───────────────
Ratio │ │ │ │
of │ │ │ │
Height │ │ │ │
to │ │ │ │
Width │ │ │ │
of │ Sand and Damp │ Saturated Top │ Damp Yellow │ Saturated
Trench │ Top Soil │ Soil │ Clay │ Yellow Clay
───────┼───────────────┴───────────────┴───────────────┴───────────────
│ Length of Load Equal to
───────┼───────┬───────┬───────┬───────┬───────┬───────┬───────┬───────
│ Width │⅒ Width│ Width │⅒ Width│ Width │⅒ Width│ Width │⅒ Width
│ of │ of │ of │ of │ of │ of │ of │ of
│Trench │Trench │Trench │Trench │Trench │Trench │Trench │Trench
───────┼───────┼───────┼───────┼───────┼───────┼───────┼───────┼───────
0.0│ 1.00│ 1.00│ 1.00│ 1.00│ 1.00│ 1.00│ 1.00│ 1.00
0.5│ 0.77│ 0.12│ 0.78│ 0.13│ 0.79│ 0.13│ 0.81│ 0.13
1.0│ 0.59│ 0.02│ 0.61│ 0.02│ 0.63│ 0.02│ 0.66│ 0.02
1.5│ 0.46│ │ 0.48│ │ 0.51│ │ 0.54│
2.0│ 0.35│ │ 0.38│ │ 0.40│ │ 0.44│
2.5│ 0.27│ │ 0.29│ │ 0.32│ │ 0.35│
3.0│ 0.21│ │ 0.23│ │ 0.25│ │ 0.29│
4.0│ 0 12│ │ 0.12│ │ 0.16│ │ 0.19│
5.0│ 0.07│ │ 0.09│ │ 0.10│ │ 0.13│
6.0│ 0.04│ │ 0.05│ │ 0.06│ │ 0.08│
8.0│ 0.02│ │ 0.02│ │ 0.03│ │ 0.04│
10.0│ 0.01│ │ 0.01│ │ 0.01│ │ 0.02│
───────┴───────┴───────┴───────┴───────┴───────┴───────┴───────┴───────
TABLE 46
WEIGHTS OR COMMON BUILDING MATERIAL WHEN PILED FOR STORAGE. POUNDS PER
CUBIC FOOT
────────────────────────────────────────┬──────────────────────────────
Brick │ 120
Cement │ 90
Sand │ 90
Broken stone │ 150
Lumber │ 35
Granite paving │ 160
Coal │ 50
Pig iron │ 400
────────────────────────────────────────┴──────────────────────────────
The pressure of the pile of brick per square foot of trench area
is, from Table 46, 120 × 6 = 720 pounds per square foot. The value
of _C_ from Table 44, is about 0.70. Therefore _L_{p}_ is 0.7 × 9
× 720 = 4536 pounds. The equivalent depth of backfill weighing 130
pounds per cubic foot is (4536)⁄130 × 9 = 3.88 foot. The total
equivalent depth of backfill is therefore 3.88 + 15 = 18.88 feet.
The ratio of depth to width is 18.88⁄9 = 2.98. The coefficient _C_
in the expression _W_ = _CwB_^2 is 2.17. The total load per foot
length of sewer is therefore _W_ = 2.17 × 130 × 81 = 22,800
pounds.
TABLE 47
WEIGHTS OF SHORT LOADS ON SEWER TRENCHES
(Adapted from Specifications of the American Bridge Company for
Bridges)
──────────────────────────────┬────────────────────────────────────────
Street railways, heavy │A load of 24 tons on 2 axles on 10 foot
│ centers.
Street railways, light │A load of 18 tons on 2 axles on 10 foot
│ centers.
For city streets, heavy │A load of 24 tons on 2 axles 10 feet
traffic │ apart and 5 foot gage.
For city streets, moderate │A load of 12 tons on 2 axles 10 feet
traffic │ apart and 5 foot gage.
For city streets, light │A load of 6 tons on 2 axles 10 feet
traffic or country roads │ apart and 5 foot gage.
│
Road rollers │Total weight 30,000 pounds. Weight on
│ front wheel, 12,000 pounds, and on
│ each of two rear wheels, 9,000 pounds.
│ Width of front wheel, 4 feet and of
│ each of two rear wheels 20 inches.
│ Distance between front and rear axles
│ 11 feet. Gage of rear wheels, 5 feet,
│ c. to c.
──────────────────────────────┴────────────────────────────────────────
=103. Stresses in Circular Ring=—In Fig. 81_a_ the loads shown indicate
the distribution ordinarily assumed in sewer design, the forces being
uniformly distributed across the diameter. To find the bending moment in
the pipe caused by this loading, let _ab_ in Fig. 81_b_ represent a
section of a pipe loaded with equally distributed horizontal and
vertical forces. Then the vertical component on a strip of differential
length _ds_ is _wds_ cos Θ and the horizontal component is _wds_ sin Θ
and resolving, the resultant normal to the surface is _wds_, in which
_w_ is the intensity per unit length of the horizontal and vertical
forces and Θ is the angle which the tangent to _ds_ makes with the
horizontal. Thus the loading of the nature shown in Fig. 81_b_ is
equivalent to a loading of equally distributed normal forces which give
no moment in the ring.
[Illustration:
FIG. 81.—Distribution of Stresses on Buried Pipe.
]
Considering a ring subjected to vertical forces only, the moments will
be as shown in Fig. 81_c_ and if loaded with horizontal forces only, the
moments will be as shown in Fig. 81_d_. Because of the symmetry of the
figure, moment (1) equals moment (4) but is opposite in direction and
moment (2) equals moment (3) but is opposite in direction. When the
horizontal and vertical forces are combined on the same ring as in Fig.
81_b_ these moments cancel each other as has been proven. Therefore
moment (1) equals moment (2) and moment (3) equals moment (4). Then in
Fig. 81_e_, _M_{a}_ = _M_{b}_. Now ∑_M_ = _O_ for conditions of
equilibrium, therefore _M_{a}_ + _M_{b}_ + (_W_⁄2)(_d_⁄4) = _O_ and
solving _M_{a}_ = (_Wd_)⁄16. This moment occurs at the ends of the
horizontal and vertical diameters and causes tension on the inside of
the pipe at the top and on the outside at the ends of the horizontal
diameter. There will also be compression at each end of the horizontal
diameter equal to one-half of the total load on the pipe. If the
material of the pipe is homogeneous, the maximum fiber stress _f_ can be
found through the expression _f_ = (_My_)⁄_I_ ± _P_⁄_A_ in which _M_ is
the bending moment, _y_ is the distance from the neutral axis to the
extreme fiber of a cross-section of the shell of the pipe of unit
length, _I_ is the moment of inertia of this cross-section about its
neutral axis, _P_ is one-half the total load on the pipe, and _A_ is the
area of the cross-section. For reinforced concrete, the standard
formulas should be used with this expression for _M_. The stresses in a
circular ring subjected to other distributions of loads are shown in
Table 48. An exhaustive study of the stresses in circular rings was
published by Prof. A. N. Talbot in Bulletin No. 22 of the Engineering
Experiment Station at the University of Illinois, 1908.
TABLE 48
MAXIMUM STRESS IN FLEXIBLE RINGS DUE TO DIFFERENT LOADINGS
(From Marston)
──────────────────┬───────────────┬───────────────┬───────────
Symmetrical │Moment at Crown│ Moment at End │Compressive
Vertical Loadings │ of Sewer │ of Horizontal │ Thrust at
│ │ Diameter │ Crown
│ │ │
│ │ │
────────────┬─────┼───────────────┼───────────────┼───────────
Character │Width│ │ │
────────────┼─────┼───────────────┼───────────────┼───────────
Concentrated│ 0°│+ .318_R__W_/12│- .182_R__W_/12│ 0.000
Uniform │ 60°│+ .207_R__W_/12│- .168_R__W_/12│ 0.000
Uniform │ 90°│+ .169_R__W_/12│- .154_R__W_/12│ 0.000
Uniform │ 180°│+ .125_R__W_/12│- .125_R__W_/12│ 0.000
────────────┴─────┴───────────────┴───────────────┴───────────
──────────────────┬────────────┬───────────┬──────────
Symmetrical │Compressive │ Shear at │ Shear at
Vertical Loadings │ Thrust at │ Crown │ End of
│ End of │ │Horizontal
│ Horizontal │ │ Diameter
│ Diameter │ │
────────────┬─────┼────────────┼───────────┼──────────
Character │Width│ │ │
────────────┼─────┼────────────┼───────────┼──────────
Concentrated│ 0°│+ .500_W_/12│0.500_W_/12│ 0.000
Uniform │ 60°│+ .500_W_/12│0.000_W_/12│ 0.000
Uniform │ 90°│+ .500_W_/12│0.000_W_/12│ 0.000
Uniform │ 180°│+ .500_W_/12│0.000_W_/12│ 0.000
────────────┴─────┴────────────┴───────────┴──────────
_R_ = the radius of the pipe, _W_ = total weight of ditch filling and
superimposed load plus ⅝ of the weight of the pipe itself (usually
neglected), expressed in pounds per foot length of pipe. Moments are
inch-pounds per inch length of pipe. Shears and thrusts are in pounds
per inch length of pipe.
=104. Analysis of Sewer Arches.=—The preceding method for the
determination of the stresses in a sewer ring has referred only to a
circular pipe uniformly loaded. Other methods must be used if the pipe
is not circular or the load is not uniformly distributed. The simplest
method, is the static or so-called vouissoir method. In this method the
arch is assumed to be fixed at both ends, presumably at the springing
line or line of intersection between the inside face of the arch and the
abutment, and it is so designed that the resultant of all the forces
acting on any section shall lie within the middle third of that section.
[Illustration:
FIG. 82.—Voussoir Arch Analysis.
]
[Illustration:
FIG. 83.—Force Polygon for Voussoir Arch Analysis.
]
To design an unreinforced sewer arch by the vouissoir method, a desired
arch is drawn to scale in apparently good proportions for the loadings
anticipated. The arch is then divided into any number of sections of
equal or approximately equal length called vouissoirs, and the line of
action of the resultant load, including the weight of the vouissoir is
drawn above each vouissoir as shown in Fig. 82. The forces are assumed
to act as shown in the figure. In symmetrically loaded sewer arches
there is no vertical reaction at the crown. The resultant _R_ is assumed
to act at the lower middle third of the skewback, which is the inclined
joint between the arch and the abutment. The upper horizontal force _H_
is assumed to act at the upper middle third of the middle or crown
section. The magnitude of _H_ is computed by equating the sum of the
moments of all forces about the point of application of _R_ at the
skewback to zero, and solving. The force polygon is then drawn as shown
in Fig. 83, and the equilibrium polygon is completed in Fig. 82 with its
rays parallel to the corresponding strings drawn from the end of _H_ as
origin in Fig. 83. If the equilibrium polygon line, called the
resistance line, lies wholly within the middle third of each vouissoir,
the arch is satisfactory to support the assumed load without
reinforcement. If any portion of the resistance line lies outside of the
middle third, an attempt should be made to find a resistance line which
lies wholly within the middle third. The true resistance line is that
which deviates the least from the neutral axis of the arch. To
approximate more nearly the true resistance line find two points at
which the resistance line already drawn deviates the most from the
neutral axis of the arch. Select points _M_ and _N_ on these joints, _M_
being nearer the crown than _N_. Then let _W_{1}_ and _W_{2}_ be the sum
of all the loads between the crown and _M_ and _N_ respectively, _y_
represent the vertical distance from the crown to _N_, and _y′_
represent the vertical distance between _M_ and _N_, and _x_{1}_ and
_x_{2}_ represent the horizontal distance from _W_{1}_ and _W_{2}_ to
_M_ and _N_ respectively. Then the horizontal thrust, _H_, and _a_, the
distance from the crown to the point of application of _H_, are,
_H_ = ((_W_{2}x_{2}_ − _W_{1}x_{1})_)⁄_y′_,
_a_ = _y_ − (_W_{2}x_{2}_)⁄_H_.[72]
A resistance line should be drawn with this new horizontal thrust. If no
resistance line can be found lying wholly within the middle third, new
sections should be designed until a resistance line can be drawn lying
wholly within the middle third—unless the arch is to be reinforced. A
number of satisfactory arches should be designed and the easiest one to
build should be selected. This method is limited in its application to
sewer arches with rigid side walls and it cannot be extended to include
the invert. Although an approximate method it is accurate within less
than 10 per cent of the true stresses and is usually quite close.
