TM 5-814-9
sediment, and water at maximum
meters). Deeper basins should be
depth. Concrete can crack and
considered in arid regions to reduce
deteriorate and can be relatively
water
surface
areas,
thereby
expensive to install and repair
minimizing evaporation.
compared with other types of liners.
(b)
Sediment volume. A depth of from 1 to
The slope of the walls will depend on
2 feet (0.3 to 0.6 meter) should be al-
the
method
of
construction.
lowed for sediment storage at the
Expansion/contraction joints are
bottom of the basin and this volume
required and may be difficult to seal
added to the water storage
tightly.
requirements.
(b)
Bentonite and natural clay. A
(c)
Dead storage volume. A depth of 2 to
bentonite or natural clay liner can be
4 feet (0.6 to 1.2 meter) should be
installed only over certain soils.
allowed directly above the sediment
Noncohesive soils that contain large
volume for dead storage. This
amounts of fines without coarse lenses
additional volume allows for
(sands and gravels) are susceptible to
submergence of the basin effluent
movement which can cause leakage in
control structure and provides a
bentonite and clay liners. A site
minimum water volume to optimize
without shallow bedrock and without
settling.
a perched water table is desirable for
installing these types of liners. When
Influent structure. A concrete trough should
(3)
these materials are used, the slope of
be installed at the inlet to prevent liner
the walls will vary, depending on local
erosion. The inlet pipe should he higher than
conditions such as the soil's angle of
the maximum water elevation in the basin to
repose. Sediment removal equipment
allow free discharge of wastewater. If the
could damage a bentonite liner, since
wastewater Influent line were submerged,
movement of the bentonite can cause
sediment could collect and eventually clog the
leakage. Therefore, a concrete ramp
line.
and floor must be installed to provide
access for removing the sediment.
Effluent control structure. The effluent struc-
(4)
Leaks may occur at each point where
ture should be located opposite the Influent
the bentonite and concrete interface.
structure thereby maximizing the water
The inlet must have an energy
detention time in the basin. If cohesive soils
dissipator such as a concrete trough
are present, float supported baffles may be re-
with rocks to prevent erosion of the
quired to increase the detention time and
bentonite walls. Bentonite-treated
minimize short circuiting of wastewater in the
basins must be protected from
basin. Stormwater diversion prior to
stormwater erosion to maintain their
treatment should be considered as standard
integrity.
procedure only if the CVWFis in a shutdown
mode. This diversion should be in operation
(c)
Synthetic materials. Various types of
after each day's shutdown and when it is
synthetic rolled material can be used to
expected that the facility will not be in use for
line the equalization basin. A synthetic
long periods of time, such as during winter.
liner is installed in the same way that a
landfill is lined. A proper base is pre-
(5)
pared and the liner is positioned, then
are designed and constructed with water-tight
covered with soil. Equipment can
walls and bottom. Several liner materials, or
move over the bottom of the basin if
combinations, can be considered for water-
the soil covering the liner is thick
tight construction. However, local experience
enough to protect it, a proper base is
with earthen basins may suggest that no lining
under the liner, and the walls are not
is needed. In-ground basins should be de-
sloped too steeply. A concrete ramp
signed and constructed in accordance with
and floor can be installed in the basin
and with approval by the state regulatory
before the rolled material is placed. A
authority in which the facility is located. The
concrete trough must be used to
most cost effective, environmentally
channel the Influent over rocks and
acceptable basin construction should be
other obstacles to dissipate the energy
provided.
of the falling water. If a synthetic liner
(a)
Concrete. A concrete liner is
material is selected, it should be
permanent and can resist freeze/thaw
resistant to petroleum products and
cycles. If a concrete lined basin is
sunlight such as high density
used, the foundation must support the
polyethylene (HDPE).
weight of the structure, concrete,
6-15
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