───────────────────────────────────┬─────┬─────┬─────┬─────┬─────┬─────
Weight in tons │ 35 │ 45 │ 55 │ 65 │ 75 │ 90
Dipper, cubic yards │ 1¼ │ 1½ │ 1¾ │ 2 │ 2½ │ 3
Coal, tons per 10 hour day │ ¾ │ 1 │ 1¼ │ 1½ │ 2 │ 2¼
Water, gallons per 10 hour day │1500 │2000 │2500 │3000 │4000 │4500
───────────────────────────────────┴─────┴─────┴─────┴─────┴─────┴─────
TABLE 53
PERFORMANCE BY STEAM SHOVELS
──────┬──────┬──────┬───────┬───────────┬──────┬──────────────┬────────
Weight│Dipper│Depth │ Width │ 10–Hour │ Cost │ Authority │Remarks
in │Cubic │ of │of Cut │Performance│ in │ │
Tons │Yards │ Cut, │ │ │Cents,│ │
│ │ Feet │ │ │ per │ │
│ │ │ │ │Cubic │ │
│ │ │ │ │ Yard │ │
──────┼──────┼──────┼───────┼───────────┼──────┼──────────────┼────────
25 │ 1 │ 9 │36 in. │ 85 │ 22.6 │R. T. Dana │ 1
│ │ │ │ │ │ Eng. Rec., │
│ │ │ │ │ │ 69:581 │
25 │ 1 │ 8 │35 in. │ 96 │ 23.5 │ do. │ 2
70 │ 2 │ 26 │16 ft. │ 569 │ 6.7 │ do. │ 3
30 │ 1 │15–18 │60 in. │ 300 │ │A. B. McDaniel│ 4
│ │ │ │ │ │ Excavating │
│ │ │ │ │ │ Machinery │
15 │ ⅝ │ 14 │134 ft.│ 400 │ │Eng. Cont’r, │ 5
│ │ │ │ │ │ 8–25–09 │
│ 8 │ 36 │ Very │16 yd. cars│ │Marion Steam │ 6
│ │ │ wide │ │ │ Shovel Co. │
55 │ │ │ │ 296 │ │H. P. │ 7
│ │ │ │ │ │ Gillette’s │
│ │ │ │ │ │ Cost Data │
65 │ 2¼ │ │ │ 280 │ │ do. │
│ │ │Greater│ 700 │ 30.6 │G. C. D. │ 8
│ │ │than 78│ │ │ Lenth, Eng. │
│ │ │ in. │ │ │ News-Record,│
│ │ │ │ │ │ 85:22 │
──────┴──────┴──────┴───────┴───────────┴──────┴──────────────┴────────
Remarks:
1. One runner at $5.00, one fireman at $2.31, two laborers
at $1.70 each, supplies at $4.50, and interest and
depreciation on 200 days per year, $4.00. Total per
day, $19.21. Material, clay and gravel.
2. Average of 11 jobs with the same shovel.
3. Cost per day, one runner at $5.00, one crane-man at $3.60,
one fireman at $2.00, 7 roller men at $1.50 each, supplies
$9.00 and interest and depreciation on $9000 at 200 days
per year $8.00. Total, $38.10.
4. Hard clay.
5. Stiff clay for the basement of a building in Chicago.
6. Stripping ore. This is a maximum record. The average was
about three hundred and twenty 16 cubic yard cars per day.
7. Blasted mica-schist.
8. General average.
=134. Drag Line and Bucket Excavators.=—A drag line excavator is shown
in Fig. 94. The back of the bucket is attached to a drum on the engine
by means of a cable passing over the wheel in the end of the long boom.
The front of the bucket is attached by another cable directly to another
drum on the engine. In operation the bucket is raised by its rear end
and dropped out to the extremity of the boom. It is then dragged over
the ground towards the machine, digging itself in at the same time. When
filled the bucket is raised by tightening up on the two cables, swung to
one side by means of the movable boom, and dumped.
[Illustration:
FIG. 94.—Drag Line at Work on Trench for Drain Tile.
]
Drag line excavators will perform as much work as steam shovels under
favorable conditions. They are less expensive in first cost and
operation, and are equally reliable but they are not adapted to the more
difficult situations where steam shovels can be used to advantage. Drag
lines are suitable only for relatively wide trenches in material
requiring no bracing, and in a locality where relatively long stretches
of trench can be opened at one time.
The bucket excavator differs from the drag line in that the bucket can
be lifted vertically only and the types of buckets used in the two types
of machine are different. The bucket may be self filling of the
orange-peel or clam-shell type, or a cylindrical container which must be
filled by hand. A drag line can be easily converted into a boom and
bucket excavator. Boom and bucket excavators are well adapted to use in
deep, closely braced trenches and shafts.
=135. Excavation in Quicksand.=[85]—A sand or other granular material in
which there is sufficient upward flow of ground water to lift it, is
known as quicksand. Its most important property, from the viewpoint of
sewer construction, is its inability to support any weight unless the
sand is so confined as to prevent flowing of the sand, or unless the
water is removed from the sand.
Excavation in quicksand is troublesome and expensive and is frequently
dangerous. The material will flow sluggishly as a liquid, it cannot be
pumped easily, and its excavation causes the sides of the trench to fall
in or the bottom to rise. The foundations of nearby structures may be
undermined, causing collapse and serious damage. These conditions may
arise even after the backfilling has been placed unless proper care has
been taken. The greatest safeguard against such dangers is not only to
exercise care in the backfilling to see that it is compactly tamped and
placed, but to leave all sheeting in position after the completion of
the work.
The ordinary method of combating quicksand and in conducting work in wet
trenches is to drive water-tight sheeting 2 or 3 feet below the bottom
of the trench, and to dewater the sand by pumping. When dry it can be
excavated relatively easily. A more primitive but equally successful
method is to throw straw, brickbats, ashes, or other filling material
into the trench in order to hold the excavation once made, or this may
supplement the attempts at pumping, or the wet sand may be bailed out in
buckets. Successful excavation in quicksand requires experience,
resourcefulness. and a careful watch for unexpected developments. The
well points described in Art. 142 are used for dewatering quicksand.
=136. Pumping and Drainage.=—Ground water is to be expected in nearly
all sewer construction and provision should be made for its care. Where
geological conditions are well known or where previous excavations have
been made and it is known that no ground water exists it may be safe to
make no provision for encountering ground water. Where ground water is
to be expected the amount must remain uncertain within certain rather
wide limits until actually encountered.
In order to avoid the necessity for pumping, or working in wet trenches
it is sometimes possible to build the sewer from the low end upwards and
to drain the trench into the new sewer. The wettest trenches are the
most difficult to drain in this manner as the material is usually soft
and the water so laden with sediment as to threaten the clogging of the
sewer. It is undesirable to run water through the pipes until the cement
in the joints has set. This necessitates damming up the trench for a
period which may be so long as to flood the trench or delay the progress
of the work. If it is not possible to drain the trench through the sewer
already constructed the amount of water to be pumped can be reduced by
the use of tight sheeting.
[Illustration:
FIG. 95. Improvised Trench Pump.
]
Pumps for dewatering trenches must be proof against injury by sand, mud,
and other solids in the water. For this purpose pumps with wide passages
and without valves or packed joints are desirable. The types of pumps
used are: simple flap valve pumps improvised on the job, diaphragm
pumps, jet pumps, steam vacuum pumps, centrifugal pumps, and
reciprocating pumps. All are of the simplest of their type and little
attention is paid to the economy of operation because of the temporary
nature of their service.
=137. Trench Pump.=—A simple pump which can be improvised on the job is
shown in section in Fig. 95. Its capacity is about 20 gallons per minute
but its operation is backaching work. It is inexpensive, quickly put
together and may be a help in an emergency. It is to be noted that the
passages are large and straight, that there are no packed joints, and
that the velocity of flow is so small that it is not liable to clogging
by picking up small objects.
[Illustration:
FIG. 96.—Diaphragm Pump
Courtesy, Edson Manufacturing Co.
]
=138. Diaphragm Pump.=—The type of pump shown in Fig. 96 is the most
common in use for draining small quantities of water from excavations.
It is known as the diaphragm pump from the large rubber diaphragm on
which the operation depends. The pump is made of a short cast-iron
cylinder, divided by the rubber diaphragm or disk to the center of which
the handle is connected. The valve is shown at the center of the disk.
As the diaphragm is lifted the valve remains closed, creating a partial
vacuum in the suction pipe and at the same time discharging the water
which passed through the valve on the previous down stroke. When the
valve is lowered the foot valve on the suction pipe closes, holding the
water in place, and the valve in the pump opens allowing the water to
flow out on top of the disk to be discharged on the next up stroke.
Table 54 shows the capacities of some diaphragm pumps as rated by the
manufacturers. The smaller sizes are the more frequently used and are
equipped with a 3–inch suction hose with strainer and foot valve. They
are not adapted to suction lifts over 10 to 12 feet. Where greater lifts
are necessary one pump may discharge into a tub in which the foot valve
of a higher pump is submerged.
TABLE 54
CAPACITIES OF DIAPHRAGM PUMPS
─────────────────┬─────────────────┬─────────────────┬─────────────────
Diameter of │ Diameter of │Length of Stroke │ Capacity per
Cylinder, Inches │ Suction, Inches │ in Inches │ Stroke, Gallons
─────────────────┼─────────────────┼─────────────────┼─────────────────
6│ 3│ 4│ 0.49
8½│ 4│ 6│ 1.47
9[86]│ 2½│ │ 0.75
12½[86]│ 3│ │ 1.25
│ Power driven by 1 horse-power │
12½[86]│ engine │ 0.58[87]
─────────────────┴───────────────────────────────────┴─────────────────
[Illustration:
FIG. 97.—McGowan Steam Jet Pump.
Courtesy, The John H. McGowan Co.
]
=139. Jet Pump.=—The simplicity of the parts of the jet pump is shown in
Fig. 97. It has a distinct advantage over pumps containing valves and
moving parts in that there are no obstructions offered to the passage of
solids as well as liquids through the pump. It is not economical in the
use of steam, however. It operates by means of a steam jet entering a
pipe at high velocity through a nozzle. This action causes a vacuum
which will lift water from 6 to 10 feet. The lower the suction lift,
however, the greater the efficiency of the work. The sizes and
capacities of jet pumps as manufactured by the J. H. McGowan Co. are
shown in Table 55.
TABLE 55
CAPACITIES OF JET PUMPS
(J. H. McGowan Co.)
──────────────┬─────────────┬─────────────┬─────────────┬──────────────
Size of Pump │ │ │ Capacity, │ Approximate
and Suction │ Discharge │ Steam Pipe, │ Gallons per │ Horse-power
Pipe, Inches │Pipe, Inches │ Inches │ Minute │ Required
──────────────┼─────────────┼─────────────┼─────────────┼──────────────
¾│ ½│ ⅜│ 8│ 2
1│ ¾│ ½│ 15│ 3
1¼│ 1│ ½│ 20│ 4
1½│ 1¼│ ¾│ 30│ 6
2│ 1½│ ¾│ 40│ 8
2½│ 2│ 1│ 50│ 10
3│ 2½│ 1│ 60│ 15
4│ 3½│ 1¼│ 85│ 25
──────────────┴─────────────┴─────────────┴─────────────┴──────────────
=140. Steam Vacuum Pumps.=—This type of pump depends on the condensation
of steam in a closed chamber to create a vacuum which lifts water into
the chamber previously occupied by the steam and from which the water is
ejected by the admission of more steam. The best known pumps of this
type are the Pulsometer, manufactured by the Pulsometer Steam Pump Co.,
the Emerson, manufactured by the Emerson Pump and Valve Co., and the Nye
Pump, manufactured by the Nye Steam Pump and Machinery Co.
[Illustration:
FIG. 98.—Pulsometer Steam Vacuum Pump.
]
A section of a Pulsometer is shown in Fig. 98. It consists of two
bottle-shaped chambers _A_ and _B_ with their necks communicating at the
top and each opening into the outlet chamber _O_ through a check valve.
Steam is admitted at the top and enters chamber _A_ or _B_ according to
the position of the steam valve _C_ as shown. This steam valve is a ball
which is free to roll either to the right or left and forms a
steam-tight joint with whichever seat it rests upon. In normal operation
chamber _A_ would be filled with water as the steam enters the cylinder.
At the same time a check valve at the top opens to admit a small
quantity of air which forms a cushion insulating the steam from the
water, reduces the condensation of the steam, and serves as a cushion
for the incoming water on the opposite stroke. The pressure of the steam
depresses the surface of the water without agitation and forces the
water through the check valve _F_ into the discharge chamber _O_. When
the water falls to the level of the discharge chamber the even surface
is broken up and the intimate contact of the steam and water condenses
the former instantaneously. This forms a vacuum in chamber _A_ which,
assisted by a slight upward pressure in chamber _B_ caused by the
incoming water, immediately pulls the ball _C_ over to the other seat
and directs the steam into chamber _B_. The vacuum in chamber _A_ now
draws up a new charge of water through the suction pipe into the
chamber.
[Illustration:
FIG. 99.—Emerson Steam Vacuum Pump.
]
A section of the Emerson pump is shown in Fig. 99. The pump consists of
two vertical cylinders _B_ and _C_. Each chamber has a suction valve _L_
at the bottom, opening upward from a common chamber from which the
discharge pipe _U_ extends. On the top of each chamber is a baffle plate
_G_ which operates to distribute the steam evenly to the two chambers
and to prevent it from agitating the surface of the water in the
chambers. A condenser nozzle _F_ is connected with the bottom of the
opposite chamber by a pipe into which a check valve opens upward. As the
pressure in the chamber alternates water will be injected through _F_
into the opposite chamber and condense the steam therein, promptly
forming a vacuum. An air valve _P_ admits a small quantity of air while
the chamber is filling with water, the air acting as an insulating
cushion as in the Pulsometer. Valve _O_, just above the top connection
_S_ is used to regulate the amount of steam that enters the pump. The
top connection _S_ has two ports, one leading to each chamber. An
oscillating valve enclosed in it admits the steam through these ports to
the two chambers alternately. This valve is driven by a small
three-cylinder engine, the crank shaft of which extends into the top
connection in the center of the bearing on which the valve oscillates. A
positive geared connection is made between the valve and the engine and
so arranged that the engine will run faster than the valve.
The action of these pumps consists of alternately filling and emptying
the two chambers. They will continue operation without attention or
lubrication so long as the steam is turned on. In view of the simplicity
of their operation and make-up, their ability to handle liquids heavily
charged with solids, and their reasonable steam consumption these pumps
are widely used for pumping water in construction work. They have an
added advantage that no foundation or setting is required for them as
they can be hung by a chain from any available support.
These pumps are manufactured in sizes varying from 25 to 2500 gallons
per minute at a 25–foot head, and with a steam consumption of about 150
pounds per horse-power hour. They reduce about 4 per cent in capacity
for each 10 feet of additional lift. They will operate satisfactorily
between heads of 5 to 150 feet, with a suction lift not to exceed 15
feet. Lower suction lifts are desirable and the best operation is
obtained when the pump is partly submerged. The steam pressure should be
balanced against the total head. It varies from 50 to 75 pounds for
lifts up to 50 feet, and increases proportionally for higher lifts. The
dryer the steam the lower the necessary boiler pressure.
=141. Centrifugal and Reciprocating Pumps.=—The details of these pumps,
their adaptability to various conditions, and their capacities are given
in Chapter VII. The centrifugal is better adapted to trench pumping as
it is not so affected by water containing sand and grit, but for clear
water, high suction lifts and fairly permanent installations,
reciprocating pumps can be used with satisfaction.