[Illustration:
FIG. 84.—Method for Dividing Arch into Proportion _I_⁄_S_.
]
The elastic method for the design of arches locates the true line of
resistance without approximations and is more accurate though not so
simple to apply as the static or vouissoir method. In this method a
desired form of arch is drawn as in the static method and subdivided
into vouissoirs so that the distance _S_ along the neutral axis between
joints is such that the ratio _I_⁄_S_ shall be the same for all
vouissoirs. _I_ is the average of the moments of inertia of the surfaces
of the two limiting joints about the neutral axis. If the thickness of
the arch is constant the distance between joints will be the same. The
method for dividing the arch into sections such that the ratio _I_⁄_S_
shall be a constant[73] is as follows: divide the half arch axis into
any number of equal parts; measure the radial depth at each point of
division; lay off the length of the arch axis to scale on a straight
line; divide this line into the same number of equal parts as the half
arch, as shown in Fig. 84; at each point erect a perpendicular equal in
length by scale to the moment of inertia at the corresponding point on
the arch section; draw a smooth curve through the tops of these lines;
draw a line _ab_ at any slope from the center of the original straight
line to the curve, and then a line _bc_ back to the straight line to
form an isosceles triangle _abc_; continue forming these triangles in a
similar manner thus dividing the original straight line in the required
ratio. The distance between joints is represented by the bases of the
triangles. By construction the altitude of the triangle represents the
average moment of inertia between the two limiting joints. The base of
each isosceles triangle is _S_, and _I_⁄_S_ = ½ tan α in which α is the
base angle of all the isosceles triangles.
[Illustration:
FIG. 85.—Elastic Arch Analysis.
]
The following steps in the procedure are taken from the second edition
of the American Civil Engineers Pocket Book, p. 634:
In Fig. 85 let the middle points of the joints be marked 1, 2, 3,
etc. and the coordinates _x_ and _y_ from the crown be found for
each by computation or measurement. For a load _W_ placed at one
of these points, let _z_ denote the distance from it, toward the
nearest skewback, to another middle point. Let ∑_zx_ be the sum of
the products of all the values of _z_ by the corresponding _x_,
and ∑_zy_ be the sum of all the products of _z_ by the
corresponding _y_; that is, each _z_ in the last two summations is
multiplied by the _x_ or _y_ of the point back of _W_ which
corresponds to _z_.
For a single load _W_ on the left semi-arch of Fig. 85 the
following formulas are deduced from the elastic theory, _n_ being
the number of parts into which the semi-arch is divided.
Horizontal thrust, _H_ = (_W_⁄2)(_n_∑_zy_ − ∑_y_·∑_z_)⁄(_n_∑_y_^2
− (∑_y_)^2) (1)
Moment at Crown, _M__{0} = (½_W_∑_z_ − _H_∑_y_)⁄_n_ (2)
Shear at Crown, _V__{0} = (½_W_∑_zx_)⁄∑_x_^2 (3)
For symmetrical loading such as _W_ on the left and _W_ on the
right the horizontal thrust and crown moment due to both loads are
double those found by the above formulas, while the crown shear
_V__{0} is zero. For several loads unsymmetrically placed the
formulas are to be applied to each in succession and the results
added algebraically, the value of _V__{0} being taken as negative
for the left semi-arch and positive for the right semi-arch.
For any joint whose middle point is at a distance _x_ from the
crown
_M_ = _M__{0} + _Hy_ + _V__{0}_x_ − ∑_Wz_,
_V_ = _V__{0} − ∑_W_,
where ∑_W_ is the sum of all the loads between the joint and the
crown and ∑_Wz_ is the sum of the moments of those loads with
respect to the middle of the joint. The components of the
resultant thrust normal and parallel to the joints are,
_N_ = _H_ cos θ − _V_ sin θ,
_F_ = _H_ sin θ + _V_ cos θ,
in which θ is the angle which the plane of the joint makes with
the vertical.
The distances from the neutral axis to the resistance line are,
at the crown, _e__{0} = _M__{0}⁄_H_,
at the joint, _e_ = _M_⁄_N_.
The resistance line should be located as in the vouissoir method and if
not within the middle third a new design should be studied.
=105. Reinforced Concrete Sewer Design.=—The method to be followed in
the design of reinforced concrete arches is similar except that the
moment of inertia should include both the concrete and the steel, that
is,
_I_ = _I_{c}_ + _nI_{s}_,
in which _I_ is the moment of inertia to be employed, _I_{c}_ is the
moment of inertia of the concrete, _I_{s}_ is the moment of inertia of
the steel, and _n_ is the ratio of their moduli of elasticity, generally
taken as 15. All of the moments of inertia are referred to the neutral
axis of the beam. The reinforcement called for in precast circular pipes
is given in Table 39. Sewers cast in place are ordinarily designed to
avoid reinforcement, except where the depth of cover is small and the
sewer may be subjected to superimposed loads.
Concrete sewers are sometimes reinforced longitudinally, with expansion
joints from 30 to 50 feet apart. This reinforcement is to reduce the
size of expansion and contraction cracks by distributing them over the
length of a section. The pipe is divided into sections to concentrate
motion due to expansion or contraction at definite points where it can
be cared for.
The amount of longitudinal reinforcement to be used is a matter of
judgment. It varies in practice from 0.1 to 0.4 per cent of the area of
the section. Since the coefficients of expansion of concrete and of
steel are nearly the same, movements of the structure are as important
as the stresses due to changes in temperature.
Because of the uncertain and difficult conditions under which concrete
sewers are frequently constructed it is advisable to specify the best
grade of concrete and not to stress the concrete over 450 pounds per
square inch in compression, with no allowable stress in tension. The
concrete covering of reinforcing steel should be thicker than is
ordinarily used for concrete building design, because of the possibility
of poor concrete allowing the sewage to gain access to the steel,
resulting in more rapid deterioration than would be caused by exposure
to the atmosphere. A minimum covering of about 2 inches is advisable,
except in very thin sections not in contact with the sewage. A minimum
thickness of concrete of about 9 inches is frequently used in design,
although crown thicknesses of 4½ inches have been used with success.
Greater thicknesses should be used near the surface, particularly in
locations subjected to heavy or moving loads.
Brick linings are often provided for the invert where moderately high
velocities of about 10 feet per second when flowing full are to be
expected. For velocities in the neighborhood of 20 feet per second the
invert should be lined with the best quality vitrified brick. Although
concrete may erode no faster than brick under the same conditions, brick
linings are more easily replaced and at a smaller expense.
CHAPTER X
CONTRACTS AND SPECIFICATIONS
=106. Importance of the Subject.=—Sewers may be constructed by day labor
or by contract. Under the day labor plan a city official or commission
is charged with the purchase of material, the hiring and firing of
employees, and the management of the work. Under the contract system a
private individual or company contracts to supply all the material and
labor necessary for the completion of the work.
Under the day labor plan all persons engaged are “working for the City.”
There is not the same sense of individual responsibility, the same
incentive to economize, the same feeling of loyalty that is inspired by
work under the personality of a contractor. Under either the day labor
or contract plan unscrupulous politics are likely to enter into the
relations of the employees of the city and the city officials or between
the contractor and the city officials. Neither the day labor nor the
contract plan offer a sure cure for unscrupulous political misdealings.
Under the contract plan the contractor is led to keep his bid as low as
possible, realizing the competition of other bidders, and during
construction he will obtain greater efficiency from his labor because of
their realization of the different conditions under which they are
working. In some states and cities it is illegal for the municipality to
do sewer construction except under the contract method.
The contract method is therefore used in the majority of cases, and it
is to the interest of the engineer that he be acquainted with the
essentials of contracts and specifications necessary for the proper
prosecution of sewer construction.
=107. Scope of Subject.=—The making of a contract is one of the most
common episodes of every day life. The contract may be an informal
verbal agreement to meet at a certain place at a certain time, or it may
be a formal document hedged about by confusing legal phraseology and
bearing varieties of penalties and dire consequences in the event of its
breach. The purpose of this chapter is to explain only those general
features of an engineering contract which have particular bearing upon
sewerage construction. Only the most essential points can be touched in
the limited space available to this subject, it being presumed that the
engineer is previously grounded in the principles of business law.[74]
=108. Types of Contracts.=—Contracts are known as lump sum, cost-plus,
unit-price, and by other titles indicating the method of payment.
A lump sum contract is one in which a stated amount is fixed upon,
before the execution of the contract, to be paid for all the work to be
done and materials to be furnished under the contract. Such an
arrangement is not advisable for a sewer contract, as the cautious
contractor will bid high enough to protect himself in the event of any
probable emergency. The principal must therefore pay whether the
emergency or unforeseen difficulty is met or not. The advantage of this
type of payment is that the principal knows exactly the cost of the work
to him before construction is commenced.
Cost-plus contracts are those in which the cost of the work to the
contractor is to be paid by the principal, plus, (_a_) a fixed sum of
money, (_b_) a percentage of the cost of the work, (_c_) a percentage of
the cost of the work but with a fixed limit, (_d_) a percentage of the
difference between the cost of the work and some fixed sum, or other
variations of this principle. Such contracts have the advantage that the
principal assumes all the risk in construction and therefore pays for
only those contingencies which actually arise. Except for the last named
form, they have the disadvantage that there is little or no incentive
for the contractor to keep the cost of the work down. They are most
successful where the contractor can be selected by the principal, but
where it is necessary to let contracts to the lowest bidder, the
“cost-plus” contract is not easily managed. In most states a
municipality cannot make a cost-plus contract.
A unit-price contract is one in which the amount to be paid is fixed in
proportion to the amount of work done or materials supplied. This type
of contract is the most suitable for sewer construction for a
municipality where the contract must be let to the lowest bidder. The
contractor is protected in the event of many unforeseen emergencies and
the principal is protected against a raise in bids to cover such
emergencies and against increase in the cost of the work in order to
increase the profits under a “cost-plus” contract.
