Page 253 - Moving the Earth_ The Workbook of Excavation

DITCHING AND DEWATERING

DITCHING AND DEWATERING 5.55

SEPTIC DRAINS

Some drains have no open discharge but depend on the porosity of the soil through which they run, or
in which they end. A common type is the septic leach field for disposal of domestic waste, a set of stan-
dards for which is shown in Figs. 5.46 and 5.47. Septic tank overflow is carried in a sealed pipe to the
head of the field, which consists of a number of lines of porous land tile with a maximum slope of 1
inch to 16 feet, with open joints on a base of graded washed gravel that is brought up the sides flush
with the top. Building paper or hay is placed over this to prevent dirt fill from sifting into the spaces.
Fluids in the pipe leak out at the joints and percolate through the gravel and subsoil down to the ground-
water. The gravel serves to widen the area over which the sewage can come in contact with the soil.
Purification is affected by microorganisms in the septic tank and in the soil.
For successful operation, a septic field must distribute the sewage evenly over an area large enough
to absorb it completely. If the system is overburdened, or if some part of it carries too large a share of the
total load, the fluids may force a channel and flow out on the surface.
Pervious soils absorb the sewage much more rapidly than tight ones, and can handle much
larger quantities per foot of tile and square foot of trench.
The size of a system is calculated on the basis of volume of flow and the rate of absorption. For this
set of specifications, it is figured there will be 100 gallons per person per day in residences. Day
schools have a rate of 15 to 35, and restaurants and business buildings 10.
A percolation test is made on the soil. Holes 1 foot square are dug to the bottom level of the
proposed trenches, and filled with water to a depth of 2 feet. The water is allowed to drain until it
is 6 inches deep. The time required to fall to 5 inches is recorded.
This process is repeated until the drop from 6 to 5 inches takes the same time on two successive
tries. This time is matched to the same or the next-higher figure in column 1, Fig. 5.47. The center
column gives the absorption rate, the right-hand one the number of square feet of trench bottom
needed.
The tank should have a capacity of not less than 100 gallons per person. Any watertight con-
struction may be permitted. Concrete block, special concrete slabs, poured concrete, and prefab-
ricated tanks are used.
Where a garbage destructor is used, 50 percent should be added to all capacities.
As with most permanent installations, it is good policy to be generous in figuring. The load
might increase, or the soil might lose absorption efficiency.
The field should not be heavily shaded, and cannot be crossed by vehicles. It must be at least
25 feet from any pond or stream, and 50 to 100 feet from any drilled well or reservoir. It must not
drain toward a surface well or spring used for household water, at any distance.
If the native soil is not suitable, or the water table is high, it may be necessary to install a cur-
tain drain to divert water, or to haul in hundreds of yards of sand to construct a filter bed in which
to lay the tile field. This can be expensive enough to be a determining factor in selecting a lot.
Local regulations should be consulted before arranging for sewage disposal, as methods vary
in different areas and exact conformance may be required.
Septic systems are used only where public sewers are not accessible, and usually are aban-
doned if a sewer is laid.
A drain may also terminate in a dry well. The well consists of a hole in the earth, filled with loose
rock, rubble, or clean gravel. The pipe discharges into it, and the fluid soaks out of the sides or bot-
tom, or becomes part of the general circulation of groundwater. This system depends on pervious
soil in contact with the dry well.

VERTICAL DRAINS

Vertical drains of sand or porous soil can be used to move water up or down. They are often made
by drilling or blasting holes in an impervious layer lying over a pervious one. Figure 5.48(A) shows
a section of ground in which opening up the hardpan would allow water to drain into underlying
gravel, and (B) shows a pond which is able to exist above the water table in a pervious sand,