=142. Well Points.=—In dewatering quicksand a method frequently attended
with success is to drive a number of well points into the sand and
connect them all to a single pump. Figure 100 shows a well point system
used on sewer work in Indiana. The well points are 3 feet apart and are
connected to a 2½-inch header which in turn is connected to six Nye
pumps, each with a capacity of 200 gallons per minute for a lift of 50
feet. The number and size of well points and pumps to use will depend on
conditions as met on the job. On a piece of work in Atlantic City[88]
the equipment consisted of two complete outfits each comprising one
hundred 1½ inch by 36–inch No. 60 well points, one hundred 6–foot
lengths of rubber hose, about 600 feet of suction main, one hundred
valved T connections, and a 7 × 8–inch Gould Triplex Pump with a
capacity of 200 gallons per minute, belted to a 7½ horse-power motor.
[Illustration:
FIG. 100.—Well Points Pumped by Nye Steam Vacuum Pump.
]
=143. Rock Excavation.=—A common definition of rock used in
specifications is: whenever the word Rock is used as the name of an
excavated material it shall mean the ledge material removed or to be
removed properly by channeling, wedging, barring, or blasting; boulders
having a volume of 9 (this volume may be varied) cubic feet or more, and
any excavated masonry. No soft disintegrated rock which can be removed
with a pick, nor loose shale, nor previously blasted material, nor
material which may have fallen into the trench will be measured or
allowed as rock.
Channeling consists in cutting long narrow channels in the rock to free
the sides of large blocks of stone. The block is then loosened by
driving in wedges or it is pried loose with bars. It is a method used
more frequently in quarrying than in trench excavation where it is not
necessary to preserve the stone intact. In blasting, a hole is drilled
in the rock, and is loaded with an explosive which when fired shatters
the rock and loosens it from its position.
[Illustration:
FIG. 101.—Plug and Feathers for Splitting Rock.
]
In drilling rock by hand the drill is manipulated by one man who holds
it and turns it in the hole with one hand while striking it with a
hammer weighing about 4 pounds held in the other hand, or one man may
hold and turn the drill while one or two others strike it with heavier
hammers. In churn drilling a heavy drill is raised and dropped in the
hole, the force of the blow developing from the weight of the falling
drill. Hand drills are steel bars of a length suitable for the depth of
the hole, with the cutting edge widened and sharpened to an angle as
sharp as can be used without breaking. The drill bar is usually about
⅛th of an inch smaller than the diameter of the face of the drill.
Wedges used are called plugs and feathers. They are shown in Fig. 101
which shows also the method of their use. The feathers are wedges with
one round and one flat face on which the flat faces of the plug slide.
=144. Power Drilling.=—In power drilling the drill is driven by a
reciprocating machine which either strikes and turns the drill in the
hole, or lifts and turns it as in churn drilling, or the drill may be
driven by a rotary machine which is revolved by compressed air, steam,
or electricity. There are many different types of machines suitable for
drilling in the different classes of material encountered and for
utilizing the various forms of power available.
A jack hammer drill is shown in Fig. 102. In its lightest form the drill
weighs about 20 pounds and is capable of drilling ⅞-inch holes to a
depth of 4 feet. Heavier machines are available for drilling larger and
deeper holes. The same machine can be adapted to the use of steam or
compressed air. When in use the point of the drill is placed against the
rock and a pressure on the handle opens a valve admitting air or steam.
The piston is caused to reciprocate in the cylinder, striking the head
of the drill at each stroke. The drill is revolved in the hole by hand
or by a mechanism in the machine. A hollow drill can be used by means of
which the operator admits air or steam to the hole, thus blowing it out
and keeping it clean. These machines have the advantage of small size,
portability and simplicity. They can be easily and quickly set up and
the drills can be changed rapidly. Their undesirable features are the
vibration transmitted to the operator and the dust raised in the trench.
[Illustration:
FIG. 102.—Jack Hammer Rock Drill.
]
[Illustration:
FIG. 103.—Tripod Drill.
]
A type of drill heavier and larger than the jack hammer drill is shown
in Fig. 103. It requires some form of support such as a tripod, or in
tunnel work it can be braced against the roof or sides. Some data on
steam and air drills are given in Table 56. The effect of the length of
the transmission pipe, temperature of the outside air, pressure at the
boiler or compressor, etc., will have a marked effect on the amount of
steam or air to be delivered to the drill. Compressed air is affected
more than steam by these outside factors, but it has an advantage in
that as it loses in pressure it increases in volume so that the loss of
power is not so marked. Gillette states:
We may assume that a cubic foot of steam will do practically the
same work in a drill as a cubic foot of compressed air at the same
pressure, because neither the steam nor the air acts expansively
to any great extent in a drill cylinder, due to the late cut-off.
This being so ... one pound of steam is equivalent to nearly 30
cubic feet of free air ... all at the same pressure of 75 pounds
per square inch. If a drill consumes at the rate of 100 cubic feet
of free air per minute ... it would therefore consume 240 pounds
of steam (at 75 pounds pressure) per hour.... Where not more than
three or four drills are to be operated, probably no power can
equal compressed air generated by gasoline. It will require 12
horse-power to compress air for each drill, hence 1½ gallons of
gasoline will be required per hour per drill while actually
drilling.
TABLE 56
DATA ON ROCK DRILLS
(From H. P. Gillette)
───────────────────────────────────┬─────┬─────┬─────┬─────┬─────┬─────
Diameter of cylinder in inches │ 2¼│ 2½│ 2¾│ 3⅛│ 3¼│ 3⅜
Length of stroke in inches │ 5│ 6│ 6½│ 6⅝│ 6⅝│ 7¼
Length of drill from end of crank │ │ │ │ │ │
to end of piston │ 36│ 43│ 50│ 50│ 50│ 52
Depth of hole drilled without │ │ │ │ │ │
change of bit, inches │ 15│ 20│ 24│ 24│ 24│ 24
Diameter of supply inlet. Standard │ │ │ │ │ │
pipe, inches │ ¾│ ¾│ ¾│ 1│ 1│ 1¼
Approximate strokes per minute with│ │ │ │ │ │
60 pound pressure at the drill │ 500│ 450│ 375│ 350│ 325│ 300
Depth of vertical hole each machine│ │ │ │ │ │
will drill easily, feet │ 6│ 8│ 10│ 14│ 16│ 20
Diameter of holes drilled, inches │ ¾ to 1½ as desired
Diameter of octagon steel, inches │ ¾ to│ ⅞ to│ 1 to│1⅛ to│1⅛ to│1¼ to
│ ⅞│ 1│ 1⅛│ 1¼│ 1¼│ 1⅜
Best size of boiler to give plenty │ │ │ │ │ │
of steam at high pressure, │ │ │ │ │ │
horse-power │ 6│ 8│ 8│ 9│ 10│ 12
Best size of supply pipe to carry │ │ │ │ │ │
steam 100 to 200 feet, inches │ ¾│ ¾│ ¾│ 1│ 1│ 1¼
Weight of drill unmounted, with │ │ │ │ │ │
wrenches and fittings, hot boxed,│ │ │ │ │ │
pounds │ 128│ 190│ 265│ 315│ 385│ 390
Weight of tripod, without weights, │ │ │ │ │ │
not boxed, pounds │ 80│ 160│ 160│ 160│ 210│ 275
Weight of holding down weights, not│ │ │ │ │ │
boxed, pounds │ 120│ 270│ 270│ 285│ 330│ 375
Cubic feet of free air per minute │ │ │ │ │ │
required to run one drill at 100 │ │ │ │ │ │
pounds │ 92│ 104│ 126│ 146│ 154│ 160
───────────────────────────────────┴─────┴─────┴─────┴─────┴─────┴─────
For more than one drill, multiply the value in the above line by the
following factors: For 2 drills, 1.8; 5 by 4.1; 10 by 7.1; 15 by 9.5;
20 by 11.7; 30 by 15.8; 40 by 21.4; 70 by 33.2.
Since gasoline air compressors are self regulating, when the drill
is not using air very little gasoline is burned by the gasoline
engine driving the compressor. A gasoline compressor possesses
other very important economic advantages over a small steam-driven
plant. First, there is the saving in wages of firemen and second,
there is the saving in hauling and pumping of water and the
hauling of fuel. The cost of gasoline is often less than the cost
of coal for operating a small plant.
An electric drill[89] operated on the principle of the solenoid does
away with motor, valves, pipes, vapor, freezing, and other difficulties
attendant on the use of steam or air.
The rates of drilling in different classes of rock are shown in Table
57. Frequent changes of drills and relocation of tripods will materially
reduce the performance of a drill, for as much as 45 minutes may be lost
in making a new set up. In this the jack hammer drills show their
advantage as no time is lost in a set up.
TABLE 57
RATES OF ROCK DRILLING
Rates in Feet per Ten-hour Shift. Vertical Holes 10–20 Feet Deep.
(From Gillette)
Hard Adirondack granite 48
Maine and Massachusetts granite 45–50
Mica-schist of New York City. Possible 60–70
Mica-schist of New York City. Average 40–50
Hard, Hudson River trap rock 40
Soft red sand stone of Northern New Jersey 90
Hard limestone near Rochester, N. Y 70
Limestone of Chicago Drainage Canal 70–80
Douglass, Indiana, syenite. Difficult set ups 36
Canadian granite on Grand Trunk R. R 30
Windmill point, Ontario limestone:
3⅝-inch drills 75
2¾-inch drills 60
2¼-inch drills 37
=145. Steam or Air for Power=.—The choice between steam or air is
dependent on the conditions of the work. Steam is undesirable in tunnels
on account of the heat produced. In open cut work it is at a
disadvantage because of the loss of power due to radiation from the hose
or pipe. The life of the hose is not so long as when air is used,
escaping steam causes clouds of vapor which obscure the work, and
serious burns may occur due to hot water thrown from the exhaust. It is
advantageous since leaks may be easily discovered and remedied, it
requires less machinery than air, and it is sometimes less expensive.
With compressed air, gasoline or electric motors can be used for
operating the compressors.
TABLE 58
ROCK BLASTING
(From Gillette)
────────────────────┬────────────────────┬─────────┬─────────┬─────────
Character of │Powder Used per Hole│ │Distance │Distance
Material │ │Depth of │ Back of │ Hole to
│ │ Hole, │ Face, │ Hole,
│ │ Feet │ feet │ feet
────────────────────┼────────────────────┼─────────┼─────────┼─────────
Limestone of Chicago│40 per cent dynamite│ │ │
Drainage Canal │ │ 12│ 8│ 8
Sandstone │200 pounds black │ │ │
│ powder │ 20│ 18│ 14
Granite │2 pounds 60 per cent│ │ │
│ dynamite │ 12│ 1½│ 4½ to 5
Pit mining, │ │ │ │
Treadwell, Mine, │ │ │ │
Alaska │ │ 12│ 2½│ 6
────────────────────┴────────────────────┴─────────┴─────────┴─────────
=146. Depth of Drill Hole.=—The depth of the hole is dependent on the
character of the work. The deepest holes can be used in open cut work
where the shattered rock is to be removed by steam shovel. The face can
be made 10 to 15 feet high. The depth of the hole in center cut tunnel
facings are from 6 to 10 or even 12 feet. In the bench the depth is
equal to the height of the bench. In narrow trenches where the rock is
to be removed by derrick or thrown into a bucket by hand, the hole
should be sufficiently deep to shatter the rock to a depth of at least 6
inches below the finished sewer. Frequently shooting to this depth at
one shot cannot be done due to the built up condition of the
neighborhood or other local factors. The depth of the hole in trench
work should not much exceed the distance between holes. Deep holes are
usually desirable as a matter of economy in saving frequent set ups, but
the holes cannot be made much over 20 feet in depth without increasing
the friction on the drill to a prohibitive amount.
=147. Diameter of Drill Hole.=—The diameter of the hole should be such
as to take the desired size of explosive cartridge. The common sizes of
dynamite cartridges are from ⅞ inch to 2 inches in diameter. In
drilling, the diameter of the hole is reduced about one-eighth of an
inch at a time as the drill begins to stick. This reduction should be
allowed for, and experience is the best guide for the size of the hole
at the start. In general the softer or more faulty or seamy the rock,
the more frequent the necessary reductions in size of bit.[90] For hard
homogeneous rock the holes can be drilled 10 feet or more without
changing the size of the drill bit.
=148. Spacing of Drill Holes.=—The spacing of holes in open cut
excavation is commonly equal to the depth of the hole. The character of
the material being excavated has much to do with the spacing of the
holes. The spacing, diameter and depth of holes used on some jobs is
shown in Table 58. Gillette states:
It is obviously impossible to lay down any hard and fast rule for
drill holes. In stratified rock that is friable, and in traps that
are full of natural joints and seams, it is often possible to
space the holes a distance apart somewhat greater than their
depth, and still break the rock to comparatively small sizes upon
blasting. In tough granite, gneiss, syenite, and in trap where
joints are few and far between, the holes may have to be spaced 3
to 8 feet apart regardless of their depth for with wider spacing
the blocks thrown down will be too large to handle with ordinary
appliances. Since in shallow excavations the holes can seldom be
much further apart than one to one and one-half times their depth
we see that the cost of drilling per cubic yard increases very
rapidly the shallower the excavation. Furthermore the cost of
drilling a foot of hole is much increased where frequent shifting
of the drill tripod is necessary.
The common practice in placing drill holes is to put down holes in
pairs, one hole on each side of the proposed trench; and if the
trench is wide one or more holes are drilled between these two
side holes[91] but in narrow trench work, such as for a 12–inch
pipe, one hole in the middle of the trench will usually prove
sufficient.
The holes are spaced about 3 feet apart longitudinally. After the holes
have been completed they should be plugged to keep out dirt and water.
SHEETING AND BRACING
=149. Purposes and Types.=—Sheeting and bracing are used in trenching to
prevent caving of the banks and to prevent or retard the entrance of
ground water. The different methods of placing wooden sheeting are
called stay bracing, skeleton sheeting, poling boards, box sheeting, and
vertical sheeting. Steel sheeting is usually driven to secure
water-tightness and if braced the bracing is similar to the form used
for vertical wooden sheeting.
=150. Stay Bracing.=—This consists of boards placed vertically against
the sides of the trench and held in position by cross braces which are
wedged in place. The purpose of the board against the side of the trench
is to prevent the cross brace from sinking into the earth. The boards
should be from 1½ × 4 inches to 2 × 6 inches and 3 to 4 feet long. The
cross braces should not be less than 2 × 4 inches for the narrowest
trenches and larger sizes should be used for wider trenches. The spacing
between the cross braces is dependent on the character of the trench and
the judgment of the foreman. Stay bracing is used as a precautionary
measure in relatively shallow trenches with sides of stiff clay or other
cohesive material. It should not be used where a tendency towards caving
is pronounced. Stay bracing is dangerous in trenches where sliding has
commenced as it gives a false sense of security. The boards and cross
braces are placed in position after the trench has been excavated.
=151. Skeleton Sheeting.=—This consists of rangers and braces with a
piece of vertical sheeting behind each brace. A section of skeleton
sheeting is shown in Fig. 104 with the names of the different pieces
marked on them. This form of sheeting is used in uncertain soils which
apparently require only slight support, but may show a tendency to cave
with but little warning. When the warning is given vertical sheeting can
be quickly driven behind the rangers and additional braces placed if
necessary. The sizes of pieces, spacing and method of placing should be
the same as for complete vertical sheeting in order that this may be
placed if necessary.