It is sometimes desirable for the principal to furnish a portion of the
materials, the bidders being notified beforehand that this material will
be furnished. In this manner the quality of material is assured,
contractors with the necessary skill but small capital may be attracted
to bid, and uncertainties in the procuring of materials is eliminated.
=109. The Agreement.=—A contract is an agreement between two or more
interested parties to do a certain thing. A contract for the
construction of a sewer is an agreement between a municipality or
individual desiring sewerage facilities and a company or individual
engaged in the construction of sewers. The latter promises to construct
a sewer in return for which the former promises to pay a certain amount
of money.
The various portions of the agreement which are bound together as the
complete contract are: I. The Advertisement, II. Information and
Instructions for Bidders, III. Proposal, IV. General Specifications, V.
Technical Specifications, VI. Special Specifications, VII. Contract,
VIII. Bond, and IX. Contract Drawings. These should be fastened together
in pamphlet form and constitute the complete instrument called the
contract. No binding contract and specifications can be drawn upon
logical deductions alone as legal precedent and tried methods must be
followed to insure success. To draw up an original contract requires the
combined knowledge of an engineer and a lawyer. The engineer of to-day
writes his specifications by copying copiously from specifications used
on work which has been completed successfully. In order that selections
may be made with judgment and discrimination some examples have been
selected from existing published specifications and contracts.
=110. The Advertisement.=—This should contain: (1) A heading indicating
the type of work, (2) A statement as to when, where and how bids will be
received and opened, (3) A brief description of the character and amount
of work to be done, (4) The method of payment, (5) The conditions under
which further information can be obtained, (6) A statement as to the
amount of money which must be deposited with the bid, and (7) Any other
pertinent facts concerning the work.[75] An example of an advertisement
follows;
Sewer Construction
Construction Turkey Creek Sewer
Kansas City, Missouri.
Bids for the construction of the Turkey Creek Sewer, two sewage
pumping stations to be used in connection therewith, and certain
laterals and extensions of existing sewers thereto, for Kansas
City, Missouri, will be received up to 2 p.m. August 19, 1919, at
the office of the Board of Public Works, City Hall, Kansas City,
Missouri.
The main sewer will be about one and one-fifth miles long, and the
laterals and extensions about three and one-half miles: the main
sewer will be constructed of reinforced concrete, the laterals and
extensions will consist of concrete, segment blocks, and clay
pipe.
This work is estimated to cost from $1,500,000 to $1,750,000.
Payment for the work will be made in four year special tax bills,
bearing 7 per cent interest, payable one-fourth each year. Time
600 working days, barring strikes, bad weather, etc.
Bidders are required to deposit $15,000 in cash or a certified
check with bid, to insure signing of contract when let. Same to be
returned on execution of the contract or rejection of bid.
Complete plans and specifications for the work may be had and all
information obtained by seeing or writing to A. D. Ludlow,
Engineer of Sewers, City Hall, Kansas City, Missouri. Twenty-five
($25.00) Dollars will be required to be deposited for a set of the
plans, but $20.00 thereof will be refunded upon return of the
plans in good condition.
BOARD OF PUBLIC WORKS,
Kansas City, Missouri,
by F. E. McCabe, Secretary.
There are usually legal restrictions which require that the
advertisement be inserted a certain number of times in specified
newspapers or other advertising mediums before the opening of bids. If
the contract is of sufficient size to attract outside contractors, the
advertisement should be inserted in engineering and contracting journals
of wide circulation. Although the advertisement appears separately from
the other portions of the contract, a copy is usually bound in as the
first page of the pamphlet containing the contract and specifications
and is made an integral part thereof.
=111. Information and Instructions for Bidders.=—This is somewhat on the
order of an introduction to the pamphlet in which the specifications,
contract, and contract drawings are published. As examples of the type
of information and instructions given to prospective bidders the
abstracts below have been taken from the “Contract, Specifications,
Bond, and Proposal for the North Shore Sanitary Intercepting Sewer” by
the Sanitary District of Chicago. The information and instructions to
bidders can be divided into the following sections: 1st. Examination of
Site, 2nd. Character and Quantity of Work, 3rd. Qualification for
Bidding, 4th. Instructions for Making out Proposal, 5th. Certified
Check, and 6th. Rejection of Bids.
REQUIREMENTS FOR BIDDING AND INSTRUCTIONS TO BIDDERS
Bidders are required to submit their bids upon the following express
conditions:
Bidders must carefully examine the entire sites of the work
and the adjacent premises, and the various means of approach
to the sites, and shall make all necessary investigations to
inform themselves thoroughly as to the facilities for
delivering and handling materials at the sites and to inform
themselves thoroughly as to all the difficulties that may be
involved in the complete execution of all work under the
attached contract in accordance with the specifications hereto
attached.
Bidders are also required to examine all maps, plans, and data
mentioned in the specifications, contract or proposal as being
on file in the office of the Chief Engineer, for examination
by bidders. No plea of ignorance of conditions that exist or
that may hereafter exist or of conditions or difficulties that
may be encountered in the execution of the work under this
contract, as the result of a failure to make the necessary
examinations and investigations, will be accepted as an excuse
for any failure or omission on the part of the Contractor to
fulfill in every detail all of the requirements of said
contract, specifications and plans, or will be accepted as a
basis for any claims whatsoever for extra compensation. Upon
application all information in the possession of the Chief
Engineer will be shown to bidders, but the correctness of such
information will not be guaranteed by the Sanitary District.
The following schedule of quantities, although stated with as
much accuracy as is possible in advance, is approximate only,
and is assumed solely for the purpose of comparing bids.
Then follows an itemized schedule of the quantity of work to be done
after which comes the following:
Bidders must determine for themselves the quantities of work that
will be required, by such means as they may prefer, and shall
assume all risks as to variations in the quantities of the
different classes of work actually furnished under the contract.
Bidders shall not at any time after the submission of this
proposal, dispute or complain of the aforesaid schedules of
quantities or assert that there was any misunderstanding in regard
to the amount or the character of the work to be done, and shall
not make any claims for damages or for loss of profits because of
a difference between the quantities of the various classes of work
assumed for comparison of bids and the quantities of work actually
performed.
Proposals that contain any omissions, erasures, or alterations,
conditions or items not called for in the contract and plans
attached hereto, or that contain irregularities of any kind, will
be rejected as informal.
Bids manifestly unbalanced will not be considered in awarding the
contract.[76]
No bid will be accepted unless the party making it shall furnish
evidence satisfactory to the Board of Trustees of the Sanitary
District of Chicago of his experience and familiarity with work of
the character specified and of his financial ability to
successfully and properly prosecute the proposed work to
completion within the specified time.
Each bid shall be accompanied by a certified check, or cash, to
the amount of ten (10) per cent of the total amount of said bid
figured on the quantities given herewith, the lowest alternative
total being allowed. Said amounts deposited with bids, shall be
held until all of the bids have been canvassed and the contract
awarded and signed. The return of said check or cash to the bidder
to whom the contract for said work is awarded will be conditioned
upon his appearing and executing a contract for the work so
awarded and giving bond satisfactory to said Board of Trustees,
for the fulfillment of each contract in the amount of fifty (50)
per cent of the amount of each contract.
The said Board of Trustees reserves the right to reject any or all
bids.
Accompanying the contract form are plans which, together with the
specifications, show the work on which said tenders are to be
made.
The proposal must not be detached herefrom or from the contract by
any bidder when submitting a bid.
=112. Proposal.=—The proposal is a blank printed form on which the
bidder is required to enter the prices for which he proposes to do the
work. The proposal blank is necessary in order that the bids may be
sufficiently uniform for proper comparison. Sewers are often paid for,
particularly for small sizes, per foot of completed sewer as measured
along the center line of the pipe parallel to the surface of the ground
with the exterior length of manholes and other structures deducted.
Sometimes, under other conditions, a different rate is allowed for each
additional two feet of depth of sewer, and special structures, such as
manholes, catch-basins, flush-tanks, etc., are paid for at a unit price
according to the depth. Water connections to flush-tanks are paid for
per foot of length of service pipe laid. In especially large or
difficult work, materials are paid for at a unit-price, for example, per
cubic yard of excavation, per cubic yard of concrete, per thousand feet
board measure of lumber, etc.
The following example is taken from the contract for the North Shore
Intercepting Sewer previously quoted, to indicate the type of Proposal
used:
PROPOSAL
FOR THE CONSTRUCTION OF THE NORTH SHORE INTERCEPTING SEWER
To the Honorable, the President and the Board of Trustees of the
Sanitary District of Chicago:
Gentlemen:__The undersigned hereby certi____ that ____ ha____
examined the specifications and form of contract and the
accompanying plans for the construction of the North Shore
Intercepting sewer, and ha____ also examined the premises at and
adjacent to the sites of the proposed work, as herein described,
and the means of approach to the said sites.
The undersigned ha____ also examined the foregoing “Requirements
for Bidding and Instructions to Bidders” and propose ____ to do
all the work called for in said specifications and contract, and
shown on said plans, and to furnish all materials, tools, labor
and all appliances and appurtenances necessary to the full
completion of said work at the rates and prices for said work as
follows, to_wit:
(1_a_) For six (6) by nine (9) foot concrete sewer, complete in
place, as specified, the sum of ____ Dollars and ____ cents ($
____ ) per linear foot.
(6_a_) For manholes, concrete, complete in place, as specified the
sum of ____ Dollars and ____ cents ($ ____ ) each.
The following plans showing the work to be performed in accordance
with the attached specifications, have been examined by the
undersigned in preparing the foregoing proposal, to-wit: ____ ____
In accordance with the requirements set forth in the attached
Information and Instructions for Bidders, there is deposited
herewith the sum of ____ ____ Dollars and ____ cents ($ ____ )
which under the terms therein mentioned entitle ____ to bid on
said work, the said sum to be refunded to ____ ____ upon the
faithful performance of all conditions set forth in the
Information and Instructions for Bidders.
Name ____
Address ____
Blanks are provided for each item. No place is left at the end for a
summary. The proposal ends with an acknowledgment that the contract has
been examined completely and all preliminary directions therein have
been complied with. A blank is prepared for inserting the amount of the
required certified check, and finally for the signature of the bidders.
=113. General Specifications.=—The specifications, both general and
technical, are occasionally incorporated in the contract form, but more
frequently they are printed separately and are bound in the pamphlet
preceding the contract. The general specifications relate to the
conditions under which all work must be performed and are as applicable
to the construction of a pumping station as to the smallest lateral,
unless otherwise specified. It is not possible to include a complete set
of General Specifications in the limited space of this text, but the
more important specifications will be emphasized by examples taken from
specifications in use.[77]
The subjects covered in General Specifications are:
(1) Definitions of doubtful terms.
(2) The Engineer to settle disputes.
(3) Duties of the Engineer.
(4) Duties of the Contractor.
(5) Hours and days of work.
(6) No work to be done in the absence of an inspector.
(7) Contractor to be represented at all times.
(8) Time of commencing and completing the work.
(9) Liquidated damages for delay in completion.
(10) The City may change the plans.
(11) The City may increase the amount of the work.
(12) Inspection and its conduct.
(13) The Contractor to be acquainted with laws relating to the work.
(14) Contractor responsible for damages to persons or property.
