On-Site Wastewater Treatment and Disposal Systems - Forced ...

7.2.6 Artificially Drained Systems
7.2.6.1 Description
High water tables that limit the use of trenches, beds or seepage pits
can sometimes be artificially lowered to permit the use of these dispo-
sal methods. Vertical drains, curtain drains and underdrains are com-
monly used subsurface drainage techniques. Soil and site conditions
determine which method is selected.
Curtain drains and vertical drains are used to lower perched water
tables. These methods are most effective where the perched water is
moving laterally under the soil absorption site. The drains are placed
upstream of the absorption area to intercept the groundwater as it flows
into the area.
Curtain drains are trench excavations in which perforated drainage pipe
is placed. These are placed around the upslope perimeter of the soil
absorption site to intercept the groundwater moving into the area (see
Figure 7-15). If the site has sufficient slope, the drains are brought
to the surface downslope to allow free drainage. On level sites, pumps
must be used to remove the collected water. If the restrictive layer
that creates the water table is thin and overlies permeable soil, verti-
cal drains may be used. These are trench excavations made through the
restrictive layer into the more permeable soil below and backfilled with
porous material (see Figure 7-16). Thus, water moving toward the ex-
cavation is able to drain into the underlying soil. Vertical drains are
susceptible to sealing by fine sediment transported by the water.
Underdrains are used where water tables exist 4 to 5 ft (1.2 to 1.5 m)
below the surface in permeable soils. The drains are similar to curtain
drains in construction, but several drains may be necessary to lower the
water table sufficiently (see Figure 7-17). Depth and spacing of the
drains are determined by the soil and water table characteristics.
7.2.6.2 Site Considerations
Successful design of artificially drained systems depends upon the cor-
rect diagnosis of the drainage problem. The source of the groundwater
and its flow characteristics must be determined to select the proper
method of drainage. Particular attention must be given to soil strati-
fication and groundwater gradients.
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