=152. Poling Boards.=—These are planks placed vertically against the
sides of the trench and held in place by rangers and braces. They differ
from vertical sheeting in that the poling board is about 3 or 4 feet
long. It is placed after the trench has been excavated; not driven down
with the excavation like vertical sheeting. An arrangement of poling
boards is shown in Fig. 105. This type of support is used in material
that will stand unsupported for from 3 to 4 feet in height. Its
advantages lie in that no driving is necessary, thus saving the trench
from jarring; no sheeting is sticking above the sides of the trench to
interfere with the excavation; and only short planks are necessary.
[Illustration:
FIG. 104.—Skeleton Sheeting.
]
[Illustration:
FIG. 105.—Poling Boards.
Showing Different Types of Cross Bracing.
]
The method of placing poling boards is as follows: Excavate the trench
as far as the cohesion of the bank will permit. Poling boards, 1½ inch
to 2 inch planks, 6 inches or more in width, are then stood on end at
the desired intervals along each side of the trench for the length of
one ranger. The poling boards may be held in place by one or two
rangers. Two are safer than one but may not always be necessary. If one
ranger is to be used it is placed at the center of the poling board.
After the poling boards are in position the rangers are laid in the
trench and the cross braces are cut to fit. If wedges are to be used for
tightening the cross braces, the cross braces are cut about 2 inches
short. If jacks are to be used the braces are cut short enough to
accommodate the jacks when closed, or adjustable trench braces may be
used as shown in Fig. 106. The use of extension braces saves the labor
of fitting wooden braces. With everything in readiness in the trench,
the cross brace is pressed against the ranger which is thus held in
place. The wedge or jack is then tightened holding the poling boards and
cross brace in position.
[Illustration:
FIG. 106.—Box Sheeting.
Showing Different Types of Cross Bracing.
]
=153. Box Sheeting.=—Box sheeting is composed of horizontal planks held
in position against the sides of the trench by vertical pieces supported
by braces extending across the trench. The arrangement of planks and
braces for box sheeting is shown in Fig. 106. This type of sheeting is
used in material not sufficiently cohesive to permit the use of poling
boards, and under such conditions that it is inadvisable to use vertical
sheeting which protrudes above the sides of the trench while being
driven. This sheeting is put in position as the trench is excavated. No
more of the excavation than the width of three or four planks need be
unsupported at any one time. In placing the sheeting the trench is
excavated for a depth of 12 to 24 inches. Three or four planks are then
placed against the sides of the trench and are caught in position by a
vertical brace which is in turn supported by a horizontal cross brace.
[Illustration:
FIG. 107.—Vertical Sheeting.
]
=154. Vertical Sheeting.=—This is the most complete and the strongest of
the methods for sheeting a trench. It consists of a system of rangers
and cross braces so arranged as to support a solid wall of vertical
planks against the sides of the trench. An arrangement of complete
vertical sheeting is shown in Fig. 107. This type can be made nearly
water-tight by the use of matched boards, Wakefield piling, steel
piling, etc. Wakefield piling is made up of three planks of the same
width and usually the same thickness. They are nailed together so that
the two outside planks protrude beyond the inside one on one side, and
the inside one protrudes beyond the two outside ones on the other side
as shown in Fig. 108. The protruding inside plank forms a tongue which
fits into the groove formed by the protruding outside planks of the
adjacent pile.
[Illustration:
FIG. 108.—Wakefield Sheet Piling.
]
[Illustration:
FIG. 109. Section through Malleable Steel Driving Cap.
]
In placing vertical sheeting the trench is excavated as far as it is
safe below the surface. Blocks of the same thickness as the sheeting are
then placed against the bank at the middle and at the ends of two
rangers on opposite sides of the trench. The ranger rest against blocks,
and are held away from the sides of the trench by them. Cross braces are
next tightened into position opposite the blocks to hold the rangers in
place. After the skeleton sheeting is in place the planks forming the
vertical sheeting are put in position with a chisel edge cut on the
lower end of the plank, with the flat side against the bank. The planks
should be driven with a maul, the edge of the plank following closely
behind the excavation. In relatively dry work the driving of the plank
is facilitated by excavating beneath the edge as it is driven. The upper
end of the sheeting should be protected by a malleable steel or iron cap
to prevent brooming of the lumber. A cap is shown in Fig. 109. A sledge
hammer may be used for driving when the lumber is protected. If the
sheeting is to start at the surface and is to be driven by hand, the
first length should not exceed 4 feet unless a platform is erected for
the driver. Succeeding lengths may be longer, the driver standing on
planks supported on the cross braces in the trench. Steam hammers and
pile drivers are sometimes used for driving sheeting.
The framework of the sheeting should be placed with a cross brace for
each end of each ranger and a cross brace for the middle of each ranger.
If the ends of two rangers rest on the same cross brace an accident
displacing one ranger will be passed on to the next and might cause a
progressive collapse of a length of trench, whereas the movement of an
independently supported ranger should have no effect on another ranger.
The cross braces should have horizontal cleats nailed on top of them as
shown in Fig. 107 to prevent the braces from being knocked out of place
by falling objects. In driving vertical sheeting a vacant place will be
left behind each cross brace corresponding to the original block placed
to hold the ranger away from the bank. This is an undesirable feature in
the use of vertical sheeting. It is ordinarily remedied by slipping in
planks the width of the slot and wedging or nailing them against the
convenient cross bracing. In extremely wet trenches, after all other
pieces of vertical sheeting are in place, the original cleat behind the
cross brace can be knocked out and a piece of sheeting slipped into this
opening and driven. Care must be taken in this event not to drive the
rangers down when driving the sheeting. If the bracing begins to drop,
it should be supported by vertical pieces between the rangers and
resting on a sill at the bottom of the trench.
[Illustration:
FIG. 110.—Steel Clamp for Pulling Wood Sheeting.
]
=155. Pulling Wood Sheeting.=—Wood sheeting is pulled after the
completion of the trench by a device shown in Fig. 110. In wet trenches
where the removal of the sheeting would permit a movement of the banks,
resulting in danger to the sewer or other structures, the sheeting
should be left in place in the trench. If sufficient saving can be made
the sheeting is cut off in the trench immediately above the danger line,
usually the ground water line. The cutting is done with an axe or by a
power driven saw devised for the purpose.
=156. Earth Pressures.[92]=—The various theories of earth pressure are
so conflicting in their conclusions as to be confusing. Rankine’s
theory, the most frequently used, assumes that the pressure increases
with the depth, whereas Meem’s theory[93] leads to an opposite
conclusion. The discussion following Meem’s article is very
illuminating. It indicates that no matter how good the theory, practical
experience together with the use of generous sizes and close spacing are
the best guides for bracing trenches and coffer dams. All are not
possessed with the desired practical experience and some basis on which
to commence work is essential. Another factor affecting computations of
sizes based on theory is the tendency in practice to use the same size
material for rangers and braces on any one job for all except very deep
trenches and other special cases. Occasionally where there is an
independent brace for each end of each ranger, the brace is made
thinner, but is of the same depth as the ranger.
The application of Rankine’s theory of earth pressure to the computation
of the sizes of rangers and braces will be shown. His formula for the
active earth pressure against a retaining wall is:
_P_ = _wh_ cosθ (cos θ − √(cos^2 θ − cos^2 φ))⁄(cos θ + √(cos^2 θ −
cos^2 φ))
in which _w_ = the weight of earth in pounds per cubic foot;
_h_ = depth in feet at point at which pressure is to be
determined;
θ = the angle of surcharge, or the angle which the surface
makes with the horizontal;
φ = the angle of repose of the earth. Usually taken as
33°–41′ = 1½ horizontal to 1 vertical;
_P_ = the intensity of pressure in pounds per square foot on
a vertical plane in a direction parallel to the
surface of the ground.
In studying the pressures for trenches the surface of the ground will be
assumed as horizontal and the formula reduces to
_P_ = (1 − sin φ)⁄(1 + sin φ)_wh_.
=157. Design of Sheeting and Bracing=.—The trench shown in Fig. 111 is
assumed to be constructed in moist sand weighing 110 pounds per cubic
foot, with an angle of repose of 30 degrees. The material used for
sheeting and bracing is yellow pine. The steps taken in the design of
the sheeting and bracing for this trench are as follows:
[Illustration:
FIG. 111.—Diagram for the Design of Wood Sheeting.
]
1. _Earth Pressure._—Substituting the units given in the data, in
Rankine’s formula for earth pressures,
_P_ = 36.7_h_.
Because the earth has been freshly cut and will not be kept open long
enough to break up the cohesiveness of the banks it is customary to
reduce the assumed pressure by dividing by 2, 3, or 4, according to the
natural cohesiveness of the material. The cohesiveness of sand is not
great, therefore the pressure will be assumed as one-half of the amount
given by the formula, or
_p_ = 18_h_.
2. _Thickness of Sheeting and Spacing of Rangers._—It is desirable to
use the same thickness of sheeting throughout the depth of the trench.
Computations should therefore be commenced at the bottom of the trench
where the pressures are the greatest and the thickest sheeting will be
required. It is necessary to determine by trial a spacing for the
rangers and a thickness of sheeting so that the sheeting is stressed to
its full working strength. Having determined the thickness of the
sheeting at the bottom, the remainder of the computations consists in
determining the spacing of the rangers.
In the example the lower ranger will be assumed as 3 feet from the
bottom of the trench and the distance to the next ranger as 4 feet.
The intensity of pressure at 22 feet 9 inches is 409.5 pounds per
square foot.
The intensity of pressure at 26 feet 9 inches is 481.5 pounds per
square foot.
The distribution of pressures is shown by the diagram on Fig. 111. The
maximum bending moment is slightly below the point midway between the
rangers and for a 12–inch strip is 10,500 inch-pounds.
Assuming 3 inch sheeting the maximum fiber stress is:
_f_ = _Mc_⁄_I_ = (10,400 × 1.5 × 12)⁄12 × 27 = 568 pounds per square
inch.
The working strength of yellow pine as given in Table 59, is 1200 pounds
per square inch. Thinner sheeting should therefore be used.
TABLE 59
WORKING UNIT STRESSES FOR TIMBER
The most used value in the Building Codes of Baltimore, Boston,
Cincinnati, Chicago, District of Columbia, and New York City
─────────────┬────────┬───────────┬───────────┬──────────┬──────┬──────
Wood │ │ │ │ │Shear │Shear
│ │ │ │ │ With │Across
│ │ │Compression│Transverse│Grain,│Grain,
│Tension,│Compression│ Across │ Bending, │ lb. │ lb.
│lb. sq. │With Grain,│Grain, lb. │ lb. sq. │ sq. │ sq.
│ in. │lb. sq. in.│ sq. in. │ in. │ in. │ in.
─────────────┼────────┼───────────┼───────────┼──────────┼──────┼──────
Yellow pine │ 1200│ 1000│ 600│ 1200│ 70│ 500
White pine │ 800│ 800│ 400│ 800│ 40│ 250
Spruce and │ │ │ │ │ │
Va. pine. │ 800│ 800│ 400│ 800│ 50│ 320
Oak │ 1000│ 900│ 800│ 1000│ 100│ 600
Hemlock │ 600│ 500│ 500│ 600│ 40│ 275
Chestnut │ 600│ 500│ 1000│ 800│ │ 150
Locust │ │ 1200│ 1000│ 1200│ 100│ 720
─────────────┴────────┴───────────┴───────────┴──────────┴──────┴──────
As published in American Civil Engineers Pocket Book.
Assuming 2–inch sheeting, the fiber stress is 1,300 pounds per square
inch. This stress is too large. By reducing the ranger spacing slightly
the stress can be brought within the required limits.
Assuming a ranger spacing of 3 feet 9 inches the depth to the upper
ranger is changed to 23 feet and the maximum stress in the 2–inch
sheeting becomes 1,140 pounds per square inch, a satisfactory result.
The results for the computations for the other ranger spacings are shown
in Table 60. The spacing of the rangers at the sheeting junctions is
controlled by convenience and is not computed so long as it is obviously
safe.
3. _Size of Rangers._—The rangers will be assumed as 16 feet long with
two end cross braces and one intermediate cross brace for each ranger.
Starting as before at the bottom of the trench.
The area of the panel below the ranger and between cross
braces is 24 square feet.
The average intensity of pressure is 28.25 × 18 = 508.5
pounds per square inch.
The load transmitted to the ranger is 6,000 pounds.
Similarly the load transmitted to the ranger from the
panel above is 6,890 pounds.
The total distributed load on the ranger is 12,890 pounds.
If _b_ is the vertical dimension of the ranger and _d_ is the horizontal
dimension in inches, then from the beam theory, using _f_ as 1,200
pounds per square inch, _bd_^2 = _M_⁄200, in which _M_ is expressed in
inch-pounds. The maximum bending moment is
(_Wl_)⁄8 = 12,200 × 8 × 12⁄8 = 155,000 inch-pounds
Therefore, _bd_^2 = 775.
An 8 × 10 inch beam will fulfill the conditions closely. Substituting
these dimensions in the beam formula
_f_ = (_Mc_)⁄_I_ = (155,000 × 5 × 12)⁄8 × 1000
= 1,160 pounds per square inch tension in outer fiber. The results of
the computations for other rangers are shown in Table 60.