(15) City to be protected against patent claims.
(16) Abandonment of contract and its remedy.
(17) Estimates of work done and moneys due.
(18) Payments for extra work.
(19) Character of workmen to be employed.
(20) City may reserve a sum for repairs during stipulated term after
completion.
(21) City may use money due Contractor to pay claims for labor or
materials used on the work and not paid for by the Contractor.
(22) The Contractor shall have no claim for damages on account of delay
or unforeseen difficulties.
(23) The Contractor may not assign nor sublet the contract without the
City’s consent.
(24) Cleaning up after completion.
(25) The Contractor’s relations to other contractors.
(26) The portions composing the contract.
The following examples cover the subjects named in the preceding titles:
1. Definitions. The word Engineer whenever not qualified shall
mean the Chief Engineer of the Commission, acting either directly
or through his properly authorized agents, such agents acting
severally within the scope of the particular duties entrusted to
them.
This article may include words that may be in dispute or ambiguous such
as: Board of Trustees, Elevation, City, Contractor, Rock, Earth, etc.,
etc.
2. Disputes. To prevent disputes and litigations, the Engineer
shall in all cases determine the amount, quality, and
acceptability of the work which is to be paid for under the
contract; shall decide all questions in relation to said work and
the performance thereof, and shall in all cases decide every
question which may arise relative to the fulfillment of the
contract on the part of the Contractor. His determination,
decision and estimate shall be final and conclusive, and in case
any question shall arise between the parties touching the
contract, such determination, decision, and estimate shall be a
condition precedent to the right of the Contractor to receive any
moneys under the contract.
3. Duties of the Engineer. The Engineer shall make all necessary
explanations as to the meaning and intentions of the
specifications and shall give all orders and directions, either
contemplated therein or thereby, or in every case in which a
difficulty or unforeseen condition shall arise in the performance
of the work. Should there be any discrepancies in or between, or
should any misunderstanding arise as to the import of anything
contained in the plans and specifications, the decision of the
Engineer shall be final and binding. Any errors or omissions in
plans and specifications may be corrected by the Engineer, when
such corrections are necessary for the proper fulfillment of their
intentions as construed by him.
4. Duties of the Contractor. The Contractor shall do all the work
and furnish all the labor, materials, tools and appliances
necessary or proper for performing and completing the work
required by the contract, in the manner called for by the
specifications, and within the contract time. He shall complete
the entire work at the prices agreed upon and fixed therefor to
the satisfaction of the Commission and its Chief Engineer and in
accordance with the specifications, the drawings, and such
detailed drawings as may be furnished from time to time, together
with such extra work as may be required for the performance of
which written orders may be given and received as hereinafter
provided.
The Contractor shall place sufficient lights on or near the work
and keep them burning from twilight to sunrise; shall erect
suitable railings, fences or other protections about all open
trenches, and provide all watchmen on the work, by day or night,
that may be necessary for the public safety. The Contractor shall,
upon notice from the Engineer that he has not satisfactorily
complied with the foregoing requirements, immediately take such
methods and provide such means and labor to comply therewith as
the Engineer may direct, but the Contractor shall not be relieved
of this obligation under the contract by any such notice or
directions given by the Engineer, or by neglect, failure, or
refusal on the part of the Engineer to give such notice and
directions.
The Contractor shall furnish such stakes and the necessary labor
for driving them as may be required by the Engineer. He shall
maintain the stakes when set, with reasonable diligence, and
stakes misplaced due to the carelessness of the Contractor or his
workmen shall be reset under the direction of the Engineer, at the
Contractor’s expense.
5. Night, Sunday, and Holiday Work:[78] No night, Sunday, nor
holiday work requiring the presence of an engineer or inspector
will be permitted except in case of emergency, and then only to
such an extent as is absolutely necessary and with the written
permission of the Engineer; provided that this clause shall not
operate in the case of a gang organized for regular and continuous
night, Sunday, or holiday work.
6. Absence of Engineer or Inspector. Any work done without lines,
levels, and instructions having been given by the Engineer or
without the supervision of an assistant or inspector, will not be
estimated or paid for except when such work is authorized by the
Engineer in writing. Work so done may be ordered removed and
replaced at the Contractor’s sole cost and expense.
7. Absence of Contractor. During the absence of the Contractor he
shall at all times have a duly authorized representative on the
work. The Contractor shall give written notice to the Commission
of the name and address of said representative and shall state
where and how such representative can be reached, at any and all
hours, whether by day or night.
Whenever the Contractor or his representative is not present at
any place on the work where it may be necessary to give orders or
directions, such orders or directions will be given by the
Engineer and they shall be received and promptly obeyed by the
superintendent or foreman who may have immediate charge of the
particular work in relation to which the order may be given.
8. Commencing Work. The Contractor agrees to begin the work
covered by this contract within —— days of the execution of the
contract and to prosecute the same with all due diligence and to
entirely complete the work within —— days.
It is understood and agreed that time is of the essence of this
contract, and that a failure on the part of the Contractor to
complete the work herein specified within the time specified will
result in great loss and damage to said Sanitary District and that
on account of the peculiar nature of such loss it is difficult, if
not impossible, to accurately ascertain and definitely determine
the amount thereof.
9. Liquidated Damages. It is therefore covenanted and agreed that
in case the said Contractor shall fail or neglect to complete the
work herein specified on or before the date hereinbefore fixed for
completion, the said Contractor shall and will pay the said
Sanitary District the sum of —— Dollars for each and every day the
Contractor shall be in default in the time of completion of this
contract.
Said sum of —— Dollars per day is hereby agreed upon, fixed and
determined by the parties hereto as the liquidated damages which
said Sanitary District will suffer by reason of such defaults, and
not by way of a penalty.
10. Changes in Plans. The Board reserves the right to change the
alignment, grade, form, length, dimensions or materials of the
sewers or any of their appurtenances, whenever any condition or
obstructions are met that render such changes desirable or
necessary. In case the alterations thus ordered make the work less
expensive to the Contractor a proper deduction shall be made from
the contract prices and the Contractor shall have no claim on this
account for damages or for anticipated profits on the work that
may be dispensed with. In case such alterations make the work more
expensive, a proper addition shall be made to the contract prices.
Any deduction or addition as aforesaid shall be determined and
fixed by the Engineer.
11. Extensions and Additions. In the event that any material
alterations or additions are made as herein specified which in the
opinion of the Engineer will require additional time for execution
of all the work under this contract, then, in that case the time
of completion of the work shall be extended by such a period or
periods of time as may be fixed by said Engineer and his decision
shall be final and binding upon both parties hereto, provided that
in such case the Contractor, within four (4) days after being
notified in writing of such alterations and additions, shall
request in writing an extension of time, but the provisions of
this paragraph shall not otherwise alter the provisions of this
contract with reference to _liquidated damages_, and the said
Contractor shall not be entitled to any damages or compensation
from the said Sanitary District on account of such additional time
required for the execution of the work.
12. Inspection. All materials of whatsoever kind to be used in the
work shall be subject to the inspection and approval of the
Engineer and shall be subject to constant inspection before
acceptance. Any imperfect work that may be discovered before its
final acceptance shall be corrected immediately, and any
unsatisfactory materials used in the work or delivered at the site
shall be rejected and removed on the requirement of the Engineer.
The inspection of any work shall not relieve the Contractor of any
of his obligations to perform proper and satisfactory work as
herein specified, and all work which, during the progress and
before the final acceptance, may become damaged from any cause,
shall be removed and replaced by good and satisfactory work
without extra charge therefor. The Engineer and his assistants
shall have at all times free access to every part of the work and
to all points where material to be used in the work is
manufactured, procured or stored and shall be allowed to examine
any material furnished for use in the work under this contract.
All inspection of any and all material furnished for use in work
to be performed under this contract shall be made at the site of
the work after the delivery of the material, provided, that, if
requested by the Contractor the Engineer may at his option
perform, or have performed, inspection of materials at points
other than the site of the work. In any such case the Contractor
shall pay the Sanitary District the extra cost of such inspection,
including the necessary expenses of the inspector for the extra
time expended in performing any such inspection at said other
points.
13. Legal Requirements. The Contractor shall keep himself fully
informed of all existing and future national and state laws and
local ordinances and regulations in any manner affecting those
engaged or employed in the work, or the materials used in the
work, or of all such orders and degrees of bodies or tribunals
having any jurisdiction or authority over the same, and shall
protect and indemnify the party of the first part against any
claim or liability arising from or based on the violation of such
law, ordinance, regulation, order or decree, whether by himself or
his employees.
14. Damages. If any damage shall be done by the Contractor or by
any person or persons in his employ to the owner or occupants of
any land or to any real or personal property adjoining, or in the
vicinity of the work herein contracted to be done or to the
property of a neighboring contractor the Engineer shall have the
right to estimate the amount of said damage and to cause the
Sanitary District to pay the same to the said owner, occupant, or
contractor, and the amount so paid shall be deducted from the
money due said Contractor under this contract. Said Contractor
covenants and agrees to pay all damages for any personal injury
sustained by any person growing out of any act or doing of himself
or his employees that is in the nature of a legal liability, and
he hereby agrees to indemnify and save the Sanitary District
harmless against all suits or actions of every name and
description brought against said Sanitary District, for or on
account of any such injuries, or such damages received or
sustained by any person or persons; and the said Contractor
further agrees that so much of the money due to him under this
contract, as shall be considered necessary by the Board of
Trustees of said Sanitary District, may be retained by the
Sanitary District until such suit or claim for damages shall have
been settled, and evidence to that effect shall have been
furnished to the satisfaction of said Board of Trustees.
15. Patents. It is further agreed that the Contractor shall
indemnify, keep and save harmless said Sanitary District from all
liabilities, judgments, costs, damages and expenses which may in
any wise come against said Sanitary District, or which may be the
result of an infringement of any patent by reason of the use of
any materials, machinery, devices, apparatus, or process furnished
or used in the performance of this contract, or by reason of the
use of designs furnished by the Contractor and accepted by the
Sanitary District, and in the event of any claim or suit or action
at law or in equity of any kind whatsoever being made or brought
against said Sanitary District, then the Sanitary District shall
have the right to retain a sufficient amount of money in the same
manner and upon the conditions as hereinafter specified.
16. Abandonment of Contract. If the work to be done under the
contract shall be abandoned by the Contractor, or if at any time
the Engineer shall be of the opinion, and shall so certify, in
writing, to the Commission, that the performance of the contract
is unnecessarily or unreasonably delayed, or that the Contractor
is willfully violating any of the conditions of the
specifications, or is executing the same in bad faith, or not in
accordance with the terms thereof, or if the work be not fully
completed within the time named in the contract for its
completion, the Commission may notify the Contractor to
discontinue all work thereunder, or any part thereof, by a written
notice served upon the Contractor, as herein provided; and
thereupon the Contractor shall discontinue the work, or such part
thereof, and the Commission shall thereupon have the power to
contract for the completion of said work in the manner prescribed
by law, or to procure and furnish all necessary materials,
animals, machinery, tools and appliances, and to place such and so
many persons as it may deem advisable to work at and complete the
work described in the specifications, or such part thereof, and to
charge the entire cost and expense thereof to the Contractor. And
for such completion of the work or any part thereof, the
Commission may for itself or its contractors, take possession of
and use or cause to be used any or all such materials, animals,
machinery, tools and implements of every description as may be
found on the line of the said work. The cost and expense so
charged shall be deducted from, and paid by the City out of such
moneys as may be due or may become due to the Contractor, under
and by virtue of the contract. In case such expense shall exceed
the amount which would have been payable under the contract, if
the same had been completed by the Contractor, he shall pay the
amount of such excess to the City. When any particular part of the
work is being carried on by the Commission, by contract or
otherwise, under the provisions of this clause of the contract,
the Contractor shall continue the remainder of the work in
conformity with the terms of his contract, and in such manner as
in no wise to hinder or interfere with the persons or workmen
employed by the Commission by contract or otherwise as above
provided, to do any part of the work or to complete the same under
the provisions hereof.
