DRAINAGE OF ROADS
=The Necessity for Drainage.=--The importance of drainage for all
roads subject to the effects of storm or underground water has always
been recognized by road builders, but during recent years constantly
increasing attention has been given to this phase of road
construction. It is unfortunate that there has in the past been some
tendency to consider elaborate drainage provisions less necessary
where rigid types of surfaces were employed. It has become apparent,
from the nature of the defects observed in all sorts of road surfaces,
that to neglect or minimize the importance of drainage in connection
with either earth roads or any class of surfaced roads is to invite
rapid deterioration of some sections of the roadway surface and to add
to maintenance costs.
The degree to which lack of drainage provisions affect the
serviceability of the road surface varies with the amount of
precipitation in the locality and the manner in which it is
distributed throughout the year. In the humid areas of the United
States, which are, roughly, those portions east of a north and south
line passing through Omaha and Kansas City, together with the northern
part of the Pacific slope, precipitation is generally in excess of 30
inches per year and fairly well distributed throughout the year, but
with seasonal variations in rate. In these areas, the effect of the
precipitation, both as regards its tendency to lower the stability of
soils and as an eroding agent, must be carefully provided against in
highway design.
Outside of the areas mentioned above, the precipitation is much less
than 30 inches per year and its effect as an agent of erosion is of
greatest significance, although in restricted areas there may be short
periods when the soil is made unstable by ground water.
=Importance of Design.=--The drainage system for a proposed road
improvement ought to be designed with as much care as any other
element, and, to do so, a study must be made of all factors that have
any bearing on the drainage requirements and the probable
effectiveness of the proposed drainage system. The well established
principles of land drainage should be followed so far as applicable.
The basic principle of road drainage is to minimize the effect of
water to such an extent that there will always be a layer of
comparatively dry soil of appreciable thickness under the traveled
part of the road. This layer should probably never be less than two
feet thick and for soils of a structure favorable to capillary action
it should be at least three feet thick. The means employed to
accomplish the requisite drainage will be as various as the conditions
encountered.
=Surface Drainage.=--The drainage method which is by far the most
nearly general in application is that which utilizes open ditches, and
the system which employs these ditches is usually referred to as
surface drainage. The full possibilities of this method of minimizing
the effects of storm water are rarely fully utilized in road
construction. Very frequently, deterioration of a road surface is
directly attributable to failure to provide adequately for the removal
of the storm water or water from the melting of snow that has fallen
on the road, or water that flows to the road from land adjacent
thereto. Surface water can usually most cheaply and expeditiously be
carried away in open ditches, although special conditions are
occasionally encountered which require supplementary tile drains. The
cross section commonly adopted for roads lends itself naturally to the
construction of drainage ditches at the sides of the traveled way, and
these are usually the principal dependence for the disposal of storm
water.
=Run-off.=--The capacity required of side ditches to insure
satisfactory surface drainage will be affected by the amount and
nature of the precipitation in the region where the road is built. The
annual rainfall in a region may amount to several feet, but may be
well distributed throughout the year with an absence of excessive
rainfall for short periods, that is, flood conditions may rarely
occur. In other areas, the annual rainfall may be comparatively small
but the precipitation occurs at a very high rate, that is, flood
conditions may be common, or it may be at a low rate extending over a
considerable period. These peculiarities must be known before an
adequate drainage system can be planned.
It is almost universally true in the United States that precipitation
at a very high rate will be for a relatively short duration, and
during these short periods, which usually do not exceed thirty
minutes, a portion of the water that falls on the areas adjacent to
the road and that drains to the road ditches will soak into the soil
and therefore not reach the ditches along the road. The extent to
which the water is taken up by the soil will vary with the porosity
and slope of the land and the character of the growth thereon.
Cultivated land will absorb nearly all of the water from showers up to
fifteen or twenty minutes duration; grass land a somewhat smaller
percentage; and hard baked or other impervious soil will absorb a
comparatively small amount. Rocky ground and steep slopes will absorb
very little storm water.
The surface of the road is designed to turn water rapidly to the
ditches, but when the material is the natural soil, there is always
considerable absorption of storm water. Surfaces such as sandclay,
gravel and macadam do not absorb to exceed 10 per cent of the
precipitation during short showers. Bituminous surfaces, brick and
concrete pavements, do not absorb an appreciable amount of storm
water.