4. _Size of Cross Braces._—The cross braces act as columns. The
dimensions of the cross braces are determined by trial in such a manner
that the vertical dimension of the brace is equal to the vertical
dimension of the ranger and the compressive stress in pounds per square
inch is computed from the expression,
_S_ ⪙ _S__{1}(1 − _l_⁄(60_d_)),[94]
TABLE 60
COMPUTATIONS FOR SHEETING AND BRACING FOR TRENCH SHOWN IN FIG. 111
Material is moist sand weighing 110 pounds per cubic foot, with an angle of
repose of 30°. Lumber is yellow pine, with working stress as given in Table
59. Working stresses for columns given as _S_(1 − _l_⁄(60_d_)).
──────────────────────────────┬───────────────────────────────────────────────
Sheeting 2 inches × 12 Inches │ Cross Braces
──────────┬───────────┬───────┼───────────┬──────┬──────┬──────────┬──────────
│ │Maximum│ │ │ │ │
│ │ Fiber │ │ │ │ │
│ │Stress,│ │ │ │ Actual │Allowable
│ Maximum │Pounds │ │ │ │Intensity,│Intensity,
│ Bending │ per │ │Total │ │Pounds per│Pounds per
│ Moment, │Square │ Depth and │Load, │Size, │ Square │ Square
Depth │Inch-Pounds│ Inch │Description│Pounds│Inches│ Inch │ Inch
──────────┼───────────┼───────┼───────────┼──────┼──────┼──────────┼──────────
│ │ │end at 26′│ │ │ │
23′–26.75′│ 9100│ 1140│ 9″│ 6,445│ 4 × 8│ 202│ 784
│ │ │int. at 26′│ │ │ │
19′–23′│ 8800│ 1100│ 9″│12,890│ 4 × 8│ 403│ 784
│ │ │end at 23′│ │ │ │
13′–17.5′│ 8550│ 1070│ 0″│ 6,393│ 4 × 8│ 200│ 784
│ │ │int. at 23′│ │ │ │
8′–13′│ 7160│ 900│ 0″│12,785│ 4 × 8│ 400│ 784
│ │ │end at 19′│ │ │ │
0′–6′│ 3000│ 375│ 0″│ 3,930│ 4 × 8│ 123│ 784
│ │ │int. at 19′│ │ │ │
│ │ │ 0″│ 7,860│ 4 × 8│ 240│ 784
│ │ │end at 17′│ │ │ │
│ │ │ 6″│ 3,566│ 4 × 8│ 112│ 684
│ │ │int. at 17′│ │ │ │
│ │ │ 6″│ 7,132│ 4 × 8│ 224│ 684
│ │ │end at 13′│ │ │ │
│ │ │ 0″│ 4,385│ 4 × 8│ 137│ 684
│ │ │int. at 13′│ │ │ │
│ │ │ 0″│ 8,770│ 4 × 8│ 274│ 684
│ │ │end at 8′│ │ │ │
│ │ │ 0″│ 2,270│ 4 × 6│ 96│ 687
│ │ │int. at 8′│ │ │ │
│ │ │ 0″│ 4,540│ 4 × 6│ 189│ 667
│ │ │end at 6′│ │ │ │
│ │ │ 0″│ 1,344│ 4 × 6│ 60│ 584
│ │ │int. at 6′│ │ │ │
│ │ │ 0″│ 2,687│ 4 × 6│ 112│ 584
│ │ │end at 0′│ │ │ │
│ │ │ 0″│ 432│ 4 × 6│ 18│ 584
│ │ │int. at 0′│ │ │ │
│ │ │ 0″│ 863│ 4 × 6│ 36│ 584
──────────┴───────────┴───────┴───────────┴──────┴──────┴──────────┴──────────
Rangers
──────┬──────┬─────────┬──────┬────────────────────┬──────┬───────────┬───────
│ Area │ │ │ │ │ │
│ of │Intensity│ │ │ │ │
│Panel │ of │ │ │ │ │Maximum
│Below │Pressure,│ │ │ │ Maximum │Stress
│ this │ Pounds │Total │ │ │ Bending │Pounds
│Depth,│ per │ Load │ │ │ Moment in │ per
│Square│ Square │ in │Load Transmitted to │Size, │ Thousand │Square
Depth │ Feet │ Inch │Pounds│the Ranger from the │Inches│Inch-Pounds│ Inch
──────┼──────┼─────────┼──────┼──────┬──────┬──────┼──────┼───────────┼───────
│ │ │ │Panel │Panel │ Both │ │ │
│ │ │ │Below │Above │Panels│ │ │
──────┼──────┼─────────┼──────┼──────┼──────┼──────┼──────┼───────────┼───────
26′ 9″│ 24│ 508.5│12,200│ 6000│ 6890│12,890│8 × 10│ 155│ 1160
23′ 0″│ 30│ 448│13,440│ 6545│ 6240│12,785│8 × 10│ 153│ 1150
19′ 0″│ 32│ 378│12,100│ 5860│ 2000│ 7,860│8 × 10│ 94.3│ 708
17′ 6″│ 12│ 328.5│ 3,942│ 1942│ 5190│ 7,132│8 × 10│ 85.6│ 636
13′ 0″│ 36│ 274.5│ 9,880│ 4690│ 4080│ 8,770│8 × 10│ 105│ 790
8′ 0″│ 40│ 189│ 7,560│ 3480│ 1060│ 4,540│6 × 8│ 54.4│ 850
6′ 0″│ 16│ 126│ 2,020│ 960│ 1727│ 2,687│6 × 8│ 32.2│ 503
0′ 0″│ 48│ 54│ 2,590│ 863│ 0│ 863│6 × 8│ 10.4│ 161
──────┴──────┴─────────┴──────┴──────┴──────┴──────┴──────┴───────────┴───────
in which _S_ = permissible crushing across the grain in a column whose
length is greater than 15 diameters;
_S__{1} = unit working compressive strength of wood;
_l_ = length of the column;
_d_ = smallest dimension of the column;
_l_ and _d_ are in the same units.
The lower intermediate cross brace supports a length of 8 feet of the
lower ranger on which the load has been found to be 12,890 pounds. The
load on the end cross brace for the same ranger is one-half of this or
6,445 pounds. The length of each brace is 4 feet 4 inches. From Table
59, _S__{1} is 1,000 pounds per square inch. From the column formula,
_S_ is 784 pounds per square inch.
A 4 × 8 inch cross brace is the smallest that is feasible. This is
stressed only 12,890 pounds or 403 pounds per square inch, which is well
within the permissible limits. The results of the other computations for
cross braces are shown in Table 60.
=158. Steel Sheet Piling.=—This is coming into more general use with the
increased cost of lumber and better acquaintance with its superiority
over wood under many conditions. Although its first cost is higher than
that of wood, the fact that with proper care it can be used almost an
indefinite number of times renders it economical to contractors who may
have an opportunity to make repeated use of it. The life of good yellow
pine sheeting with the best of care may be as much as three or four
seasons. With no particular care it will be destroyed at the first
using. Fig. 112 shows various sections of steel piling used for trench
sheeting. These forms are practically water-tight and aid materially in
maintaining dry trenches. The piling can be made water tight by slipping
a piece of soft wood between the steel sections when they are being
driven, or by pouring in between the piles some dry material which will
swell when wet. The piling is generally driven by a steam hammer and is
pulled by attaching a ring through a bolt hole in the pile, or by
grasping the pile with a clutch that tightens its grasp as the pull
increases. An inverted steam hammer attached to the pile is sometimes
used in pulling it. The impulses of the hammer together with a steady
pull on the cable serve to drag out the most stubborn piece of piling.
[Illustration:
FIG. 112.—Sections of Lackawanna Steel Sheet Piling.
]
LINE AND GRADE
=159. Locating the Trench.=—In order to locate a trench a line of stakes
should be driven at about 50–foot intervals along the center line of the
proposed sewer before excavation is commenced. Reference stakes or
reference points to this line are located at some fixed offset or easily
described point, or the stakes marking the center line of the trench may
be driven at some constant offset distance one side of the trench, in
order to avoid danger of loss or disturbance of the stakes. Grade or cut
is seldom marked on the line of preliminary stakes, although the
approximate cut may be indicated.
For hand excavation the foreman lays out the trench from these stakes.
In machine work the operator guides the machine so as to follow the line
of the stakes.
=160. Final Line and Grade.=—After the excavation of the trench has
proceeded to within a foot or two of the final depth, the grade and line
are transferred to markers supported over the center of the trench. The
markers are horizontal boards spanning the trench and held in position
either by nails driven into stakes at the side of the trench, by nails
driven into the sheeting, or by weights holding the boards on the
ground. Two stakes driven in the ground at the side of the trench as
shown in Fig. 113 are the common method of support. If the banks are too
weak to stand under the jarring of the driving of the stakes, or
pavement or other causes prevent their use the horizontal cross piece
may be weighted down by bricks or a bank of earth. The cross pieces are
located about every 25 feet along the trench and at any convenient
distance above the surface of the ground. The nearer the ground the
stronger the support but the greater the interference with work in the
trench. The center line of the sewer is marked on the cross pieces after
they are set, and vertical struts are nailed on them with one edge of
the strut straight, vertical, and on the center line as shown in Fig. 1.
The corresponding edge should be used on all struts in order to avoid
confusion. The edge is placed in a vertical position by means of a plumb
bob or carpenter’s level.
[Illustration:
FIG. 113.—Methods for the Support of the Grade Line.
]
The cut to the invert of the sewer is recorded to an even number of feet
where practicable by driving a nail in the upright strut so that the top
edge of the nail is at the desired elevation above the sewer, or the
upright is nailed with its top at the proper number of feet above the
sewer invert. The cut is marked on the upright in feet, tenths, and
hundredths from the recorded point to the elevation of the invert.
The inspector should watch these grade markers with care by sighting
back along them to see that they are in line and have not moved. In
quicksand or caving material the marks may move during the setting of
the pipes and the levelman should be on the job constantly.
When excavation is being done by machine the depth of the excavation is
controlled by the operator who maintains a sighting rod on the machine
in line with the grade marks on the uprights.
[Illustration:
FIG. 114.—Diagram Showing the Use of the Grade Rod for Fixing the
Elevation of a Sewer.
]
=161. Transferring Grade and Line to the Pipe.=—In transferring grade
and line to the sewer a light strong string is stretched tightly from
nail to nail on the uprights marking the line and grade. A rod with a
right angle projection at the lower end, as shown in Fig. 114, is marked
with chalk or a notch at such a distance from the end that when the mark
is held on the grade cord the lower portion of the rod which projects
into the pipe will rest on the invert. The pipe is placed in line by
hanging a plumb bob so that the plumb bob string touches the grade and
center line cord. These marks are taken only as frequently as may be
necessary to keep the sewer in line. An experienced workman can maintain
the line by eye for considerable distances. Measurements should never be
taken to the top of the pipe in order to determine position and grade as
the variations in the diameter of the pipe may cause appreciable errors.
The position and elevation of the forms for brick, concrete, and unit
block sewers are located by reference to the grade line, or they may be
placed under the immediate direction of the survey party, or by
specially located stakes. For large sewers requiring deep and wide
excavation the grade and line stakes are driven in the bottom of the
trench about a foot above the finished grade. This requires the constant
presence of an engineer who is usually available on work of such
magnitude.
=162. Line and Grade in Tunnel.=—In tunnels, line and grade are given by
nails driven in the roof, the progress of excavation or the shield being
followed by eye and the forms set by direct measurement to the nails.
TUNNELING
=163. Depth.=—The depth at which it becomes economical to tunnel depends
mainly upon the character of the material to be excavated and on the
surface conditions. In soft dry material with unobstructed working space
at the surface, open cut may be desirable to depths as great as 35 or 40
feet. Tunnels are cut in rock at depths of 15 feet or less. In some very
wet and running quicksand encountered in the construction of sewers for
the Sanitary District of Chicago it was found economical to tunnel at
depths of 20 feet and less. Crowded conditions on the surface, expensive
pavements, or extensive underground structures near the surface may make
it advantageous to tunnel at shallower depths than would otherwise be
economical. Winter is the best season for tunneling as the workmen are
protected from the elements and labor is more plentiful.
=164. Shafts.=—In sinking a shaft in soft material, the excavation is
usually done by hand, the material being thrown into a bucket which is
hoisted to the surface and dumped. The size of the shaft is independent
of the size of the sewer and depends principally on the machinery which
it is necessary to lower into the tunnel. Ordinarily a shaft 6 feet in
the clear is satisfactory. A method of timbering a shaft is shown in
Fig. 115. Because of the timbering the shaft must be started
sufficiently large at the top to finish with the desired dimensions at
the bottom. This excess size is sometimes obviated by driving the
sheeting at an angle to maintain the same size of shaft from top to
bottom.
In timbering a shaft as shown in Fig. 115 the upper frame is staked
securely in position at the surface of the ground. This frame is
composed of timbers fastened together in the form of a square with the
ends of the timbers extending about 12 inches on all sides. The
protruding ends are used to hold the frame in position. Excavation is
begun inside the frame, and sheeting is driven around the outside of it
as excavation progresses. Only two or three men can work advantageously
at one time in these small shafts. The second frame is made up of the
same size timbers, but all are cut off flush with the outside of the
square. The outside dimensions of this frame are such as to allow
sheeting to be slipped in between it and the sheeting already driven.
The frame is lowered into position and supported from the upper frame by
vertical struts nailed to it. The lower end of the sheeting already
driven is held out from the lower frame by blocks of the thickness of
the next length of sheeting. These blocks are removed as the next length
of sheeting is placed and driven. The driving of the sheeting is
facilitated by excavating beneath it as it descends.
[Illustration:
FIG. 115.—Section of Shaft Timbering.
Abbot, Journal Western Society of Engineers, Vol. 22.
]
The sizes of sheeting and timbering should be computed on the same basis
as that for trench sheeting except that for depths greater than 30 to 35
feet Rankine’s Theory is not applicable and judgment must be relied on
for computing the sizes for deep shafts. In stiff dry material the
pressures will change very little as the depth increases. Sheeting is
needed in shaft excavation in rock only to protect the workmen from
falling fragments, but in sand, particularly in quicksand and in wet
ground, the pressures increase directly with the depth and the sheeting
should be computed accordingly. Care must be taken to prevent the
formation of cavities behind the sheeting, to fill them if formed, and
to see that all pieces of the sheeting and bracing have a firm bearing.
It is difficult to prevent the collapse of the shaft once the movement
of earth against the sheeting has commenced.
Shafts are also sunk in soft ground by constructing a concrete or metal
shell resting on a cutting shoe on the surface. The material inside is
dug out and the shell sinks of its own or added weight. The first
section of the shell may be from 5 to 10 feet long. As this section
sinks other sections are added. This is called the caisson method. It is
advantageous in wet ground and when the shafts are to be left as a
permanent manhole. If a permanent shaft is to be left in an excavation
being braced with wood, the permanent lining should follow within 20 to
30 feet of the shaft excavation. This is done to avoid the difficulty of
maintaining a great length of temporary wood shaft with the danger of
collapse, or of blocks or other objects falling on the workers below.
The distance between shafts is controlled by the depth and size of the
tunnel, surface conditions, and the character of the material being
tunneled. Except where surface conditions are crowded the shallower the
cover to the tunnel the more frequent the shafts. The advantage of
frequent shafts lies in the possibility of removing excavated material
from the tunnel promptly, and in making ventilation of the tunnel
easier. The saving made by the construction of numerous shafts must be
balanced against the extra cost of the shafts. For the shallowest
tunnels the shafts are seldom placed closer than every 500 feet.
=165. Timbering.=—After the shaft has been excavated to the proper grade
the tunnel is struck out either by cutting through the wooden sheeting
or by removing portions of the caisson lining. Practically all tunnels
except those in solid rock must be framed to some extent. Some of the
types of frames used in tunnel construction are shown in Fig. 116.
Different combinations of these may be used in different classes of
materials. In solid rock which remains firm on exposure no timbering is
necessary. Where the roof only need be supported and the sides are
strong enough to be used for support, a timber “hitch” or frame
supported on the sides of the tunnel may be used. This is suitable for
loose rock roofs with solid rock sides. Timbering such as is shown in
the lower left hand corner of Fig. 116 becomes necessary in extremely
soft, wet, or swelling material, where the bottom and sides as well as
the roof tend to push in. The remaining frame in Fig. 116 shows a form
frequently used and lying between the two extremes indicated. In wet
tunnels a channel may be cut in the bottom below the sill for drainage
purposes as shown in this form. The needle beam method of timbering is
also shown in Fig. 116. This method of timbering is used mainly near the
heading because of the speed and ease with which it can be installed,
but it is undesirable because of the space occupied.
The distance between frames is dependent on the size of the tunnel and
the character of the material. It is seldom greater than 6 feet and the
frames are sometimes placed touching each other. The size of the
timbering is a matter of experience and is generally determined by the
judgment of the responsible person in charge of the construction as the
result of observation during the progress of the work.
The sheeting between frames is called poling boards, or spiling or
lagging according as it is sharpened and driven ahead of the excavation
or placed after the excavation has progressed. The horizontal strips
placed between the frames to keep them apart are called wales.
[Illustration:
FIG. 116.—Types of Frames and Timbering for Tunnels.