17. Estimates. The Engineer shall from time to time as the work
progresses, on or about the last day of each month, make in
writing an estimate, such as he shall believe to be just and fair,
of the amount and value of the work done and the materials
incorporated into the work by the Contractor under the
specifications, provided however that no such estimate shall be
required to be made when, in the judgment of the Engineer the
total value of the work done and the materials incorporated into
the work since the last preceding estimate is less than ——
dollars. Such estimates shall not be required to be made by strict
measurements, but they may be approximate only.
The Contractor shall not be entitled to demand from the Commission
as a right, a detailed statement of the measurements or quantities
entering into the several items of the monthly estimates, but he
will be given such opportunities and facilities to verify the
estimates as may be deemed reasonable by the Commission.
When in the opinion of the Engineer, the Contractor shall have
completely performed the contract on his part, the Engineer shall
make a final estimate, based on actual measurements, of the whole
amount of the work under and according to the terms of the
contract, and shall certify to the Commission in writing, the
amount of the final estimate at the completion of the work. After
the completion of the work the City shall pay to the Contractor
the amount remaining after deducting from the total amount or
value of the work, as stated in the final estimate, all such sums
as have theretofore been paid to the Contractor under any of the
provisions of the contract, except such sums as may have been paid
for extra work, and also any sum or all sums of money which by the
terms thereof the City is or may be authorized to reserve or
retain; provided that nothing therein contained shall affect the
right of the City, hereby reserved, to reject the whole or any
portion of the aforesaid work, should the said certificate be
found or known to be inconsistent with the terms of the contract
or otherwise improperly given. All monthly estimates upon which
partial payments have been made, being merely estimates, shall be
subject to correction in the final estimate, which final estimate
may be made without notice thereof to the Contractor, or of the
measurements upon which it is based.
18. Extra Work. The Contractor shall do any work not herein
otherwise provided for, when and as ordered in writing by the
Engineer or his agents specially authorized thereto in writing,
and shall when requested by the Engineer so to do, furnish
itemized statements of the cost of the work ordered and give the
Engineer access to accounts, bills, vouchers, etc. relating
thereto. If the Contractor claims compensation for extra work not
ordered as aforesaid, or for any damages sustained, he shall
within one week after the beginning of any such work or the
sustaining of any such damage, make a written statement of the
nature of the work performed or the damage sustained, to the
Engineer, and shall, on or before the fifteenth day of the month
succeeding that in which any such extra work shall have been done
or any such damage shall have been sustained, file with the
Engineer an itemized statement of the details and amount of any
such work or damage; and unless such statement shall be made as so
required, his claim for compensation shall be forfeited and he
shall not be entitled to payment on account of any such work or
damage.
For all such extra work the Contractor shall receive the
reasonable cost of said work, plus fifteen (15) per cent of said
cost.
19. Competent Employees. The Contractor shall employ only
competent skillful men to do the work; and whenever the Engineer
shall notify the Contractor, in writing, that any man employed on
the work is, in his opinion unsatisfactory, such man shall be
discharged from the work and shall not again be employed on it,
except with the consent of the Engineer.
20. Money Retained. Upon the completion of the work and its
acceptance by the City, the City shall reserve and retain five (5)
per cent of the total value of the work done under the contract as
shown by the final estimate, over and above any and all other
reservations which the city by the terms thereof is entitled or
required to retain and shall hold the said five (5) per cent for a
period of nine (9) months from and after the date of completion
and acceptance, and the City shall be authorized to apply such
part of said five (5) per cent so retained to any and all costs of
repairs and renewals as may become necessary during such period of
nine (9) months, due to improper work done or materials furnished
by the Contractor, if the Contractor shall fail to make such
repairs or renewals within twenty-four (24) hours after receiving
notice from the City so to do.
Upon the expiration of said nine (9) months from and after the
completion and acceptance of the work, the City shall pay to the
Contractor the said five (5) per cent hereby retained, less such
sums as may have been retained hereunder.
21. Unpaid Claims against Contractor. The Contractor shall furnish
the City with satisfactory evidence that all persons who have done
work or furnished materials under the contract, and have given
written notices to the City, before and within ten (10) days after
the final completion and acceptance of the whole work under the
contract, that any balance for such work or materials is due and
unpaid, have been fully paid or satisfactorily secured. And in
case such evidence is not furnished as aforesaid, such amount as
may be necessary to meet the claims of the persons aforesaid shall
be fully discharged or such notices withdrawn.
22. Delays and Difficulties. The Contractor shall not be entitled
to any claims for damages on account of postponement or delay in
the work occasioned by forces beyond the control of the City, nor
for postponement or delay in the work where ten (10) days written
notice has been given the Contractor of such postponement or
delay, nor where unforeseen difficulties are encountered in the
prosecution of the work. In the event of a postponement or delay
ordered in writing by the City the time of completion of the
contract shall be extended a number of days equal to the number of
days that the work has been postponed or delayed.
23. Assignment of Contract. The Contractor shall not assign by
power of attorney or otherwise, nor sublet the work or any part
thereof, without the previous written consent of the party of the
first part, and shall not either legally nor equitably assign any
of the moneys payable under this agreement or his claim thereto
unless by and with the consent of the party of the first part.
24. Cleaning Up. On or before the completion of the work, the
Contractor shall, without charge therefor, tear down and remove
all buildings and other structures built by him, shall remove all
rubbish of all kinds from any grounds which he has occupied, and
shall leave the line of the work in a clean and neat condition.
25. Access to Work and Other Contractors. The Commission and its
engineers, agents and employees may at any time and for any
purpose enter upon the work and the premises used by the
Contractor, and the Contractor shall provide proper and safe
facilities therefor. Other contractors of the Commission may also
when so authorized by the Engineer, enter upon the work and the
premises used by the Contractor for all the purposes which may be
required by their contracts. Any differences or conflicts which
may arise between this Contractor and other contractors of the
Commission in regard to their work shall be adjusted and
determined by the Engineer.
26. The Contract. It is understood and agreed by the City and the
Contractor that the terms of this contract are embodied and
included in the Advertisement, Information and Instructions to
Bidders, Proposal, Specifications of every nature, the Bond and
the contract drawings hereto attached.
These few articles have been given as examples of some of the essential
subjects to be treated in general specifications. It is to be understood
that these examples do not represent a complete set of general
specifications and items have been omitted the absence of which in a
complete contract might be injurious to the successful completion of the
work.
=114. Technical Specifications.=—These ordinarily follow the general
specifications and have to do with the quality of materials, the manner
of putting them together, and the method of doing the work. The subject
headings in the Technical Specifications on the Baltimore Sewerage
Commission are:
Excavation
Tunneling
Rock Excavation
Sheeting
Sheet Piling
Sheeting and Bracing
Piles
Blasting
Pumping and Drainage
Foundations
Refilling
Repaving
Underdrains
Buildings
Inlets and Catch-Basins
Cement
Mortar
Concrete
Brick
Masonry
Reinforced Concrete
Vitrified Pipe
Concrete and Brick Sewers
Vitrified Pipe Sewers and Drains
Manholes
Iron Castings
House Connections
Obstructions
Fences
Flush-Tanks
Each of these subjects is treated in the appropriate section of this
book.
An important part of each section of the technical specifications is the
clause providing for the method of payment for the work specified. This
is usually the last clause in the section. For example, the last clause
in the Baltimore Specifications relating to Rock Excavation, is:
“Payment will be made for the number of cubic yards of rock
measured and allowed as above specified at the price of four
dollars and fifty cents ($4.50) per cu. yd., measured in place.
Payment for rock excavation will be made in addition to the prices
bid for excavation.”
=115. Special Specifications.=—These have to do with problems, methods
of construction, or materials peculiar to certain contracts or certain
portions of the work. It frequently occurs that the construction of
sewerage works will be let out under a number of contracts, or bids will
be called for on different alternatives to which the entire
Advertisement, Information and Instructions for Bidders, Proposal, and
General Specifications are applicable. The special specifications will
apply only to the contract in question, e.g., in some work done under
the direction of the author, the sewer on one contract came within
twelve inches of the surface of a highway. The special specification
relating to this piece of construction, was:
“Where crossing under the Chicago Road the pipe sewer shall be
embedded in concrete as shown on the contract drawings. The
concrete for this purpose shall be mixed in the proportions of one
(1) part cement, three (3) parts fine aggregate, and six (6) parts
coarse aggregate. Payment for the concrete so used will be made at
the unit price stated in the accompanying Proposal.”
In order to avoid confusion the special specifications are either
incorporated directly in the Contract form, or follow the Technical
Specifications and are grouped according to the contracts to which they
apply.
=116. The Contract.=—The contract is a brief instrument which includes a
simple statement of the obligations of each party involved. The
following is an example of a form in successful use:
CONTRACT
This agreement made and entered into this ____ day of ____ in the
year one thousand nine hundred and ____ by and between the City of
____ by its duly constituted or elected authorities herein acting
for the City of ____ without personal liability to themselves,
party of the first part, hereinafter designated as the City, and
____ party of the second part hereinafter designated as the
Contractor.
WITNESSETH, that the parties to these presents each in
consideration of the undertakings, promises and agreements on the
part of the other herein contained, have undertaken, promised and
agreed, and do hereby undertake, promise and agree, the party of
the first part for itself, its successors and assigns, and the
part ____ of the second part for ____ and ____ heirs, executors,
administrators and assigns as follows, to-wit:
Art. I. To be bounden by all the articles of the General,
Technical, and Special Specifications applicable, and by the terms
of the Advertisement, Information and Instructions for Bidders,
Proposal and Contract Drawings hereto attached, and which are
understood and acknowledged to be an integral part of this
contract.