Generally it is best to assume that if a rain lasts for forty-five
minutes or more, all of the water will run off, as the soil will reach
a state of saturation in that time. This is not true of deep sand, but
is for nearly all other soils.
The ditch capacity needed will therefore depend upon the area drained,
the character of the soil, the slopes and the rainfall characteristics
of the region, and upon the nature of the road surface.
For a required capacity, the cross section area of the ditch will vary
inversely as the grade, because the velocity of flow increases with an
increase in the grade of the ditch. If the surface water must be
carried along the road for distances exceeding five or six hundred
feet, the ditch must be constructed of increasing capacity toward the
outlet in order to accommodate the accumulated volume of water.
The velocity of flow varies not only with the grade, but with the
shape of the cross section, cleanness of the channel, the depth of the
water in the channel, alignment of the channel and the kind of
material in which the channel is formed. It is not necessary to go to
great refinement in the design of the side ditches for the ordinary
case where the water is carried along the road for only a few hundred
feet. The ditches are made of ample capacity by using the commonly
accepted cross section for a road, which will be discussed in a later
paragraph. But where large areas must be drained by the road ditches,
it is desirable carefully to design the side ditches. The basis for
that design is too lengthy to be included herein, and reference should
be made to a standard treatise on the subject.
=Ordinary Design of Ditches.=--For grades of one per cent or less on
roads in the humid area, the bottom of the ditch should be at least
three and one-half feet lower than the traveled surface of the road,
except for very sandy soil. For grades greater than one per cent, this
depth may be decreased one foot, and for grades of four per cent and
upward, the depth may be still less. These general rules for depth are
susceptible of variation but are believed to be the minimum except in
arid or semi-arid climates. It is far better to be too liberal in
ditch allowance than to be too conservative. In arid or semi-arid
regions, the ditch design will be based on the necessity of providing
for flood flow and preventing damage through erosion. Ordinary
drainage requirements will be satisfactory with the ditch about one
foot deep.
If the topography is such that it is evident considerable storm water
will flow from the adjacent land to the road ditches, the design must
be modified to take this into account. Sometimes such water can be
diverted by ditches well back from the road, and thus prevented from
flowing into the side ditches along the roadway. It is especially
desirable to divert water, which would otherwise flow down the slope
of a cut, by means of a ditch on the hill-side above the upper edge of
the slope of the cut.
Ditches are not effective unless they afford a free flow throughout
their length and have an outlet to a drainage channel of ample
capacity. Therefore, ditch grades should be established by survey,
especially if the gradient is less than one per cent, and the
construction work should be checked to insure that the ditch is
actually constructed as planned. A few high places in the ditch will
greatly reduce the effectiveness, although these may appear at the
time of construction to be slight. Constricted places, such as might
be due to a small amount of loose earth left in the ditch, are always
to be avoided.
Where the side ditch passes from a cut to the berm alongside a fill,
the ditch should be excavated throughout in the undisturbed natural
soil, five feet or more from the toe of the slope of the fill, and
along the filled portion of the road there should be a berm of three
or four feet between the toe of the slope of the fill and the near
edge of the ditch.
=Underground Water.=--In a preceding paragraph, mention was made of
the fact that only a part of the storm water runs off over the surface
of the ground, the larger part being absorbed by the soil. The water
thus absorbed flows downward through the pores in the soil until it is
deflected laterally by some physical characteristic of the soil
structure. The movement of underground water is affected by many
circumstances, but only two conditions need be discussed herein.
Underground water, like surface water, tends to attain a level
surface, but in so doing it may need to flow long distances through
the pores of the soil, and to overcome the resistance incident to so
doing some head will be required. That is to say, the water will be
higher at some places than at others. If a cut is made in grading the
road, the road surface may actually be lower than the ground water
level in the land adjoining the road. As a result, the water will seep
out of the side slopes in the cut and keep the ditches wet, or even
furnish enough water to occasion a flow in the ditch. Similarly, the
higher head of the underground water near the top of a hill may result
in ground water coming quite close to the surface some distance down
the hill. The remedy in both cases is tile underdrains alongside the
road to lower the ground water level so that it cannot affect the road
surface.