]
In cutting out from the shaft in soft materials requiring support, where
the width of the tunnel is the same or smaller than that of the shaft, a
frame with a maximum width four thicknesses of sheeting less than the
width of the tunnel is set up against the lining of the shaft. The
vertical side pieces of the tunnel frame rest on the bottom frame of the
shaft as a sill and are securely wedged into position. As the lining of
the shaft at the top is cut away the top poling boards of the tunnel are
slipped in between the cap of the first tunnel frame and the shaft frame
immediately above it. The poling boards are driven with an upward pitch
so that there may be room to slip the second length of boards between
the next tunnel frame and the first length of boards. The placing of the
side sheeting follows in a similar manner. Excavation is then started
and the poling boards driven to keep pace with it. The next frame is
placed in position and the previous sheeting or boards wedged out a
sufficient distance to allow the advance lining to be slipped in when
the wedges are removed. Waling pieces are nailed firmly between the
frames to hold them in position. The various phases in the driving of a
12–foot sewer tunnel in Seattle are shown in Fig. 117.
[Illustration:
FIG. 117.—Stages of Sewer Tunneling.
Eng. Record, Vol. 69, 1914, p. 195.
]
In soft or running material it may be necessary to protect the face of
the tunnel by horizontal boards, called breast boards, wedged back to
the last frame placed. The excavation is performed by removing one board
at a time, excavating behind it and then replacing it in the advance
position. The advance is made from the top downwards. This represents
the method pursued in the most difficult material where wooden sheeting
without a shield is used. The timbering during the advance may be
modified in any manner that the character of the material will permit.
The timbering may lag behind the excavation a distance of two or more
frames, or it may be omitted altogether. Heavier timbering may be
necessary in soft, slipping or shattered rock.
[Illustration:
FIG. 118.—Shield for Driving Milwaukee Sewer Tunnel.
Eng. News-Record, Vol. 80, 1918, p. 669.
]
=166. Shields.=—Shields are used in tunneling in soft wet material and
are particularly suitable for work under air pressure. They are used in
rock tunnels where water is anticipated or air pressure is used. The
shields often save the expense and difficulty of timbering as the
masonry of the sewer follows closely behind the shield. Fig. 118 shows
the arrangement for a shield for tunneling in soft material in the
construction of the Milwaukee sewers. The shield has an exterior
diameter of 9 feet 4 inches and an overall length of 9 feet 8⅛ inches.
The cutting edge section is 20 inches long. The shell is made of one
inch plate to the back of the jack chambers and one-half inch plate in
the tail. The shield is driven by ten 60–ton hydraulic jacks. The jacks
are shown in position in the figure. These jacks rest against the
finished tunnel lining and serve to consolidate it at the same time that
they push the shield into the material to be excavated. The face of the
tunnel is cut with a pick and shovel while the jacks are removed one at
a time and a new ring of lining is put in place. The lining may be
temporary timbering to receive the thrust of the jacks, but it is
usually desirable that the permanent lining follow immediately behind
the shield. Since the shield is larger than the outside of the lining
the space left by its passage should be grouted immediately after it has
passed.
=167. Tunnel Machines.=—Tunnel machines have been used successfully on
sewer tunnels in soft materials, but not in rock.[95] The machines are
of different types, but in general consist of a revolving cutting head,
equipped with knives, and driven by an electric motor. The bearing on
which the shaft for the cutting head rests is supported against the
sides of the tunnel. The muck is carried away by means of a conveyor and
dumped into muck cars without rehandling. Rapid progress can be made
with these machines in suitable conditions.
[Illustration:
FIG. 119.—Method of Drilling and Loading Rock Tunnel Face.
Courtesy, Aetna Power Co.
]
=168. Rock Tunnels.=—Tunnels in rock are advanced by drilling into the
face as shown in diagrammatic form in Fig. 119. The holes near the
center are driven in at an angle towards the center and to depths from 6
to 15 feet. The harder the rock the greater the angle with the tunnel.
This is called the center cut. Other holes are driven near the outer
edge of the tunnel and parallel to its axis. When fired, the wedge of
rock between the center cut holes is thrown back into the tunnel and a
delayed explosion then throws the sides into the hole thus made. A final
delay thrusting shot throws the muck so formed away from the face of the
tunnel. For tunnels up to 6 or 8 feet in height the entire bore is cut
out in this fashion. For larger tunnels, the upper portion called the
heading, is taken out in this way, and the remainder, called the bench,
is taken out by drilling and blowing holes normal to the axis of the
tunnel. The amount of powder necessary in the bench holes is much less
than that required in the heading.
=169. Ventilation.=—No tunnel more than 50 feet long should be built
without ventilation. A fair amount of air for ordinary conditions is 75
cubic feet of free air per minute per person in the tunnel, and double
this amount for each animal. Where explosive gases are met, or under
conditions where the tunnel is hot, five or six times as much air may be
needed in order to cool the tunnel or to dilute the gases. In order that
the air may be fresh and cool at the face of the tunnel where work is
going on it should be conducted to the tunnel face in a pipe and blown
out into the tunnel. Immediately following a blast at the face the
current should be reversed so as to draw the poisonous gases out of the
tunnel through the duct. The high pressure air line leading to the
drills should be opened at the same time to create a current towards the
face in order to accelerate the clearing of the air at the heading. The
capacity of the air machines should be sufficient to exhaust four times
the volume of the gases created by the explosion, in 15 minutes. This
will ordinarily call for a capacity of about 4,000 cubic feet of free
air per minute. If the same blower is to be used for exhausting the
gases as for ventilation while work is going on, it should have a high
overload capacity to care for this situation. The air line should be
arranged to allow for reversal of flow.
The diameter of the air pipe should be determined by a study of the
saving of the cost and operation of the air equipment compared to the
increased cost of a larger pipe line. Other factors affecting the size
of the pipe line to be used are: the available space in the tunnel, the
temporary character of the installation, the use of the exhaust from
high-pressure air machines for the purpose of ventilation, etc.
Cast-iron, spiral-riveted galvanized sheet iron, and canvas pipes have
been used for conducting low-pressure ventilating air.
Ventilation in tunnels working under air pressure is supplied from the
compressors, and the air is delivered near the face of the heading,
except that being used in the locks. In tunnels using air drills, the
air for the drills is conducted through a separate pipe as it is not
economical to compress the ventilating air to the pressure necessary to
operate the drills.
=170. Compressed Air.=—Compressed air is used in tunnel work to prevent
the entrance of water into the tunnel and to keep the work dry. The
pressure of air used is closely that of the pressure of the ground water
but in a large tunnel or a tunnel with a weak roof the pressure may be
somewhat lower on account of the danger of blowing through the roof. It
is evident that the water pressure cannot be balanced at the top and the
bottom of the tunnel. To balance it at the bottom makes a blow out near
the top more probable. To balance the pressure at the top may leave the
bottom wet. Judgment and care must be exercised during construction and
if the pressure is balanced at or near the bottom the roof must be
carefully guarded by grouting and puddling with clay, or the surface,
particularly if under water, may be covered with a clay bank. If the
cavities in the tunnel lining are large, sawdust can be mixed with the
grout to advantage, the mixture being pumped through holes in the roof
by hand or power operated force pumps. “Blows” must be carefully guarded
against as they endanger the lives of the workmen and threaten the loss
of the tunnel. The pressure and volume of air supplied for some large
subaqueous tunnels is shown in Table 61.
Labor under compressed air is arduous and dangerous with the best of
safeguards.[96] Pressure more than about 43 pounds per square inch
cannot be used and at this high pressure men cannot work more than four
hours at a time. Little or no distress is noted at pressures less than
15 pounds.
TABLE 61
VOLUME AND PRESSURE OF COMPRESSED AIR IN TUNNELS
(American Civil Engineers Pocket Book)
──────────┬────────┬───────┬─────────┬─────────┬───────────────────────
Tunnel │ │ │ Maximum │ Average │ Conditions and Cubic
│Maximum │ │ Air │ Air │ Feet of Free Air per
│Distance│ │Pressure,│Pressure,│ Minute
│ High │ │ Pounds │ Pounds │
│Water to│Minimum│ per │ per │
│Invert, │ Cover │ Square │ Square │
│ Feet │in Feet│ Inch │ Inch │
──────────┼────────┼───────┼─────────┼─────────┼───────────────────────
City and │ │ │ │ │In water bearing-sand.
South │ │ │ │ │ 1660 cubic feet per
London │ │ │ │ │ minute per face. When
│ │ │ │ │ grouted 1000 to 1300
│ │ │ │ │ cubic feet per minute
│ 34│ 42│ 15│ │ per face
──────────┼────────┼───────┼─────────┼─────────┼───────────────────────
Blackwall │ │ │ │ │10,000 cubic feet per
│ │ │ │ │ minute per face in
│ │ │ │ │ open ballast for some
│ 80│ 5│ 37│ 35│ time
──────────┼────────┼───────┼─────────┼─────────┼───────────────────────
Baker St. │ │ │ │ │In gravel, 3300 cubic
and │ │ │ │ │ feet of air per
Waterloo│ │ │ │ │ minute per face.
│ │ │ │ │ Parallel tunnel 1650
│ │ │ │ │ cubic feet per min.
│ 70│ 18│ 35│ 28│ per face
──────────┼────────┼───────┼─────────┼─────────┼───────────────────────
Greenwich │ │ │ │ │Average 83.5 per man
│ │ │ │ │ per minute. Never
│ 70│ 30│ 28│ 20│ less than 66.7
──────────┼────────┼───────┼─────────┼─────────┼───────────────────────
Battery, │ │ │ │ │In sand. Two working
East │ │ │ │ │ faces. Maximum 32,000
River. │ │ │ │ │
N. Y. │ 94│ 12│ 42│ 26│
──────────┼────────┼───────┼─────────┼─────────┼───────────────────────
East │ │ │ │ │Maximum for one face
River, │ │ │ │ │ 25,000 cubic feet per
N. Y., │ │ │ │ │ minute for 24 hours.
Penn. │ │ │ │ │ Capacity of plant for
R.R. │ │ │ │ │ 8 faces, 80,400 cubic
│ 93│ 8│ 42│ 27│ feet per minute
──────────┼────────┼───────┼─────────┼─────────┼───────────────────────
North │ │ │ │ │Maximum in gravel
River, │ │ │ │ │ 10,000 cubic feet per
N. Y., │ │ │ │ │ man per hour.
Penn. │ │ │ │ │ Generally ranged
R.R. │ 98│ 20│ 37│ 26│ between 1500 and 5000
──────────┴────────┴───────┴─────────┴─────────┴───────────────────────
Entrance and exit to the tunnel are gained through air locks. These are
sheet iron cylinders concreted into the lining of the tunnel or shaft.
Air-tight iron doors are provided at both ends, which open inwards
towards the tunnel. On entering the lock from the outside the door to
the tunnel is found tightly closed. The outside door is then closed by
hand, the air valve is opened and air is admitted to the lock until the
pressure on the lock side of the tunnel door equalizes that on the
tunnel side and the tunnel door is swung open by hand. When the lock is
open to the tunnel the pressure in the tunnel keeps the outside door
closed. In order to leave the tunnel the process is reversed. Materials
are passed through the lock by the lock tender or tenders who pass
through the lock with the material if the pressure is low, or who
manipulate the air outside of the lock if the pressure is high. If
pressures of 30 to 40 pounds are being used, two or even three locks may
be necessary.
EXPLOSIVES AND BLASTING[97]
=171. Requirements.=—The desirable features in an explosive to be used
in trenching and tunneling in rock are: (1) stability in make up so as
not to deteriorate in strength or to become dangerous during storage,
(2) imperviousness to ordinary variations in temperature and moisture,
(3) insensibility to ordinary shocks received in transportation and
handling, (4) not too difficult of detonation, (5) convenient form for
transportation and loading and for making up charges of different
weights, (6) the non-formation of poisonous gases when fired, (7)
imperviousness to water and usefulness in wet holes, (8) power without
bulk, etc.
=172. Types of Explosives.=—Explosives which fill some or all these
requirements can be divided into two classes, deflagrating and
detonating. A deflagration is an explosion transmitted progressively
from grain to grain. A detonation is a sudden disruption caused by
synchronous vibrations of a wave-like character. The deflagrating
explosives are represented by gun-powders and contractors’ powders. They
must be carefully tamped in the hole to develop their full power and
they must be ignited by a fuse or flame. They are valueless in water or
moist holes. These powders are used mainly for loosening frozen earth,
soft sandstone, cemented gravels and similar materials where a thrusting
action rather than a disruption is desired. The detonating explosives
are most commonly represented by the dynamites. These are exploded by a
shock usually caused by another explosive which has been ignited by a
fuse or electric spark, and which is known as the “detonator.”
Detonating explosives are more powerful than deflagrating explosives and
are used in all but the softest materials.
_Gunpowder._—This is a mechanical mixture of sulphur, charcoal, and
saltpeter generally in the proportions of 10 parts sulphur, 15 parts
charcoal, and 75 parts saltpeter (sodium nitrate). It weighs about 62½
pounds per cubic foot and produces about 280 times its own volume in gas
at a pressure of 4.68 tons per square inch at a temperature of 32
degrees F., which amounts to a pressure of approximately 38 tons per
square inch at the temperature of explosion of 4,000 degrees F.
_Blasting Powder._—This is a mixture of 19 parts sulphur, 15 parts
charcoal, and 66 parts saltpeter. These powders are made in different
size angular polished grains, from the size of a pin head to sizes just
passing a ⅜ to ½ inch hole. The larger the grains the slower the action
of the powder.
_Nitro-Substitution Compounds._—These compounds are formed by the action
of nitric acid on hydrocarbons. Triton, T.N.T., or trinitrotoluene, made
famous during the war, is an example of these compounds. It is made by
the successive nitration of toluene, a coal tar derivative. It melts at
80 degrees C., is very stable, and is of great explosive strength. It is
manufactured in a convenient form, being compressed into blocks about 2
inches square by about 4 inches long with a specific gravity of about
1.5. The blocks are usually copper plated to protect the T.N.T. from
moisture. The more dense it is the less its sensitiveness. It is also
put up in crystalline form in cartridges like dynamite, in which
condition it is practically equal to 40 per cent dynamite. It can be cut
with a knife, pounded with a hammer, and will burn freely and slowly in
small quantities in the open air without exploding. It is suitable for
all but the hardest rocks. It creates poisonous gases on detonation
which are quickly dissipated in the open air but which render it
unsuitable for use in tunnel work.
_Nitro-glycerine._—This is formed by the action of nitric and sulphuric
acids on animal compounds such as gelatine or glycerine. Nitro-glycerine
is a yellowish, oily, highly unstable explosive liquid with a specific
gravity of about 1.6. It will burn quietly when ignited in the open air.
It will freeze at 41 degrees F., and will explode at 388 degrees F., or
on concussion at a lower temperature. It develops about 1,500 times its
volume in gas, which due to the heat of combustion is increased to about
10,000 times its volume. It is a very dangerous explosive to handle, and
is unsuitable for use in the liquid form.
_Blasting Gelatine._—This is made by soaking guncotton in
nitro-glycerine. Gelatine dynamite is a combination of blasting gelatine
and an absorbent. Forcite is a gelatine dynamite in which the blasting
gelatine, forming 50 per cent of the compound, contains 90 per cent
nitro-glycerine and 2 per cent guncotton; and the absorbent, forming the
other 50 per cent of the compound, contains 76 per cent of sodium
nitrate, 3 per cent sulphur, 20 per cent of wood tar, and 1 per cent of
wood pulp.