Art. II. The work to be completed under this contract is ____
Art. III. The City shall pay and the Contractor shall receive as
full compensation for everything furnished and done by the
Contractor under this contract, including all work required but
not specifically mentioned in the following items, and also for
all loss or damage arising from the nature of the work aforesaid,
or from the action of the elements, or from any unforeseen
obstruction or difficulty encountered in the prosecution of the
work and for well and faithfully completing the work as herein
provided, as follows:
Then follows a copy of the Proposal with the prices bid. The contract
closes with the final clause:
In witness whereof the said City of ____, party of the first part
have hereunto set their hands and seals, and the Contractor has
also hereunto set his hand and seal and the party of the first
part and the Contractor have executed this agreement in duplicate,
one part to remain with the party of the first part and one to be
delivered to the Contractor this ____ day of ____ in the year one
thousand nine hundred and ____
City of ____
____
____
Contractor ____
____
____
=117. The Bond.=—The bond called for in the Information and Instructions
for Bidders is bound in the pamphlet following the Contract. No uniform
practice is followed in the amount of the bond required. It varies from
50 to 100 per cent of the contract price and may be stated as a lump sum
before the contract price is known. There is a possibility that the
Contractor may fail before he has commenced work and the City may be
unable to procure another contractor to take up the work. The City
should then be protected by a 100 per cent bond. Such a contingency is
remote. The Contractor seldom fails until work is well under way, and
other contractors are usually available, although the failure of one
contractor tends to increase the bids of other contractors for the same
work. In fixing the amount of the bond the judgment of the Engineer is
called into play in order that the amount may be as low as possible in
fairness to the Contractor, and high enough to protect the interests to
the City. By reducing the amount of the bond the expense to the City is
also reduced as the City ultimately must pay its cost.
Upon the acceptance of the bond and the execution of the Contract, the
Engineer’s duties take him out of the designing office and into the
construction field.
CHAPTER XI
CONSTRUCTION
=118. Elements.=—The principal elements in construction are: labor,
materials, tools, and transportation. The lack of or inadequateness of
any one of these detracts from the effectiveness of the others. The
engineer should assure himself of the completeness of his plans or those
of the contractor on each of these points. The disposition of labor and
the handling of materials to obtain the largest amount of good with the
least expenditure of money and effort are problems which must be solved
by the engineer or the contractor during construction.
WORK OF THE ENGINEER
=119. Duties.=—The duties of the engineer during construction consist in
giving lines and grades; inspecting materials; interpreting the
contract, specifications and drawings; making decisions when unexpected
conditions are encountered; making estimates of work done; collecting
cost data; making progress reports; keeping records; and in guarding the
interests of the City.
=120. Inspection.=—In the inspection of workmanship and materials, the
engineer is assisted by a corps of inspectors and assistants who act
under his direction. The duties of the inspector are to be present at
all times that work is in progress and to act for the engineer in
enforcing the terms of the contract, the details of the drawings, and
the tests applicable to the workmanship and materials that he is
delegated to inspect. He should have a copy of the contract, or that
portion of it which pertains to his work, available at all times. He
should examine all materials as they are delivered on the job and see
that rejected materials are removed at once. An ordinary recourse of
some foremen will be to place rejected material to one side until a
brief absence of the inspector will present the opportunity for the use
of the rejected material. The methods to be followed in the inspection
of materials and workmanship should be such as to discover discrepancies
between the specifications and the materials delivered or the work done.
Other duties of the inspector are: to record the location of house
connections or to drive a stake over them for subsequent location by the
engineer; to see that plugs are put in the branches left for future
house connections; to inspect the workmanship in the making of joints in
pipe sewers; to protect the line and grade stakes from displacement; to
check the size, depth, and grade of sewers and elevations of special
structures, etc.
Dishonest and unscrupulous workmen have many tricks to get by the
inspector. These tricks are best learned by experience as no academic
list can impress them properly on the memory. The position of the
inspector is not always enviable. He must hold the respect of the
workmen, of the contractor, and of the engineer. To do this he must not
be unreasonable or arbitrary in his decisions, but when a decision is
once made he must be firm in following up its enforcement. He must be
careful not to give directions whose fulfillment he cannot enforce, nor
for which he cannot give adequate reason to his superiors. His integrity
must never be questioned. He must not allow himself to become under
obligations to the contractor by the acceptance of favors he cannot
return except at the expense of his employer, yet at the same time he
must not appear priggish by the refusal of all favors or social
invitations. In brief he must be friendly without being intimate,
independent without being aloof, and firm without being arbitrary.
The engineer must support his inspectors in their decisions or discharge
them if he cannot.
=121. Interpretation of Contract.=—In interpreting the contract,
specifications and drawings, the engineer is supposedly an impartial
arbiter between the interests of the city and the contractor. His
decisions, as to the meaning of the contract, must be founded on his
engineering judgment, and should aim to produce the best results without
demanding more from the contractor than, in his honest opinion, it is
the intention of the contract to demand. However conscientiously he may
attempt to remain impartial, and in spite of the honesty of the
contractor, his position, as an employee of the city will almost
invariably cause him to favor the city in his decisions on close points.
The experienced contractor knows this and fixes his bid accordingly, the
personality of the engineer sometimes acting as an important factor in
the amount of the bid. The situation arises through the character of the
contract, and not through a lack of moral integrity on the part of
anyone concerned.
=122. Unexpected Situations.=—When unexpected or uncertain conditions
are encountered in construction the engineer should visit the spot at
once and should advise or direct, according to the terms of the
contract, the procedure to be followed. Such conditions may be the
encountering of other pipes, quicksand, rock, etc. Each case is a
problem in itself. Water, gas, telephone and electric wire conduits can
be moved above or below the sewer being constructed with comparative
ease. Other sewers, if smaller, may be permitted to flow temporarily
across the line of the sewer under construction and finally discharge
into the completed sewer, or one sewer must be made to pass under the
other, either as an inverted siphon or by changing the grade of one of
the sewers. Rock, or other material for which a special rate of payment
is allowed, must be measured as soon as uncovered in order to avoid
delaying the work or losing the record of the amount removed. When
quicksand is met special precautions must be taken to safeguard the
sewer foundation and to insure that the sewer will remain in place until
after the backfilling is completed. These precautions are described in
Art. 135.
=123. Cost Data and Estimates.=—Cost account keeping and the making of
monthly or other estimates are closely connected. Cost accounts are of
value in estimating the amount of work done to date, and in making
preliminary estimates of the cost of similar work. Although the engineer
is not always required to keep such accounts, they are usually of
sufficient value to pay for the labor of keeping them. Under some
contracts the contractor’s accounts are open to examination by the
engineer. Usually, however, he must depend on reports from the
inspectors for information concerning the man-hours required on
different pieces of work, and on his own measurements of materials used
and his knowledge of their unit costs, in order to make up an estimate
of total cost.
The measurement of a completed structure and a summary of the materials
used in its construction may act as a check on the use of proper
materials as called for in the contract. For example, if it is known
that 2,000 bricks are required for the construction of a manhole and if
only 15,000 have been used in the construction of ten manholes, it is
probable that some or all of the manholes have been skimped. Similar
conditions may show in the proportions of concrete, backfilling in
tunnels, sheeting to be left in place, etc.
The statement of a few principles of cost accounting, and the
illustration of a few blanks in use should be sufficiently suggestive to
lead a resourceful engineer in the right direction.[79] Costs should be
divided into four general classifications: labor, materials, equipment,
and overhead. Labor should be subdivided under its several different
classifications arranged in accordance with rates of pay. The number of
laborers under each classification and the amount of work done per day
should be recorded. Fig. 86 is an example of a form which may be used
for such a purpose.
[Illustration:
FIG. 86.—Foreman’s Daily Payroll Report.
From Engineering and Contracting, 1907.
]
Materials may be recorded as they are delivered on the job, as they are
used, or in both cases. Measurements are usually easier to make at the
time of delivery, but records made at the time materials are used are
more serviceable. For example, 100 barrels of cement may be delivered on
a job in November, 50 of them are used before the job freezes up and the
other 50 are held over until spring. It would be misleading to charge
100 barrels used in November. Fig. 87 is a form in use for an
inspector’s report on materials. The total cost must be made up in the
office from these records and a knowledge of unit costs.
[Illustration:
FIG. 87.—Foreman’s Daily Material Report.
From Engineering and Contracting, 1907.
]
Equipment consists of tools, animals, machinery, and apparatus used in
construction. Only equipment that is actually used should be charged to
the job and a credit should be made at the completion of the job for the
fair value of the equipment remaining after the completion of the work.
Overhead charges include the expense of the office force,
superintendence, and miscellaneous items such as insurance, rent,
transportation, etc., which cannot be charged to any particular portion
of the work but are equally applicable to all portions. It happens
frequently that many jobs are handled in the same main office. The
division of overhead becomes more difficult and is frequently arranged
on an arbitrary basis, e.g., each job may be charged the proportion of
overhead that its contract price bears to the total contract prices
being performed under that office. This rule may be modified when it
becomes evident that some job is taking distinctly more than its share
of the overhead.
Estimates of work done in any period can be made with the above data in
hand by subtracting the total costs of the work up to the beginning of
the period from the total costs up to the end of the period. Fig. 88
shows a sample blank from the final estimate sheets used at Scarsdale,
N. Y.
=124. Progress Reports.=[80]—These are kept by the engineer in order
that he may see that the work is progressing as called for in the
contract, and any portion which is lagging behind without reason may be
pushed. Such reports are most useful when the information is expressed
graphically, as the eye quickly catches points where the work is falling
behind schedule.
=125. Records.=—The contract drawings are supposed to show exactly where
and how construction is to be done. Due to unexpected contingencies
changes occur, of which a record should be made and preserved. These
records may be kept in a form similar to the contract drawings, or if
the changes are not extensive, they can be recorded on the original
contract drawings. The location of house and other connections should be
recorded in a separate note book available for immediate consultation.
The engineer should keep a diary of the work in which are recorded
events of ordinary routine as well as those of special interest and
importance. This diary should be illustrated by photographs showing the
condition of the streets before and after construction, methods of
construction, accidents, etc. Such accounts are of great value in
defending subsequent litigation and their existence sometimes prevents
litigation. A contractor may wait a year or so after the completion of a
piece of work until the engineer and other city officials have broken
their connection with the city. Suit is then brought against the city
and unless good records are available the administration may be forced
to buy the claimant off or may elect to enter court, only to be beaten.
[Illustration:
FIG. 88.—Samples of Cost Record Forms.
From Engineering and Contracting, 1909.
]
Excavation
=126. Specifications.=—The following abstracts have been taken from the
specifications on Excavation by the Baltimore Sewerage Commission as
illustrative of good practice. In conducting the work the contractor
shall:
... remove all paving, or grub and clear the surface over the
trench, whenever it may be necessary and shall remove all surface
materials of whatever nature or kind. He shall properly classify
the materials removed, separating them as required by the
Engineer; and shall properly store, guard, and preserve such as
may be required for future use in backfilling, surfacing, repaving
or otherwise. All macadam material removed shall be separated and
graded into such sizes as the Engineer may direct and materials of
different sizes shall be kept separate from each other and from
any and all other materials.
All the curb, gutter, and flag-stones and all paving material
which may be removed, together with all rock, earth and sand taken
from the trenches shall be stored in such parts of the carriageway
or such other suitable place, and in such manner as the Engineer
may approve. The Contractor shall be responsible for the loss of
or damage to curb, gutter and flag-stones and to paving material
because of careless removal or wasteful storage, disposal, or use
of the same.
... When so directed by the Engineer the bottom of the trench
shall be excavated to the exact form of the lower half of the
sewer or of the foundation under the sewer.