Sometimes the ground water encounters an impervious stratum as it
flows downward through the soil, or one that is less pervious than the
surface soil. When such is the case, the water will follow along this
stratum, and should there be an outcrop of the dense stratum, a spring
will be found at that place. This may be on a highway. The impervious
stratum may not actually outcrop but may lie only a few feet under the
surface of the road, in which case, the road surface will be so water
soaked as to be unstable. The so-called "seepy places" so often noted
along a road are generally the result of this condition. This
condition can be corrected by tile laid so as to intercept the flow at
a depth that precludes damage to the road. Commonly, the tile will be
laid diagonally across the road some distance above the section where
the effect of the water is noted, and will be turned parallel to the
road at the ditch line and carried under one of the side ditches to an
outlet.
=Tile Drains.=--Where the soil and climatic conditions are such that
the roadway at times becomes unstable because of underground water
rising to a level not far below the road surface, the ground water
level is lowered by means of tile underdrains. The function of the
tile drains in such cases is precisely the same as when employed in
land drainage; to lower the ground water level.
=Laying Tile.=--The tile lines are usually laid in trenches parallel
to the center line of the road near the ditch line and at least 4 feet
deep so as to keep the ground water level well down. They must be
carefully laid to line and grade. A good outlet must be provided and
the last few joints of pipe should be bell-and-spigot sewer pipe with
the joints filled with cement mortar. The opening of the tile should
be covered with a coarse screen to prevent animals from nesting in the
tile.
It is frequently necessary to lay a line of tile at the toe of the
slope in cuts to intercept water that will percolate under the road
from the banks at the sides. In some cases, it is desirable to
back-fill the tile trench with gravel or broken stone to insure rapid
penetration of surface water to the tile. In other instances, it is
advantageous to place catch basins about every three or four hundred
feet. These may be of concrete or of tile placed on end or may be
blind catch basins formed by filling a section of the trench with
broken stone. When a blind catch basin is used, the top should be
built up into a mound, and for a tile or concrete catch basin, a
grating of the beehive type should be used, so that flow to the tile
will not be obstructed by weeds and other trash that is carried to the
catch basin.
=Culverts.=--Culverts and bridges are a part of the drainage system
and the distinction between the two is merely a matter of size.
Generally, structures of spans less than about eight feet are classed
as culverts, but the practice is not uniform. In this discussion
culverts will be defined as of spans of 8 feet or less.
Numerous culverts are required to afford passage for storm water and
small streams crosswise of the road, and their aggregate cost is a
large item in the cost of road improvement. The size of the waterway
of a culvert required in any location will be estimated by an
inspection of the stream and existing structure, and by determining
the extent and physical characteristics of the drainage area.
Sometimes there is sufficient evidence at the site to indicate quite
closely the size required, but this should always be checked by
run-off computations. The drainage area contributing water to the
stream passing through the culvert under consideration is computed
from contour maps or from a survey of the ground, and the size of
culvert determined by one of the empirical formulas applicable to that
purpose. In these formulas, the solution depends upon the proper
selection of a factor "C" which varies in accordance with the nature
of the drainage area. Two of these that are quite widely used are as
follows:
_Myers' Formula: a = CA_
Where _a_ = area of cross section of culvert in square feet. _A_ =
area in acres of the drainage area above culvert. _C_ a factor varying
from 1 for flat country to 4 for mountainous country or rocky soil,
the exact value to be selected after an inspection of the drainage
area.
_Talbot's Formula_: Area of waterway in square feet =
_C_ [Square root of] ((Drainage area in acres)^3)
Transcriber's Note: The above formula used the mathematical
square root symbol in the original. One should read it as "C
times the square root of the Drainage area in acres cubed."
_C_ being variable according to circumstances thus:
"For steep and rocky ground _C_ varies from 2/3 to 1. For rolling
agricultural country, subject to floods at times of melting snow, and
with length of valley three or four times its width, _C_ is about 1/3,
and if stream is longer in proportion to the area, decrease _C_. In
districts not affected by accumulated snow, and where the length of
valley is several times its width, 1/5 or 1/6 or even less may be
used. _C_ should be increased for steep side slopes, especially if the
upper part of the valley has a much greater fall than the channel at
the culvert. The value of _C_ to be used in any case is determined
after an inspection of the drainage area."