Blasting gelatine is packed in a jelly-like mass in metal lined wooden
boxes. It is less sensitive than straight dynamite and is one of the
most powerful explosives known. It can be made up to equal 100 per cent
dynamite. It is suitable for use in the hardest rocks and for subaqueous
work as it is not affected by moisture. It is suitable for use in
tunnels as the amount of carbon monoxide, peroxide of nitrogen, hydrogen
sulphide and other dangerous gases is comparatively low when fully
detonated. Gelatine dynamite[98] is sold as 30 per cent to 70 per cent
dynamite, the actual percentage of nitro-glycerine being less than the
nominal quantity given.
_Dynamite._—The dynamites are made by soaking nitro-glycerine in some
absorbent. If the absorbent is some neutral substance such as infusorial
earth the combination is known as a true dynamite. The false or active
dynamites are those in which the absorbent is also an explosive
compound. The false dynamites form the best known contractors’
explosives. Among the materials mixed with the nitro-glycerine are:
magnesium carbonate, sulphur, wood meal, wood pulp, wood fiber, wood
tar, nut galls, kieselguhr, sawdust, resin, pitch, sugar, charcoal, and
guncotton. The strength of dynamites is noted by the per cent of
nitro-glycerine and nitro substitutes contained. Dualin and Hercules
powder both contain 40 per cent nitro-glycerine. Dualin contains 30 per
cent sawdust and 30 per cent potassium nitrate, but the Hercules powder,
which is stronger, contains 16 per cent sugar, 3 per cent potassium
chlorate, 31 per cent potassium nitrate, and 10 per cent magnesium
carbonate.
Dynamite is the most common explosive used on construction work. It is
supplied in cylindrical sticks wrapped in paper, the diameter of the
sticks varying between ⅞ and 2 inches. They are about 8 inches long.
Forty per cent dynamite is the common strength found on the market. It
is suitable for ordinary work in all but very hard rocks or very soft
material. Direct contact with water separates the nitro-glycerine from
the base and is dangerous when the explosive is used in wet places
unless it is fired immediately after the hole is loaded. It freezes at
about 42 degrees F., or at even higher temperatures and in the frozen
state it is highly dangerous, requiring powerful detonators for firing,
but exploding spontaneously from a slight jar, or the breaking of the
stick. Special low-freezing dynamites are made that will not freeze
above 35 degrees F.
_Ammonia Compounds._—Ammonia dynamite is a combination of
nitro-glycerine, ammonium nitrate and such other ingredients as sodium
nitrate, calcium carbonate and combustible material. This form of
explosive is advantageous for underground work because, like gelatine
dynamite, its explosion does not create large quantities of poisonous
gases. It has a low freezing point and is relatively low in cost. It is
seriously affected by moisture, however, and can not be used in wet
places. Ammonium nitrate explosives which do not contain nitro-glycerine
include 70 per cent to 95 per cent ammonium nitrate and some combustible
material. Ammonal is a special type of this class formed by a mixture of
ammonium nitrate, aluminum, and triton. All of these explosives are
deliquescent, insensitive to shock, and are cheaper than the dynamites.
=173. Permissible Explosives.=—As specified by the United States Bureau
of Mines explosives whose rapidity, detonation, and temperature of
explosion will not ignite explosive mixtures of pit gases and air are
known as permissible explosives. They include nitrate explosives,
ammonia dynamite, and others.
Gunpowder, triton, picric acid, blasting gelatine, dynamite, guncotton,
etc., are not classed as permissible explosives.
=174. Strength.=—The relative weights for equal strength of various
explosives are given in Table 62.
TABLE 62
RELATIVE WEIGHTS OF EXPLOSIVES WITH THE SAME STRENGTH AS A UNIT WEIGHT
OF 40 PER CENT DYNAMITE
───────────────────────────────────────────────────────┬───────────────
Explosive │Relative Weight
───────────────────────────────────────────────────────┼───────────────
Picric acid │ 0.86
Gun powder (well tamped) │ 3.10
Straight dynamite, 15% │ 1.45
Straight dynamite, 20 │ 1.33
Straight dynamite, 25 │ 1.28
Straight dynamite, 30 │ 1.18
Straight dynamite, 35 │ 1.07
Straight dynamite, 40 │ 1.00
Straight dynamite, 45 │ 0.93
Straight dynamite, 50 │ 0.86
Straight dynamite, 55 │ 0.83
Straight dynamite, 60 │ 0.78
│
Low-freezing dynamites are the same as straight │
dynamites │
Smokeless powder, well tamped │ 0.74
│
Triton │ 0.86
Blasting gelatine │ 0.43
Gelatine dynamite, 30% │ 1.28
Gelatine dynamite, 35 │ 1.21
Gelatine dynamite, 40 │ 1.14
Gelatine dynamite, 50 │ 1.04
Gelatine dynamite, 55 │ 0.97
Gelatine dynamite, 60 │ 0.90
Gelatine dynamite, 70 │ 0.83
│
Ammonia dynamites are the same as gelatine dynamites. │
Chlorates (sprengle) Rack-a-rock │ 1.33
Guncotton │ 0.72
───────────────────────────────────────────────────────┴───────────────
=175. Fuses and Detonators.=—The explosion of gunpowder and other
deflagrating explosives is caused by the direct application of a flame
led to the charge by a powder fuse, or they may be fired by a blasting
cap which is itself exploded by the heat from a fuse or an electric
spark. The powder fuse is a cord made up of a train of powder securely
wrapped in a number of thicknesses of woven cotton or linen threads and
usually made waterproof. Ordinary fuse burns at about 2 feet per minute
but there may be wide variations from this rate due to the quality of
the fuse, moisture, temperature, or pressure. Moisture tends to retard
the rate, pressure to increase it. Instantaneous fuse will burn at about
120 feet per second. It is distinguished from the ordinary safety fuse
both by eye and touch due to the rough red braid with which it is
covered. It is used in firing a number of charges simultaneously. Powder
fuses are lighted by the application of a flame or smoldering torch to
the freshly cut or opened end exposing the powder grains. Cordeau
Bickford is lead tubing filled with triton, in which the flame travels
at about 17,000 feet per second. This is also used for igniting charges
simultaneously.
The detonation of an explosive is caused by the shock or heat of the
explosion of a more sensitive substance which has been exploded by a
powder fuse or electric spark. The common method of detonating explosive
charges is by the firing of a blasting cap. These caps are copper
cylinders, closed at one end, about 1½ inches long and ¼ to ⅜ of an inch
in diameter, or larger. They contain a mixture of about 85 per cent
fulminate of mercury and 15 per cent potassium chlorate held in place by
a wad of shellac, collodion, or paper. The strength of detonators is
based on the weight of fulminate of mercury and is designated as shown
in Table 63.
TABLE 63
STRENGTH OF BLASTING CAPS
────────────────────────────────────────┬──────────────────────────────
Blasting Cap, Commercial Grade │ Grains Fulminate of Mercury
────────────────────────────────────────┼──────────────────────────────
3X or Triple │ 8.3
4X or Quadruple │ 10.0
5X or Quintuple │ 12.3
6X or Sextuple │ 15.4
7X or Number 20 │ 23.1
8X or Number 30 │ 30.9
Single strength │ 12.3
Double strength │ 15.4
Triple strength │ 23.1
Quadruple strength │ 30.9
────────────────────────────────────────┴──────────────────────────────
The force of the explosion is markedly affected by the strength of the
caps, the effect being greater for low-grade powders. For 40 per cent
dynamite the explosion caused by a 5X cap is 15 per cent stronger than
that caused by a 3X cap. For 60 per cent dynamite the difference is only
6 per cent. The deterioration of the caps will reduce the strength of an
explosion noticeably. With straight dynamite, 3X caps are generally
used, but with gelatine dynamite 6X or heavier caps must be used. Caps
may be tested by exploding them in a confined space and noting the
report and the effect on the shell. A full strength cap will tear the
shell into minute pieces, while a deteriorated cap will merely tear it
into three or four large pieces. An ordinary blasting cap is shown in
Fig. 120 together with other equipment for blasting.
Firing by electricity is generally safer and more satisfactory than by
the use of ordinary caps and powder fuses. The explosion is more certain
and its exact time is under the control of the operator. Fig. 121 shows
a section through an electric blasting cap or detonator, commonly called
an electric fuse. Delayed action electric detonators are made by
inserting a slow-burning substance between the platinum bridge and the
detonating substance. The time of delay is controlled by the depth of
the slow-burning substance. Delayed action detonators are useful in
tunnel work where it is desired to explode the charge in three or four
stages in order that the debris from one charge may be out of the way of
the following, and that the forces of the explosions may not serve to
nullify each other.
[Illustration:
FIG. 120.—Blasting Supplies.
Courtesy, Aetna Powder Co.
]
=176. Care in Handling.=—Some of the don’ts in the handling of
explosives recommended by the U. S. Army Engineer Field Manual are: in
the use of nitro-glycerine explosives of all kinds—
(_a_) Don’t store detonators with explosives. Detonators should be
kept by themselves.
(_b_) Don’t open packages of explosives in a store house.
(_c_) Don’t open packages of explosives with a nail puller, pick
or chisel. Packages should be opened with a hard wood wedge and
mallet, outside of the magazine and at some distance from it.
(_d_) Don’t store explosives in a hot or damp place. All
explosives spoil rapidly if so stored.
(_e_) Don’t store explosives containing nitro-glycerine so that
the cartridges stand on end. The nitro-glycerine is more likely to
leak from the cartridges when they stand on end than it is when
they lie on their sides.
(_f_) Don’t use explosives that are frozen or partly frozen. The
charge may not explode completely and serious accidents may
result. If the explosion is not complete the full strength of the
charge is not exerted and larger quantities of harmful gases are
given off.
[Illustration:
FIG. 121.—Electric Fuse.
Full size.
]
(_g_) Don’t thaw frozen explosives in front of an open fire, nor
in a stove, nor over a lamp, nor near a boiler, nor near steam
pipes, nor by placing cartridges in hot water. Use a commercial or
improvised thawer.
(_h_) Don’t put hot water or steam pipes in a magazine for thawing
purposes.
(_i_) Don’t carry detonators and explosives in the same package.
Detonators are extremely sensitive to heat, friction, or blows of
any kind.
(_j_) Don’t handle detonators or explosives near an open flame.
(_k_) Don’t expose detonators or explosives to direct sunlight for
any length of time. Such exposure may increase the danger in their
use.
(_l_) Don’t open a package of explosives until ready to use the
explosive, then use it promptly.
(_m_) Don’t handle explosives carelessly. They are all sensitive
to blows, friction, and fire.
(_n_) Don’t crimp a detonator (blasting cap) around a fuse with
the teeth. Use a cap crimper, which is supplied for this purpose.
(_o_) Don’t economize by using a short length of fuse.
(_p_) Don’t return to a charge for at least one-half hour after a
miss fire. Hang fires are likely to happen.
(_q_) Don’t attempt to draw nor to dig out the charge in case of a
miss fire.
Some of the positive rules in connection with the handling of explosives
are: build the magazine on an earth foundation remote from any other
structures, protect it with earth embankments that will direct the force
of the explosion upwards, and build it of materials that will supply as
few missiles as possible. Hollow tile brick, double-walled galvanized
iron filled with sand, and similar constructions are satisfactory. The
magazine may be heated by steam or hot-water pipes so located that
explosives cannot come in contact with them, or by a cluster of
incandescent bulbs, but if the explosives become frozen they must not be
thawed out by turning on the steam or hot water. If powder or
nitro-glycerine is dropped on the floor the magazine should be emptied,
washed out with a hose and spots of nitro-glycerine scrubbed with a
brush and a mixture of ½ gallon of wood alcohol, ½ gallon of water and 2
pounds of sodium sulphide. Frozen explosives may be thawed by spreading
out on special shelves in a warm thaw house—not in the magazine proper,
by burying in a manure pile so that the explosive may not become
moistened, or more commonly by heating slowly in a water bath. This is a
dry kettle in which the explosives are placed and covered. The kettle is
then put in another containing water which is heated gently to about 120
degrees F. It should not be boiled.
In case of a miss fire, instead of digging out the old charge put a new
charge on top of the old and fire the two simultaneously.
=177. Priming, Loading, and Firing.=—Priming is the act of placing the
cap or detonator in the cartridge of explosive. The primer is either the
cap or the cap and cartridge which are to be detonated by the fuse. If a
cap and safety fuse are to be used the paper at the upper end of the
cartridge is opened, a hole is poked in the explosive with the finger or
a piece of wood, the cap and the attached fuse are pushed into the hole
and gently embedded in the explosive so that the end of the cap is
exposed sufficiently to prevent the fuse from igniting the dynamite
directly. The paper is then folded up and tied firmly around the fuse
with a piece of string. The result is shown in Fig. 122.
[Illustration:
FIG. 122.—Dynamite Cartridge, Safety Fuse, and Cap.
]
In placing the fuse in the cap the end of the fuse is cut off square,
and inserted in the open end of the cap, care being taken not to spill
the loose grains of powder or to grind the fuse down on top of the cap.
When the fuse is shoved firmly into place the upper portion of the
copper cap is pressed or crimped with the cap crimpers shown in Fig.
120.
The number of primers to be used is dependent on the size and location
of the charge, but in practically all sewer work only one primer is used
to each hole. In bulky charges the primer should be placed near the
center of the charge and the fuse so protected that it will not ignite
the charge prematurely. In drill holes the primer is put in last with
the cap end down.
In loading a hole, it is first pumped and cleaned out. This can be done
satisfactorily with the end of a stick frayed out into a broom.
Cartridges which very nearly fill the hole are dropped in one at a time
and are pressed firmly together, with a light wooden tamping bar. They
should not be pounded. After the primer is placed, a wad of clay or
similar material is pressed gently into the hole against it and the hole
is then filled with well-tamped clay. In tunnel work tamping is not so
essential as an overcharge of powder is usually used and the time of
tamping, which is worth more than two or three sticks of dynamite, is
saved. In handling bulk explosives, such as gunpowder, they are poured
into the hole, the fuse is set in the upper portion and the remainder of
the hole is tamped with clay as for dynamite cartridges.
[Illustration:
FIG. 123.—Methods for Cutting Safety Fuse for Splicing.
]
If a large number of charges are to be fired simultaneously with a
safety fuse, the length of the fuse to each charge should be made equal
or a safety fuse used to a common center and approximately equal lengths
of instantaneous fuse or Cordeau Bickford used from there to the charge.
In splicing the fuses for such connections they are cut diagonally as
shown in Fig. 123 and bound together firmly with tape. Electric
connections are particularly advantageous under such conditions as they
avoid the dangers incidental to spliced fuses and are less expensive. In
tunnel work simultaneous electric detonation is not desirable as the
holes should be fired progressively: 1st, the cuts; 2nd, the relievers;
3rd, the backs; 4th, the sides; and 5th, the lifters. Different lengths
of safety fuse, or delayed action electric fuses can be used for these
delay shots.
In igniting a safety fuse an open flame such as that furnished by a
match or candle is the most satisfactory. For electric fuses the current
is generated by a magneto shown in Fig. 120. Pressing vigorously down on
the handle closes the circuit and generates an electric current which
heats the platinum bridges and explodes the charges. For the small
number of charges used in ordinary construction they are connected in
series so that if there is a broken connection anywhere no charge will
be exploded. If many charges are to be fired and a line circuit is to be
used, the final connection should not be made until just before the
charge is to be fired in order to obviate the danger of stray currents
firing the charge prematurely. Care should be taken to see that all
connections are good and that there are no broken wires on the line.
=178. Quantity of Explosive.=—The quantity of explosive to be used can
be determined satisfactorily only by experience on the job in question,
as the factors affecting the necessary quantity are so diverse. The
figures in Table 64 indicate the relative amounts needed under different
conditions.