The bottom width of the trench for a brick or concrete sewer shall
be ... not less in any case than the overall width of the sewer,
as shown on the plans. In case the trench is sheeted this minimum
width will be measured between the interior faces of the sheeting
as driven, but in no case shall bracing, stringers, or waling
strips be left within any portion of the masonry of the sewer
except by permission of the Engineer; and such braces, stringers
and waling strips shall not, in any case, be allowed to remain
within the neat lines of the masonry as shown on the plans. In
case that the distance between faces of the sheeting is less than
that called for by the width of the sewer to be laid in the
trench, the Engineer may direct the sheeting to be drawn and
redriven, or otherwise changed and altered; or he may direct that
the sewer be reinforced in such manner and to such an extent as he
may deem necessary without compensation to the Contractor, even
though such narrower trench was not caused by negligence or other
fault on the part of the Contractor.
Trenches for vitrified pipe shall be at all points at least six
inches wider in the clear on each side than the greatest external
width of the sewer, measured over the hubs of the pipe.... Bell
holes shall be excavated in the bottoms of trenches for vitrified
pipe sewers wherever necessary.
Not more than three hundred feet of trench shall be opened at any
one time or place in advance of the completed building of the
sewer, unless by written permission of the Engineer and for a
distance therein specified....
The excavation of the trench shall be fully completed at least
twenty feet in advance of the construction of the invert, unless
otherwise ordered.
During the progress of construction the Contractor will be
required to preserve from obstruction all fire hydrants and the
carriageway on each side of the line of the work.
The streets, cross-walks, and sidewalks shall be kept clean,
clear, and free for the passage of carts, wagons, carriages and
street or steam railway cars, or pedestrians, unless otherwise
authorized by special permission in writing from the Engineer. In
all cases a straight and continuous passageway on the sidewalks
and over the cross walks of not less than three feet in width
shall be preserved free from all obstruction.
Where any cross walk is cut by the trench it shall be temporarily
replaced by a timber bridge at least three feet wide, with side
railings, at the Contractor’s expense. The placing of planks
across the trench without proper means of connection or
fastenings, or pipe or other material, or the using of any other
makeshift in place of properly constructed bridges, will not be
permitted.
This is equally applicable to certain wagon bridges to be fixed upon by
the Engineer, on the basis of traffic requirements.
In streets that are important thoroughfares or in narrow streets
the material excavated from the first one hundred feet of any
opening or from such additional length as may be required, shall
upon the order of the Engineer, be removed by the Contractor, as
soon as excavated. The material subsequently excavated shall be
used to refill the trench where the sewer has been built.
The preceding specifications are applicable to open-trench excavation.
Rigid restrictions are placed about tunneling because of the greater
difficulty of doing good work, the greater danger to life and property
and the possibility of later surface subsidence if the backfilling is
done improperly. A common clause in specifications is:
All excavations for sewers and their appurtenances shall be made
in open trenches unless written permission to excavate in tunnel
shall be given by the Engineer.
=127. Hand Excavation.=—Earth excavation by pick and shovel is the
simplest and most primitive mode of excavation. Only small jobs are
handled in this manner in order to save the investment necessary in
machines or the expense of hiring and moving one to the work. The tools
used in the hand excavation of trenches are: picks, pickaxes,
long-handled and short-handled pointed shovels, square-edged long- and
short-handled shovels, scoop shovels, axes, crowbars, rock drills,
mauls, sledges, etc. The excavating gangs are divided up into units of
20 to 50 men under one foreman or straw boss, and among the men may be a
few higher priced laborers who set the pace for the others. Each laborer
on excavation should be provided with a shovel, the style being
dependent on the character of the material being excavated and the depth
of the trench. In stiff material and deep trenches requiring the lifting
of the material in the shovel, long-handled pointed shovels should be
used. In loose sandy material loaded directly into buckets
short-handled, square pointed shovels are satisfactory. Picks are used
in cemented gravels or where hard obstructions prevent cutting down with
the edge of the shovel. Very stiff but not hard material can be cut out
in chunks with a pickaxe and thrown from the trench or into a bucket
with a scoop shovel. Scoop shovels are also useful in wet running
quicksand. The number of picks, axes, crowbars, and other tools must be
proportioned according to the material being excavated. Under the worst
conditions of excavation in a hard cemented gravel it may be necessary
to provide each man with a pick as well as a shovel, whereas in sand
only a shovel is necessary. Two or three crowbars, axes, a length of
chain, two or three screw jacks, etc., are provided per gang in case of
an unexpected encounter with an obstruction in the trench, such as a
boulder, a tree stump, a length of pipe, etc.
In laying out the work the foreman marks the outlines of the trench on
the ground by means of a scratch made with a pick, chalk marks, tape, or
other devices. These marks are measured from offset or center stakes set
by the engineer. Center stakes are less conducive to error but are more
likely to be disturbed before use than are offset stakes, but careless
foremen make more errors with offset than with center stakes. The
inspector should assist or be present at the laying out of the trench.
After the trench has been laid out each laborer should be given a
certain specific portion of it to dig and this portion is marked out on
the ground. In this way a check can be kept upon the performance of each
laborer and the knowledge of this fact tends to a uniformly better
performance. The amount of work that can be performed by one man with a
pick and shovel is as shown in Table 49. Some men may exceed these
rates, many will not attain them. The allotted task must be gaged on the
character of the ground in order that the tasks may be equal and a
spirit of competition fostered. The hard worker will set the pace for
the lazy man. Some contractors have adopted the expedient of dismissing
laborers for the day as soon as the allotted task is done.
TABLE 49
AMOUNT OF MATERIAL MOVED BY ONE MAN WITH A PICK AND SHOVEL
(From H. P. Gillette)
────────────────────────────────────────┬──────────────────────────────
Material │ Cubic Yard per hour
────────────────────────────────────────┼──────────────────────────────
Hardpan │ 0.33
Common earth │ 0.8 to 1.2
Stiff clay │ 0.85
Clay │ 1.00
Sand │ 1.25
Sandy soil │ 0.8 to 1.2
Clayey earth │ 1.3
Sandy soil (frozen) │ 0.75
────────────────────────────────────────┴──────────────────────────────
The opening of the trench may be facilitated by breaking ground with a
plow. In hard ground or on paved roads it may be necessary to cut
through the surface crust with a hammer and drill, although in some
cases a plow can be used successfully. Frozen ground can be thawed by
building fires along the line of the trench, or greater economy may be
achieved by placing steam pipes along the surface with perforations
about every 18 inches and either boxing them on the top and sides or
burying them in the frozen earth with a covering of sand. Another
arrangement is to blow steam into a line of bottomless boxes in which
each box is about 8 feet long. Holes are left in the top of the boxes
into which the pipe is shoved, and after its withdrawal the holes are
covered. Blasting of frozen earth is sometimes successful but cannot be
resorted to in built up districts where it is unsafe unless properly
controlled. Once the frost crust is broken through it can be attacked
from below and frequently broken down by undermining.
A laborer cannot dig and raise the earth much more than to the height of
his head, and preferably not quite so high, without tiring quickly.
After the trench has passed a depth of 4 feet he cannot throw the earth
clear of the trench. An additional laborer is needed then at the surface
to throw the earth back. He should shovel the earth from a board
platform placed at the edge of the trench as a protection to the bank.
When the trench passes the 6–foot depth a staging is put in about 4 feet
from the top on which the lowest laborer piles his materials. It is then
passed up to the surface by a second laborer on the staging, and a third
laborer on the surface throws the material back clear of the trench.
Stagings are put in about every 5 or 6 feet for the full depth of the
trench.
When the trench has come within half the diameter of the pipe of the
final grade, if the material is sufficiently firm, the remainder of the
trench should be cut to conform to the shape of the lower half of the
outside of the pipe, with proper enlargements for each bell.
=128. Machine Excavation.=—On work of moderately large magnitude
excavation by machine is cheaper than by pick and shovel alone. In
comparing the cost of excavation by the two methods all items such as
sheeting, pipe laying, backfilling, etc., should be included, since
these items will be affected by the method of excavation. The cost of
setting up and reshipping the machine must be included as this is
frequently the item on which the use of the machine depends. Because of
the cost of setting up and shipping, which must be distributed over the
total number of yards excavated, the cost per cubic yard of excavating
by machine varies with the number of cubic yards excavated. The point of
economy in the use of a machine is reached when the cost by hand and by
machine are equal. For all work of greater magnitude, excavation by
machine will prove cheaper.[81] Items favoring the use of machinery
which may cause its adoption for small jobs are: its greater speed,
reliability, ease in handling, economy in sheeting, economy in labor,
and small amount of space needed making it useful in crowded streets.
Continuous bucket machines, drag lines, and occasionally steam shovels
are not adapted to conditions where rocks, pipes and other underground
obstacles are frequently met.
The following problem is an example of the work necessary in making a
comparison of the relative economy of machine and hand excavation:
It is assumed that a man can excavate 15 feet of trench 30 inches
wide and 8 feet deep in 10 hours. He receives 55 cents per hour
for his work. A machine costing $10,000 has a life of 6 years. It
can be kept busy 150 days in the year. When operating it costs
$1.25 per hour for the operator, fuel and repairs. It will
excavate 800 linear feet of 30 inch trench to a depth of 8 feet in
10 hours. It is assumed that capital is worth 10 per cent on such
a venture and that the sinking fund will draw 10 per cent. If the
cost of moving and setting up the machine is $1,800, how many
cubic yards of excavation must there be to make excavation by
machine economical? Costs of sheeting, pumping, etc., are assumed
to be the same for machine or hand work.
_Solution._—For hand work the man excavated 1.11 cubic yard per
hour at 55 cents. The relative cost of hand excavation is then 50
cents per cubic yard.
The cost of machine work will be divided into: interest on first
cost; operation and repairs; and sinking fund for renewal. The
interest on the first cost of $10,000 at 10 per cent is $1,000 per
year. The machine works 1,500 hours in the year. Therefore the
cost per hour is $0.67.
The sinking fund payment, as found from sinking fund tables or the
accumulation of $10,000 in. 6 years, is $1,300 per year or per
hour for 1,500 hours is $0.87.
The cost of operation per hour is given as $1.25.
The total cost per hour is therefore $2.79.
The machine excavated 59.3 cubic yards per hour which makes the
cost, exclusive of moving, equal to $0.47 per cubic yard. In order
to equalize the cost of machine and hand excavation the cost of
moving the machine must be divided among a sufficient number of
cubic yards so that the cost per cubic yard shall be 3 cents. The
cost of moving is given as $1,800. This amount divided among
60,000 cubic yards equals 3 cents per cubic yard. Therefore the
job must provide at least 60,000 cubic yards of excavation in
order that the use of the machine shall be justifiable from the
viewpoint of economy alone.
=129. Types of Machines.=—Machines particularly adapted to the
excavation of sewer and water pipe trenches are of four types: (1)
continuous bucket excavators; (2) overhead cableway or track excavators;
(3) steam shovels; and (4) boom and bucket excavators. Other types of
excavating machinery can be used for sewer trenches under special
conditions. Machines are ordinarily limited to a minimum width of trench
of 22 inches. Between widths of 22 inches and 36 inches the limit of
depth for the first class of machines is about 25 feet. For other types
of machines there is no definite limit, though the economical depth for
open cut work seldom exceeds 40 feet.