[Illustration: Fig. 2. Design of Pipe Culvert and Bulkhead]
=Length of Culvert.=--The clear length between end walls on a culvert
should be at least equal to the width of the roadway between ditches.
This is a minimum of 20 feet for secondary roads and ranges from 24 to
30 feet for main roads. The headwall to the culvert should not be a
monument, but should be no higher than needed to prevent vehicles from
leaving the roadway at the culvert.
=Farm Entrance Culverts.=--At farm entrances, culverts are required to
carry the farm driveway across the side ditch of the road. These
culverts are usually about 16 feet along, and should be of a size
adequate to take the flow of the side ditch. The farm entrance culvert
should be of such design that it can be easily removed to permit
cleaning out the ditches with a road grader.
TYPES OF CULVERTS
Culverts constructed of concrete and poured in place are called box
culverts because of the rectangular form of the cross section.
Culverts of pre-cast pipe are known as pipe culverts. Several forms of
pipe culvert are in general use.
[Illustration: Fig. 3.--Typical Concrete Box Culvert]
=Metal Pipe.=--These may be of cast iron, steel or wrought iron. The
cast iron pipe is very durable but expensive and heavy to handle and
is not widely used in highway construction. Steel pipe has been
employed to a limited extent but its durability is questioned. At
least it is known that the pipe made from uncoated, light sheet steel
is not very durable. Sheet iron and sheets made from alloy iron
coated with spelter have been extensively used and seem to be durable,
especially when laid deep enough to eliminate possibility of damage
from heavy loads. To insure reasonable resistance to corrosion, the
metal sheets should be coated with at least one and one-half ounces of
spelter per square foot of sheet and the sheets should not be lighter
than 16 gauge for small sizes and should be heavier for the larger
sizes.
=Clay and Cement Concrete Pipe.=--The ordinary burned clay bell and
spigot pipe that is employed for sewer construction is sometimes used
for culverts. It must be very carefully bedded, preferably on a
concrete cradle and the joints filled with cement mortar. Culverts of
this type have a tendency to break under unusual loads, such as
traction engines or trucks. They may be damaged by the pressure from
freezing water, particularly when successive freezing and thawing
results in the culvert filling with mushy snow, which subsequently
freezes.
=Concrete Pipe.=--Reinforced concrete pipe is a satisfactory material
for culverts, if the pipe is properly designed. The pipe should be
carefully laid on a firm earth bed with earth carefully back-filled
and tamped around the pipe. The joints in the pipe should be filled
with cement mortar, or should be of a design that will be tight.
=Endwalls for Culverts.=--A substantial retaining wall is placed at
each end of the culvert barrel, whatever the type. This is to prevent
the end of the culvert from becoming choked with earth and to retain
the roadway at the culvert. It also indicates to the drivers the
location of the end of the culvert. The endwall extends a foot or more
below the floor of the culvert to prevent water from cutting under the
barrel. Plain concrete or stone masonry are most commonly used for
culvert endwalls.
[Illustration: Fig. 4.--Two Types of Drop Inlet Culvert]
=Reinforced Concrete Box Culverts.=--The pipe culvert is limited in
application to the smaller waterways. Reinforced concrete is
extensively used for culverts of all sizes, but especially for the
larger ones. These are usually constructed with endwalls integral with
the barrel of the culvert. Culverts of this type must be designed for
the loads anticipated to insure suitable strength and stability, and
must be constructed of a good quality of concrete. Figs. 2 and 3 show
designs for pipe and box culverts.
[Illustration: Fig. 5.--Drop Inlet Culvert]
=Drop Inlet Culverts.=--In some locations erosion has begun in the
fields adjacent to a culvert and it will probably continue until the
stream above the culvert has eroded to about the level of the floor of
the culvert. This is a reason for placing the culvert as high as the
roadway will permit, so long as the area above the culvert will be
properly drained. Considerable reclamation of land is possible if the
culvert is constructed with a box at the inlet and as shown in Fig. 4.
The area up-stream from the culvert will not erode below the level of
the top of the box at the inlet end.
Where the stream crossing the road has eroded to considerable depth or
has considerable fall, as would sometimes be the case on side hill
roads, the culvert barrel would follow the general slope of the ditch
but should have a drop inlet. This type of culvert is shown in Fig.
5.