TABLE 64
QUANTITIES OF EXPLOSIVES
───────────┬──────┬────────┬─────────┬─────────────┬─────────┬──────────
Kind of │Drift │Feet[99]│Black[99]│Dynamite[99],│Grade of │
Rock │ in │of Hole │ Powder, │ Pounds │Dynamite,│ Remarks
│ Feet │ │ Pounds │ │Per Cent │
───────────┼──────┼────────┼─────────┼─────────────┼─────────┼──────────
Limestone, │ │ │ │ │ │
Chicago │ 12│ 0.40│ │ 0.75│ 40│Gillette
Drainage │ │ │ │ │ │
Canal │ │ │ │ │ │
Limestone │ │ │ │ │ │
for │ 6│ 1.00│ │ 0.70│ 40│Gillette
crushing │ │ │ │ │ │
Limestone │ │ │ │ │ │
for │ 20│ │ │ 0.37│ 50│Gillette
cement │ │ │ │ │ │
Limestone, │ │ │ │ │ │
holes │ 15│ 0.40│ │ 0.26│ 50│Gillette
sprung │ │ │ │ │ │
Sandstone, │ 20│ 0.10│ 1.0│ 0.10│ 40│Gillette
side cut │ │ │ │ │ │
Sandstone, │ │ │ │ │ │
thorough │ 20│ 0.20│ 2.0│ 0.20│ 40│Gillette
cut │ │ │ │ │ │
Shale, soft│ 24│ 0.08│ 0.7│ 0.03│ 40│Gillette.
side cut │ │ │ │ │ │ Open cut
Shale, hard│ │ │ │ │ │
thorough │ 24│ 0.20│ 1.5│ 0.10│ 40│Gillette
cut │ │ │ │ │ │
Granite for│ 16│ 1.36│ │ 0.20│ 60│Gillette
rubble │ │ │ │ │ │
Gneiss, New│ 12│ 1.33│ │ 0.60│ 40│Gillette
York City│ │ │ │ │ │
Gneiss, New│ 14│ 0.63│ │ 0.50│ 40│Gillette
York City│ │ │ │ │ │
Syenite, │ │ │ │ │ │
Treadwell│ 12│ 1.70│ │ 0.67│ 40│Gillette
Mine │ │ │ │ │ │
Magnetic │ 12½│ 0.32│ │ 0.44│ 52│Gillette
iron ore │ │ │ │ │ │
Trap, seamy│ 14│ 0.35│ │ 0.20│ 75│Gillette
Trap, │ 17│ 1.00│ │ 0.70│ 40│Gillette
massive │ │ │ │ │ │
│ │ │ │ │ │50%
Granite, │ │ │ │ │ │ dynamite
Grand │ 25│ 0.10│ │ 0.80│ 50│ used to
Trunk │ │ │ │ │ │ spring
│ │ │ │ │ │ holes
Clay, rock │ │ │ │ │ │
and │Tunnel│ │ │ 1.00│ │
Gypsum │ │ │ │ │ │
│ │ │ │ │Grade │
│ │ │ │ │ varied │
│ │ │ │ │ ⅗ at │
Hard shale │Tunnel│ │ │ 2.07│ 45%, ⅕ │
│ │ │ │ │ at 60%,│
│ │ │ │ │ some at│
│ │ │ │ │ 100% │
Hard rocky │Tunnel│ 1.60│ │ 3.57│ │
slate │ │ │ │ │ │
Hard rocky │Tunnel│ 1.46│ │ 3.57│ │
slate │ │ │ │ │ │
Mill Creek │ │ │ │ │ │Mun.
sewer, │Tunnel│ │ │ 4.00│ 60│ Eng’g.
St. Louis│ │ │ │ │ │ Vol. 52,
│ │ │ │ │ │ p. 14
───────────┴──────┴────────┴─────────┴─────────────┴─────────┴──────────
PIPE SEWERS
=179. The Trench Bottom.=—It is customary to dig the bottom of the
trench to conform to the shape of the lower 45 degrees to 90 degrees of
the sewer if the character of the material will allow such construction.
In soft material which will not hold its shape the sewer may be encased
in concrete or a concrete cradle may be prepared for the pipe. In rock
the trench is excavated to about 6 inches below grade and refilled with
well-tamped earth so as to form a cradle giving bearing to 60 to 90
degrees of the pipe circumference. For large sewers to be constructed in
the trench special foundations are sometimes built.
=180. Laying Pipe.=—Before the pipe is lowered into the trench the
sections which are to be adjacent should be fitted together on the
surface and the relative positions marked by chalk so that the same
position can be obtained in the trench.
Small pipes are lowered into the trench and swung into position on a
hook as shown in Fig. 124. Pipes up to 15 or 18 inches in diameter can
be handled by the pipe layer and helper in the trench without
assistance. Heavier pipes may be lowered into the trench by passing
ropes around each end of the pipe. One end of the rope is fastened at
the surface and the ropes are paid out by the men at the surface as the
pipe is lowered. If the pipes have been fitted together and marked at
the surface it is undesirable to use this method of lowering as the
position in which the pipes arrive in the bottom of the trench can not
be easily predicted. A cradle may be used for shoving the pipe into
position as is shown in Fig. 125.
[Illustration:
FIG. 124.—Hook for Lowering and Placing Sewer Pipe.
]
[Illustration:
FIG. 125.—Cradle for Placing Sewer Pipe.
]
Pipes above 24 to 27 inches in diameter are too large to be handled from
the side of the trench. A hook as shown in Fig. 124 is placed in the
pipe so that it will be in the proper position when lowered. It is
raised by a rope passing through a block at the peak of a stiff-legged
derrick which spans the trench, or by a crane. If a derrick is used the
rope passes to a windlass on the opposite side of the trench from the
pipe. Mechanical power may be used for raising pipes too heavy to be
raised by hand. The pipe is then lowered and swung into position while
supported from the derrick. Excessive swinging is prevented by holding
back on the guide rope as the pipe is raised and lowered.
Pipes are usually laid with the bell end up grade as it is easier to fit
the succeeding pipe into the bell so laid and to make the joint,
particularly on steep grades. The Baltimore specifications state:
The ends of the pipe shall abut against each other in such a
manner that there shall be no shoulder or unevenness of any kind
along the inside of the bottom half of the sewer or drain. Special
care should be taken that the pipe are well bedded on a solid
foundation.... The trenches where pipe laying is in progress shall
be kept dry, and no pipe shall be laid in water or upon a wet bed
unless especially allowed in writing by the Engineer. As the pipe
are laid throughout the work they must be thoroughly cleaned and
protected from dirt and water, no water being allowed to flow in
them in any case during the construction except such as may be
permitted in writing by the Engineer. No length of pipe shall be
laid until the preceding length has been thoroughly embedded and
secured in place, so as to prevent any movement or disturbance of
the finished joint.
The mouth of the pipe shall be provided with a board or stopper,
carefully fitted to the pipe, to prevent all earth and any other
substances from washing in.
=181. Joints.=—Pipes may be laid with open joints, mortar joints, cement
joints, or poured joints. Open joints are used for storm sewers in dry
ground close to the surface. Mortar and cement joints are commonly used
on all sewers except in special cases. Cement joints are more carefully
made than mortar joints and result in a greater percentage of
water-tight joints. Poured joints are used in wet trenches where it is
necessary to exclude ground water from the sewer.
A specification used in some cities for open joints is:
Pipes laid with open joints are to be laid with their inverts in
the same straight line and shall be firmly bedded throughout their
length on the bottom of the trench. No cement or mortar is to be
used in the joints. Not more than ⅛ inch shall be left between the
spigot end of the pipe and the shoulder of the hub of the pipe
into which it fits. The joints shall be surrounded with cheese
cloth, burlap, broken pipe, gravel or broken stone.
The purpose of the cheese cloth, etc., is to prevent fine earth from
sifting into the pipe until the cheese cloth or other material has
rotted away, by which time the earth has become arched over the opening.
Mortar joints are specified by Metcalf and Eddy as follows:
Before a pipe is laid the lower part of the bell of the preceding
pipe shall be plastered on the inside with stiff mortar of equal
parts of Portland cement and sand, of sufficient thickness to
bring the inner bottoms of the abutting pipe flush and even. After
the pipe is laid the remainder of the bell shall be thoroughly
filled with similar mortar and the joint wiped inside and finished
to a smooth bevel outside.
In some work a wood block or a stone is embedded in the mortar at the
bottom of the joint to bring the spigot in place concentric with the
next pipe.
Cement joints are specified in the Baltimore specifications as follows:
Cement joints shall be made with a narrow gasket of hemp or jute
and cement mortar, and special care shall be taken to secure tight
joints. The gasket shall be soaked in Portland cement grout and
then carefully inserted between the bell and the spigot, and well
calked with suitable hardwood or iron calking tools. It shall be
in one continuous piece for each joint, and of such thickness as
to bring the inverts of the two pipes smooth and even. The
remainder of the joint shall be filled with cement mortar all
around, on the bottom, top and sides, applied by hand with rubber
mittens, well pressed into the annular space and beveled off from
the outer edge of the bell to a distance of two inches therefrom,
or to an angle of 45 degrees. The inside of each joint shall be
thoroughly cleansed of all surplus mortar that may squeeze out in
making the joint; and to accomplish this some suitable scraper or
follower, or form shall be provided and always used immediately
after each joint is finished.
Cement joints so made, form the most satisfactory joint for ordinary
conditions and are the most frequently used. They are not always
water-tight and can be penetrated by roots. Some roots are able to
penetrate holes of almost microscopic size and to form growths in the
sewer or to split the joints.
Poured joints are made by pouring some jointing compound, while in a
fluid state, into the joint in which it hardens, thus sealing the joint.
Water-tightness in sewer lines to exclude ground water has also been
attempted by using the ordinary cement joint and surrounding the pipe
with a layer of cement or concrete. This has not always been successful
as it is difficult to obtain the proper class of workmanship in wet
sewer trenches.
The requisite qualities of a poured jointing material are:
(1) It should make a joint proof against the entrance of water and
roots.
(2) It should be inexpensive.
(3) It should have a long life.
(4) It should not deteriorate in sewage which may be either acid
or alkaline.
(5) It should adhere to the surface of the pipe.
(6) It should run at a temperature below about 400° F., as too
high temperatures will crack the pipe.
(7) It should neither melt nor soften at temperatures below 250°
F. in order to maintain the joint if hot liquids are poured into
the sewer.
(8) It should be elastic enough to permit slight movements of the
pipes.
(9) It should not require great skill in using as it must be
handled ordinarily by unskilled workers.
The materials used for poured joints are: cement grout; sulphur and
sand; and asphalt or some bituminous compound made of vulcanized linseed
oil, clay, and other substances the resulting mixture having the
appearance of vulcanized rubber or coal tar. The bituminous materials
most nearly approach the ideal conditions.
Cement grout is made up of pure cement and water mixed into a soupy
consistency. Its main advantages are its cheapness and ease in handling
in wet trenches or difficult situations. The result is no better than a
well made cement joint. There is no elasticity to the joint and a
movement of the pipe will break it.
Sulphur and sand are inexpensive, comparatively easy to handle, and make
an absolutely water-tight and rigid joint which is stronger than the
pipe itself. It frequently results in the cracking of the pipe and is
objected to by some engineers on that account. In making the mixture,
powdered sulphur and very fine sand are mixed in equal proportions. It
is essential that the sand be fine so that it will mix well with the
sulphur and not precipitate out when the sulphur is melted. Ninety per
cent of the sand should pass a No. 100 sieve and 50 per cent should pass
a No. 200 sieve. The mixture melts at about 260° F. and does not soften
at lower temperatures. For making a joint in an 8 inch pipe about 1½
pounds of sulphur, 1½ pounds of sand, ½ pound of jute, and 0.4 pound of
pitch are used. The pitch is used to paint the surface of the joint
while still hot in order to close up any possible cracks.
Among the better known of the bituminous joint compounds are: “G.K.”
Compound made by the Atlas Company, Mertztown, Pa., Jointite and
Filtite, manufactured by the Pacific Flush Tank Co., Chicago and New
York, and some of the products of the Warren Brothers Co., Boston. These
compounds fill nearly all of the ideal conditions except as to cost and
ease in handling. They are somewhat expensive and if overheated or
heated too long become carbonized and brittle. In cold weather they do
not stick to the pipe well unless the pipe is heated before the joint is
poured. On some work joints have been poured under water with these
compounds, but success is doubtful without skillful handling. An
overheated compound will make steam in the joint causing explosions
which will blow the joint clean, and an underheated compound will harden
before the joint is completed.
The materials should be heated in an iron kettle over a gasoline furnace
or other controllable fire, until they just commence to bubble and are
of the consistency of a thin sirup. Only a sufficient quantity of
material for immediate use should be prepared and it should be used
within 10 to 15 minutes after it has become properly heated. The ladle
used should be large enough to pour the entire joint without refilling.
There are other important points to be considered in pouring joints
which can be learned best by experience.
The quantity of material necessary for making these joints, as announced
by the manufacturers, is shown in Table 65.
TABLE 65
QUANTITY OF COMPOUND NEEDED FOR POURED JOINTS
───────────┬───────────────────────────────────────────────────────────
Diameter of│
Pipe, in │ Quantity of Material in Pounds per Joint
Inches │
───────────┼─────────────────────────────┬─────────────────────────────
│ Standard Socket │ Deep and Wide Socket
───────────┼─────────┬─────────┬─────────┼─────────┬─────────┬─────────
│Jointite │ Filtite │ G. K. │Jointite │ Filtite │ G. K.
───────────┼─────────┼─────────┼─────────┼─────────┼─────────┼─────────
6│ 0.82│ 0.72│ 0.42│ 1.46│ 1.28│ 0.72
8│ 1.06│ 0.95│ 0.73│ 1.82│ 1.60│ 1.25
10│ 1.30│ 1.15│ 0.89│ 2.26│ 1.98│ 1.52
12│ 2.08│ 1.82│ 1.42│ 2.65│ 2.32│ 1.80
15│ 2.52│ 2.20│ 1.74│ 3.20│ 2.80│ 2.20
18│ 3.02│ 2.64│ 2.58│ 3.75│ 3.29│ 3.25
20│ 3.44│ 3.00│ 2.86│ 4.30│ 3.78│ 3 60
22│ 3.62│ 3.16│ 3.13│ 4.62│ 4.07│ 3.97
24│ 4.03│ 3.50│ 3.41│ 4.91│ 4.31│ 4.27
───────────┴─────────┴─────────┴─────────┴─────────┴─────────┴─────────
In making a poured joint the pipes are first lined up in position. A
hemp or oakum gasket is forced into the joint to fill a space of about ¾
of an inch. An asbestos or other non-combustible gasket such as a rubber
hose smeared with clay is forced about ½ inch into the opening between
the bell and the spigot and the compound is poured down one side of the
pipe through a hole broken in the bell, until it appears on the other
side, and the hole is filled. Occasionally the non-combustible gasket is
wrapped tightly around the spigot of the pipe and pressed or tied firmly
to the bell. In pouring cement grout joints a paper gasket is used which
is held to the bell and spigot by draw strings. Greater speed in
construction and economy in the use of materials are obtained by joining
two or three lengths of pipe on the bank and lowering them into the
trench as a unit. The pipes are set in a vertical position on the bank
with the bell end up, one length resting in the other. The joint is
calked with hemp and poured without the use of the gasket. The joint
should always be poured immediately after being calked so that the hemp
can not become water soaked. The asbestos gasket should be removed as
soon as possible after the joint is poured in order to prevent sticking
with resultant danger of breaking of the joint when attempting to pull
the gasket free.