=130. Continuous Bucket Excavators.=—Continuous bucket excavators are of
the types shown in Figs. 89 and 90. The buckets which do the digging and
raising of the earth may be supported on a wheel as in Fig. 89 or on an
endless chain as in Fig. 90. The support of the wheel or endless chain
can be raised or lowered at the will of the operator so as to keep the
trench as close to grade as can be done by hand work. In some machines
the shape of the buckets can be made such as to cut the bottom of the
trench, in suitable material, to the shape of the sewer invert. In
operation, the buckets are at the rear of the machine and revolve so
that at the lowest point in their path they are traveling forward. The
excavated material is dropped on to a continuous belt which throws it on
the ground clear of the trench, into dump wagons, or on to another
continuous belt running parallel with the trench to the backfiller, by
means of which the excavated material is thrown directly into the
backfill without rehandling. The body of the machine supporting the
engine travels on wheels ahead of the excavation and is kept in line by
means of the pivoted front axle. When obstacles are encountered the
excavating wheel or chain is raised to pass over the obstacle, and
allowed to dig itself in on the other side.
[Illustration:
FIG. 89.—Buckeye Wheel Excavator.
Courtesy, Buckeye Traction Ditcher Co.
]
[Illustration:
FIG. 90.—Buckeye Endless-chain Excavator.
Courtesy, Buckeye Traction Ditcher Co.
]
[Illustration:
FIG. 91.—Movable Sheeting Fastened to Traction Ditcher.
From Eng. News-Record, Vol. 82, 1919, p. 740.
]
Wheel excavators are not adapted to the excavation of sewer trenches
over 3 to 4 feet in width and 6 to 8 feet in depth. The endless-chain
excavators are suitable for depths of 25 feet with widths from 22 to 72
inches, and due to the arrangement permitting buckets to be moved
sideways they will cut trenches of different widths with the same size
buckets. This is an advantage where there are to be irregularities in
the width of the trench such as for manholes or changes in size of pipe.
With excavating machines pipe can be laid within 3 feet of the moving
buckets and the trench backfilled immediately, thus making an
appreciable saving in the amount of sheeting. In the construction of
trenches for drain tile at Garden Prairie, Illinois, the sheeting was
built in the form of a box or shield fastened to the rear of the machine
and pulled along after it as is shown in Fig. 91.
The performance of this type of excavating machine under suitable
conditions is large. A remarkable record was made by Ryan and Co. in
Chicago,[82] with an excavating machine. 1338 feet of 32–inch trench
were excavated to an average depth of 8½ feet in 7 hours, or an average
of 160 cubic yards per hour. More could have been accomplished if it had
not been for delays in supplies. Another crew at Greeley, Colorado,[83]
with a Buckeye endless-chain ditcher weighing 17 tons and costing $5200,
averaged 232 cubic yards per day for 300 days, and the cost was 10.7
cents per cubic yard. A 15–ton Austin excavator can be expected to
remove 300 to 500 cubic yards per day.
The cost of operation of the machines is made up of items listed in
Table 50. The figures given are merely suggestive.
TABLE 50
COST OF OPERATING DITCHING MACHINE
─────────────────────────────────────────────────────────┬──────┬──────
│ Per │
│ Day │Total
─────────────────────────────────────────────────────────┼──────┼──────
Labor: │ │
1 Operator at $150 per month │ $6.00│
1 Assistant Operator at $120 per month │ 4.00│
4 laborers at 4.00 per day │ 16.00│
│——————│
│ │$26.00
│ │
Fuel: │ │
20 Gallons of gasoline at 28 cents │ 5.60│ 5.60
│ │
Miscellaneous: │ │
Oil, waste, etc. │ 1.20│
Repairs and maintenance │ 10.00│
Interest, 6 per cent on $10,000 for 150 days │ 4.00│
Depreciation, 200 working days per year and an 8 year │ │
life │ 11.11│ 26.31
│——————│——————
Total cost per day │ │$57.91
─────────────────────────────────────────────────────────┴──────┴──────
TABLE 51
COMPARISON OF COST OF HAND EXCAVATION AND MACHINE EXCAVATION WITH
CONTINUOUS-BUCKET EXCAVATOR
───────────────────────────┬───────┬───────────────────────────┬───────
Hand Work │ Per │ Machine Work │ Per
│ Day, │ │ Day,
│Dollars│ │Dollars
───────────────────────────┼───────┼───────────────────────────┼───────
Foreman │ 4.00│Engineer │ 4.00
Timberman │ 3.00│Fireman │ 2.50
Helper │ 2.50│Coal │ 5.00
4 Laborers at $2.00 │ 80.00│Team │ 4.00
│ │Foreman │ 4.00
│ │Pipe layer │ 3.00
│ │Helper │ 2.50
│ │2 Teams backfilling │ 8.00
│ │2 Helpers │ 4.00
│ │Interest, depreciation and │
│ │ repairs │ 10.00
│ ——————│ │ ——————
Total │ 95.00│ Total │ 54.50
───────────────────────────┴───────┴───────────────────────────┴───────
In making a comparison of the cost of hand and machine excavation the
figures given in Table 51 are from “Excavating Machinery” by McDaniel,
who quotes the cost of machine excavation from the manufacturers of the
Parsons machine issued as the result of several years’ experience with
their excavator. In the comparison the hand crew is assumed to dig 315
linear feet of trench 28 inches wide by 12 feet deep in a day of 10
hours. This assumes that each man will excavate 7 cubic yards per day.
The machine is assumed to excavate 250 feet of the same trench. The
comparison indicates that an excavator will work at about 50 per cent of
the cost of hand excavation, if the cost of moving the machine is not
included.
[Illustration:
FIG. 92.—Carson Excavating Machine on Trench Excavation in South
Milwaukee.
Courtesy, Mr. C. F. Henning.
]
=131. Cableway and Trestle Excavators.=—Cableway and trestle excavators
are most suitable for deep trenches and crowded conditions. They should
not be used for trenches much less than 8 feet in depth. They differ
from the continuous bucket excavators in that the actual dislodgment of
the material is done by pick and shovel, the excavated material being
thrown by hand into the buckets of the machine. A machine of the Carson
type is shown in Fig. 92. The machine consists of a series of
demountable frames held together by cross braces and struts to form a
semirigid structure. An I beam or channel extending the length of the
machine is hung closely below the top of the struts. The lower flange of
this beam serves as a track for the carriages which carry the buckets.
All the carriages are attached to each other and to an endless cable
leading to a drum on the engine. This cable serves to move the buckets
along the trench. The buckets are attached to another cable which is
wound around another drum on the engine and serves to lower or raise all
the buckets at the same time. In operation there are always at least two
buckets for each carriage, one in the trench being filled and the other
on the machine being dumped. There should be a surplus of buckets to
replace those needing repairs.
The machines may be from 200 to 350 feet in length, and the number of
buckets which can be lifted at one time varies from one to a dozen or
more. On trenches over 5 to 6 feet in width a double line of buckets is
sometimes used. The entire machine rests on rollers and straddles the
trench. It is moved along the trench by its own power, either by gearing
or chains attached to the wheels, or by a cable attached to a dead-man
ahead.
The Potter trench machine differs from the Carson in that only 2 buckets
are used at a time and these are carried on a car which travels on a
track on top of the trestle. The movement of the buckets and the car are
controlled by 2 dump men who ride on the car and who can raise or lower
the buckets independently.
The organization needed to operate these machines is: a lockman who
locks and unlocks the buckets on the cable, a dumper, as many shovelers
as there are buckets on the machine, and an engineman who is usually his
own fireman. From 50 to 400 cubic yards of material can be excavated in
a day with one of these machines, dependent on the character of the
material and the depth of the trench. H. P. Gillette in his Handbook of
Cost Data reports that about 190 cubic yards were excavated per day with
a Potter machine. The machine was 370 feet long. Six ¾-yard buckets were
used, 4 in the trench and 2 on the carrier. The trench was 10½ feet wide
and 18 feet deep in wet sand and soft blue clay. The organization
consisted of an engineman, a fireman, 2 dumpmen on the carrier, and from
17 to 21 excavating laborers depending on the kind and the amount of the
excavation. In general the capacity of such machines is limited by the
amount of material which can be shoveled into them by hand.
=132. Tower Cableways.=—These are essentially of the same class as the
trestle cableway machines. They differ in that the carriage supporting
the buckets travels on a cable suspended between 2 towers instead of on
a track supported on a trestle. As a rule only one bucket is handled in
the machine at a time. They are used in sewer work only in exceptional
cases as the towers must be taken down and re-erected each time that
there is an advance in the trench greater than the distance between the
towers.
=133. Steam Shovels.=—The use of steam shovels for the excavation of
sewer trenches is becoming more prevalent because of their growing
dependability and durability as compared with other machines, their
adaptability for small trenches, and the relatively large number of
widely different uses to which they can be put. In excavating a trench
the shovel straddles the trench and runs on tractors, wheels, or rollers
on either side of it. The shovel cuts the trench ahead of it. As a
result it is difficult to set sheeting and bracing close to the end of
the trench while the shovel is operating. Steam shovels are therefore
not suitable for excavation in unstable material, unless the sheeting is
driven ahead of the excavation. It is only in the softest ground that
ordinary wood sheeting can be driven ahead of the excavation. Steel
sheet piling is more suitable for such use. Fig. 93[84] shows a shovel
at work on a trench in Evanston, Illinois.
Shovels are equipped with extra long dipper handles to adapt them to
trench excavation. The dipper handle in the picture is longer than the
standard for this type of machine. The method of supporting the shovel
can be seen in the picture under the machine and the method of bracing
and of finishing the trench by hand work are also shown. The excavated
material is taken out in the shovel and dropped on the bank or into
wagons.
The limiting depth to which trenches can be excavated by steam shovels
is about 20 to 25 feet, where the trench is too narrow for the shovel to
enter. Wider trenches are cut in steps of about 15 feet, the shovel
working in the trench for additional depths. Shovels are now made to cut
trenches as narrow as a man can enter to lay pipe. The greatest width
that can be cut from one position of the shovel is from 15 to 40 feet,
dependent on the size of the shovel. Occasionally a combination of a
drag line and a steam shovel can be used, as on the construction of the
Calumet sewer in Chicago. On this work the first step was cut by a steam
shovel. It was followed by a drag line resting on the step thus
prepared, and excavating the remaining distance to grade. The depth of
the trench in this work averaged about 25 to 30 feet.
[Illustration:
FIG. 93.—Steam Shovel at Work on Sewer Trench for North Shore
Intercepting Sewer, Evanston, Illinois.
]
Steam shovels are rated according to their tonnage and the capacity of
the dipper in cubic yards. Both are necessary as the size of the dipper
is varied for the same weight of machine, dependent on the character of
the material being excavated. For rock the dipper is made smaller than
for sand. Gillette in his Hand Book of Cost Data gives the coal and
water consumption of steam shovels as shown in Table 52. The performance
of steam shovels is recorded in Table 53. The conditions of the work
have a marked effect on the output of the shovel. A shovel in a thorough
cut, i.e., in a trench just wide enough for the shovel to turn 180
degrees but too narrow to run cars or wagons along side of it, will
perform less than one-half of the work that it can perform in a side
cut, i.e., where the cars can be run along side the shovel which turns
less than 90 degrees.
TABLE 52
COAL AND WATER CONSUMPTION BY STEAM SHOVELS