Environmental Engineering Reference
In-Depth Information
Deep wells installed first
Initial
GWL
Lowered
GWL
Wellpoints installed
after excavation
depth extended
Turbine
pump
FIGURE 8.46
Dewatering with deep wells and wellpoints in free-draining materials.
United States because of its relatively high cost, and perhaps because of the sparseness of
suitable conditions.
Method
Two electrodes are installed to the depth of dewatering in saturated soil and a direct cur-
rent applied. The induced current causes the porewater to flow from the anode to the cath-
ode from which it is removed by pumps. The electrodes are arranged so that the seepage
pressures are directed away from the exposed face of an excavation and hence add to the
stability.
8.4.4
Drains
General
Purpose
Drains provide a controlled path along which flow can occur, thereby reducing seepage
pressures in earth dams, slopes, retaining structures, pavements, side-hill fills, tunnels,
and spillways. In combination with pumps, drains reduce uplift forces on structures
below the water table such as basements, concrete tanks for sewage treatment plants, and
dry docks. Some types of drains are incorporated into design, whereas others are used as
corrective measures.
Description
Drains can be described by their shape and form as either blanket, trench, triangular or
trapezoidal, or circular. Some types are constructed only of soil materials, some combine
soils with a pipe to collect flow, and a few are open. Drains of large dimensions, such as
toe drains for dams, are usually constructed of free-draining materials such as gravel or
even stone.
The thickness of drainage layers depends on the flow quantities to be controlled and the
permeability of the material available. A drain can be constructed of sand, but the thick-
ness required may be several times greater than if pea gravel were used. Most drains are
provided with filters (see
Section 8.4.5
)
to protect against clogging and to prevent the
migration of finer materials from adjacent soils. Descriptions and applications are
described in
Section 8.4.6.