One man can pour about 33 eight-inch joints, and two men can complete
about 26 twelve-inch joints per hour on the bank where conditions are
more or less fixed.
=182. Labor and Progress.=—The labor required for the laying of pipe
sewers, exclusive of excavation, bracing and backfilling, consists of
pipe layers and helpers. For pipes 24 to 27 inches in diameter or
smaller one pipe layer and one or more helpers are necessary, dependent
on the size of the pipe and the depth of the trench. For larger pipes
two pipe layers can work economically each working on one-half of the
pipe and making half of the joint. The speed of pipe laying is
ordinarily limited by the speed of the excavation, but on a job in
Topeka, Kan.,[100] where the average day’s progress with a machine
excavator was 200 to 500 feet of trench per day, the pace was limited by
the speed of the pipe laying gang. This gang consisted of two pipe
layers in the trench and two helpers on the surface. The sizes of pipes
handled were from 8 to 27 inches.
BRICK AND BLOCK SEWERS
=183. The Invert.=—In good firm ground the excavation is cut to the
shape of the sewer and the bricks are laid directly on the ground, being
embedded in a thick layer of mortar. After the foundation has been
prepared and before the bricks are laid, two wooden templates, called
profiles, are prepared, similar to that shown in Fig. 126, to conform to
the shape of the inside and outside of the sewer. Each course of bricks
is represented by a row of nails in the profile and each nail
corresponds to a joint in the row. The two profiles are set true to line
and grade. A cord is stretched tightly between the two lowest nails on
opposite templates and a row of bricks is laid. The bricks are laid
radially and on edge with their long dimension parallel to the axis of
the sewer and with one edge just touching the string. As each one or two
or three rows are completed the guide line is moved up to the next
nails. When the bricks are laid on the ground all but large depressions
are filled in with tamped sand or mortar by the masons. Approximately
the same number of rows of bricks is kept completed on either side of
the center line. The succeeding courses follow within three to five rows
of each other, the only bond between courses being the mortar joint.
This is called row lock bond and with few exceptions has been used on
all brick sewers in the United States. As the sides of the sewer become
higher during the construction, platforms must be built for the masons.
These platforms are built of wood and rest directly on the green
brickwork. They should be designed to spread the load as much as
possible. The brickwork of the invert is continued up in this way to the
springing line. As soon as one section is completed one profile is moved
10 to 20 feet ahead along the trench according to the standard length of
sections, and set in position. The line is then strung from it to nails
driven or pushed into the cement joints of the last completed section.
Between work done on separate days the bricks are racked back in courses
to provide a satisfactory bond.
[Illustration:
FIG. 126.—Profile for Brick Sewers.
]
In ground too soft to support the brickwork directly a cradle is
prepared by placing profiles in position in the sewer and nailing 2–inch
planks to these profiles, first firmly tamping earth under the planks.
The bricks are laid in this cradle in a manner similar to that explained
for sewers with a firm foundation. In still softer ground it may be
necessary to construct a concrete cradle to support the bricks.
=184. The Arch.=—The arch centering consists of a wooden form made up of
wooden ribs as shown in Fig. 127. The center conforms to the shape of
the inside of the arch with allowance for the thickness of the lagging.
The lagging is nailed on the ribs in straight strips parallel to the
axis of the sewer. The center is supported on triangular struts resting
against the sides and on the bottom of the sewer and is lifted into
position by wedges driven between it and the support. The centers may be
placed immediately after the completion of the invert, or a day or two
may be allowed to pass to give the invert an opportunity to set. After
the centers are fixed in place the arch brick are carried up evenly on
each side and are pounded firmly into place. The center is usually, but
not always “struck” immediately, and the arch brick are cleaned and
pointed up from the inside. The outside is covered with a layer of ¼ to
¾ of an inch of cement mortar and may be backfilled to the top of the
arch in order to maintain the moisture of the mortar during setting and
to press the bricks of the arch together firmly. The centers are
sometimes made collapsible so that they can be carried or rolled through
the finished brickwork to the advanced position. In “striking” the
centers the wedges are removed and the wings folded in.
[Illustration:
FIG. 127.—Centering for Brick Sewer.
]
In tunneling, the invert of the sewer is constructed in the same fashion
as for open cut work. The arch centering is made in short sections and
the bricks are put in position by reaching in over the end of the
centering. All of the timbering of the tunnel is removed except the
poling boards or lagging against which the bricks or mortar are tightly
pressed, the boards being bricked in permanently.
=185. Block Sewers.=—Sewers made of unit blocks of concrete or vitrified
clay are constructed in a similar manner to brick sewers. Fig. 128 shows
the construction of a block sewer at Clinton, Iowa. In this sewer there
are two rings; an inside one of solid blocks and an outside one of
hollow blocks. Block sewers do not demand the skill in construction that
is demanded by brick sewers, as the blocks are so cast that the joints
are radial, whereas only experienced masons can lay bricks radially.
[Illustration:
FIG. 128.—Segmental Block Sewer at Clinton, Iowa.
]
=186. Organization.=—The number of men employed on a brick or block
sewer is proportioned according to the size of the sewer and the working
conditions. The number of men working on different tasks usually bears
the same ratio to the number of masons employed, regardless of the size
of the work. These proportions are shown for different jobs, in Table
66.
TABLE 66
ORGANIZATIONS FOR THE CONSTRUCTION OF BRICK AND BLOCK SEWERS
─────────────┬──────────────┬────────┬────────┬────────┬────────┬──────────
│ │ │ │ │ 84– to │
│General Ratio │15–foot,│66–inch │84–inch │108–inch│ 42–inch
Type of Work │ on Basis of │ 5–ring │Circular│Circular│ Sewer │Lock-Joint
│ Four Brick │ Brick, │ Brick, │ Brick, │Brick in│Tile Block
│ Layers │Chicago │ Gary │ Gary │Detroit │
│ │ │ │ │ Tunnel │
─────────────┼──────────────┼────────┼────────┼────────┼────────┼──────────
Foreman │ 1│ 1│ 1│ 1│ 1│ 1
Brick layers │ 4│ 12│ 6│ 6│ 5│ 2
Helpers │ 2│ 11│ 3│ 3│ │ 1
Scaffold men │ 2│ 21│ 3│ │ │
Brick tossers│ 2│ 7│ │ 15│ │ 2
Brick │ 2│ 2│ │ │ │ 2
carriers │ │ │ │ │ │
Cement mixers│ 2│ 6│ 6│ 5│ │ 1
Cement │ 2│ 10│ │ 8│ │
carriers │ │ │ │ │ │
Form setters │ 1│ │ 3│ 3│ │
Laborers │ 1│ 8│ 19│ 3│ 14│ 7
│ Municipal │
Source of │ Engineering, │ H. P. Gillette, Handbook of Cost Data
Information│ Vol. 54, p. │
│ 228 │
─────────────┴──────────────┴──────────────────────────────────────────────
=187. Rate of Progress.=—In a general way it can be assumed that the
laying of 1,000 bricks will require 3⅓ hours of the time of one mason,
10 man-hours for helpers and laborers, 2 barrels of cement, 0.6 cubic
yard of sand, and about 10 feet board measure of centering. One thousand
bricks will make about 2 cubic yards of brickwork. To the costs, as
estimated on the basis of materials and labor, must be added about 15
per cent for overhead and an additional amount for the contractor’s
profit. The number of bricks required in various size sewers is shown in
Table 67. A mason can lay more bricks per hour in a large sewer than in
a small one as there is a smaller percentage of face work, there is more
room to work, and it is easier to lay the bricks radially. The number of
bricks laid and the rate of progress on various jobs are shown in Table
68.
TABLE 67
BRICK MASONRY IN CIRCULAR SEWERS. CUBIC YARDS PER LINEAR FOOT
(From H. P. Gillette)
─────────────────┬─────────────────┬─────────────────┬─────────────────
Diameter, │ One Ring │ Two Ring │ Three ring
Feet and Inches │ (4½ Inches) │ (9 Inches) │ (13½ Inches)
─────────────────┼─────────────────┼─────────────────┼─────────────────
2 0│ 0.103│ 0.240│
2 6│ 0.125│ 0.280│
3 0│ 0.147│ 0.327│
3 6│ 0.169│ 0.371│
4 0│ 0.191│ 0.415│
4 6│ 0.213│ 0.458│
5 0│ 0.234│ 0.501│ 0.802
5 6│ 0.256│ 0.545│ 0.867
6 0│ 0.278│ 0.589│ 0.933
6 6│ │ 0.633│ 1.000
7 0│ │ 0.677│ 1.063
7 6│ │ 0.720│ 1.128
8 0│ │ 0.763│ 1.193
8 6│ │ 0.807│ 1.260
9 0│ │ 0.851│ 1.325
9 6│ │ 0.895│ 1.390
10 0│ │ 0.938│ 1.456
─────────────────┴─────────────────┴─────────────────┴─────────────────
CONCRETE SEWERS
=188. Construction in Open Cut.=—In the construction of sewer pipe of
cement and concrete one of two methods may be employed; 1st, to
manufacture the pipe in a plant at some distance from the place of final
use, or 2nd, to manufacture the pipe in place. The methods of the
manufacture of cement and concrete pipe which are to be transported to
the place of use are treated in Chapter VIII. The process of
constructing the pipes in place is ordinarily used for pipes 48 inches
or more in diameter. For smaller sizes, brick, vitrified clay, and
precast cement pipes are usually more economical.
The preparation of the foundation of a concrete sewer is similar to that
for a brick sewer. If the ground is suitable the trench is shaped to the
outside form of the sewer and the concrete poured directly on it. In
soft material which would give poor support to a sewer with a rounded
exterior, the bottom of the trench is cut horizontal and a concrete
cradle of poorer quality than that in the finished sewer is poured on
the soft ground, on a board platform, on piles, or on cribbing supported
on piles.
If the invert of the sewer is so flat that the concrete will stand
without an inside form the shape of the invert is obtained by a screed
or straight-edge which is passed over the surface of the concrete and
guided on two centers, or on one center and the face of the finished
work. The construction of a flat invert sewer at Baltimore is shown in
Fig. 1. The center for the concrete is shown in the foreground. When the
concrete for the next section is poured it will be smoothed to shape by
a screed or straight-edge resting on the face of the finished concrete
and the center. The center is shaped to conform to that of the finished
concrete. It is firmly staked in position and acts as a bulkhead for the
concrete as it is poured, as well as a guide for the screed.
TABLE 68
RATE OF PROGRESS ON BRICK SEWER CONSTRUCTION
(Based on 8–hour day)
────────┬────────┬──────┬──────┬──────┬────────
│ │ │ │Bricks│
Diameter│ │Number│Number│ per │ Number
of Sewer│ Shape │Rings,│Masons│Mason │Laborers
│ │Brick │ │ per │
│ │ │ │ Day │
────────┼────────┼──────┼──────┼──────┼────────
7′ 0″│Circular│ │ │ │
8′ 11″│ and │ 2½ │ 6│ 4710│ 39
│ Oval │ │ │ │
│ │ │ │ │
│ │ │ │ │
4′ 0″│Circular│ 2 │ 3│ 2500│
│ │ │ │ │
│ │3 arch│ │ │
6′ 8″│Circular│ 1 │ 18│ │ 62
│ │invert│ │ │
│ │ │ │ │
│ │1 arch│ │ │
2′ 9″│Egg │ 2 │ 2│ │ 3
│ │invert│ │ │
│ │ │ │ │
5′ 6″│Circular│ 2 │ 6│ 4570│ 35
6′ 6″│Circular│ │ 4│ 4800│
│ │ │ │ │
│ │ │ │ │
2′ 9″│Circular│ 2 │ 2│ 2080│ 5
│ │ │ │ │
16′ 0″│Circular│ 5 │ 8│ 5 cu.│
│ │ │ │ yd.│
16′ 0″│Circular│ 5 │ 12│ │ 70–75
3′ 6″│Egg │ │ │ 2300│
9′ 6″│Circular│ │ │ 3000│
│ │ │ │ │
│ │ │ │ │
3′ 6″│Circular│blocks│ 2│ │ 13
│ │ │ │ │
│ │ │ │ │
────────┴────────┴──────┴──────┴──────┴────────
────────┬────────┬────────────┬─────────┬───────────
│ │ │ │
Diameter│ Feet │ │ │
of Sewer│Progress│ Location │Authority│ Remarks
│per Day │ │ │
│ │ │ │
────────┼────────┼────────────┼─────────┼───────────
7′ 0″│ │ │ │
8′ 11″│ 60│Gary │Gillette │9–hour day
│ │ │ │
│ │ │ │
│ │ │Metcalf │General
4′ 0″│ 36│ │ and │ average
│ │ │ Eddy │
│ │ │ │Concrete
6′ 8″│ │Denver │Gillette │ invert
│ │ │ │
│ │ │Eng. │
│ │Springfield,│ Con., │
2′ 9″│ │ Mass. │ Jan. │
│ │ │ 16, │
│ │ │ 1907 │
5′ 6″│ 110│Gary │Gillette │
6′ 6″│ │ │Gillette │Exceptional
│ │ │ │ speed
│ │ │ │Tunnel
2′ 9″│ 13.9│Syracuse │Gillette │ 12–hour
│ │ │ │ day
16′ 0″│ 22│Chicago │Gillette │First year
│ │ │ │
16′ 0″│ 35│Chicago │Gillette │Second year
3′ 6″│ │St. Louis │Gillette │
9′ 6″│ 12.5│Chicago │H. R. │
│ │ │ Abbott │
│ │ │ │Lock joint
3′ 6″│ 30│ │ │ and tile.
│ │ │ │ 10–hour
│ │ │ │ day
────────┴────────┴────────────┴─────────┴───────────
If inside forms are to be used they are made as units in lengths of 12
or 16 feet for wooden forms, and 5 feet for steel forms. The inside form
is supported by precast concrete blocks placed under it and which are
concreted into the sewer. It is held in position by cleats nailed to the
outside form, to the sheeting, or wedged against the outside of the
trench. In some cases, particularly where steel forms are used, the
inside form is hung by chains from braces across the trench as is shown
in Fig. 129. The form is easily brought to proper grade by adjustment of
the turnbuckles and is then wedged into position to prevent movement
either sideways or upwards during the pouring of the concrete. It may be
necessary to weight the forms down to prevent flotation. Cross bracing
in the trench which interferes with the placing of the form is removed
and the braces are placed against the form until the concrete is poured.
They are removed immediately in advance of the rising concrete.
[Illustration:
FIG. 129.—Blaw Standard Half Round Sewer Form, Suspended from Overhead
Support.
Courtesy, Blaw Steel Form Co.
]
The sewer section may be built as a monolith, in two parts, or in three
parts. In casting the sewer as a monolith the complete full round inside
form is fixed in place by concrete blocks and wires. The full round
outside form is completed as far as possible without interfering too
much with the placing and tamping of the concrete. The concrete is
poured from the top, being kept at the same height on each side of the
form, and tamped while being poured. The remaining panels of the outside
form are placed in position as the concrete rises to them. An opening is
left at the top of the outside arch forms which is of such a width that
the concrete will stand without support. The casting of sewers as a
monolith is difficult and is usually undesirable because of the
uncertainty of the quality of the work. It has the advantage, however,
of eliminating longitudinal working joints in the sewers which may allow
the entrance of water or act as a line of weakness.
[Illustration: