TM 5-818-5/AFM 88-5, Chap 6/NAVFAC P-418
(assume 0.98 x 10-4 feet per second per volt
groundwater completely to the bottom of the aquifer,
per foot)
A combination of deep wells and a single stage of well-
le = electrical gradient between electrodes, volts
points may permit lowering to the desired level. The
per foot
advantages of a combined system, in which wells are
a = effective spacing of wellpoints, feet
essentially used in place of the upper stages of well-
z = depth of soil being stabilized, feet
points, are as follows:
Current requirements commonly range between 15
(a) The excavation quantity is reduced by the
and 30 amperes per well, and power requirements are
elimination of berms for installation and operation of
generally high. However, regardless of the expense of
the upper stages of wellpoints.
installation and operation of an electroosmotic de-
(b) The excavation can be started without a de-
watering system, it may be the only effective means of
lay to install the upper stages of wellpoints.
dewatering and stabilizing certain silts, clayey silts,
(c) The deep wells installed at the top of the
and clayey silty sands. Electroosmosis may not be
excavation will serve not only to lower the groundwa-
applicable to saline soils because of high current re-
ter to permit installation of the wellpoint system but
quirements, nor to organic soils because of environ-
also to intercept a significant amount of seepage and
mentally objectionable effluents, which may be un-
thus reduce the flow to the single stage of wellpoints.
sightly and have exceptionally high pH values.
A design example of a combined deep-well and well-
point system is shown in figure D-4.
d. Deep-well systems
(2) Sand drains with deep wells and wellpoints.
(1) The design and analysis of a deep-well system
Sand drains can be used to intercept horizontal seep-
to dewater an excavation depends upon the configura-
age from stratified deposits and conduct the water
tion of the site dewatered, source of seepage, type of
vertically downward into a pervious stratum that can
flow (artesian and gravity), penetration of the wells,
be dewatered by means of wells or wellpoints. The lim-
and the submergence available for the well screens
iting feature of dewatering by sand drains is usually
with the required drawdown at the wells. Flow and
the vertical permeability of the sand drains itself,
drawdown to wells can be computed or analyzed as dis-
which restricts this method of drainage to soils of low
cussed in paragraph 4-2b.
permeability that yield only a small flow of water.
(2) Methods are presented in paragraphs 4-2b and
Sand drains must be designed so that they will inter-
4-3 whereby the flow and drawdown to a well system
cept the seepage flow and have adequate capacity to
can be computed either by analysis or by a flow net as-
allow the seepage to drain downward without any back
suming a continuous slot to represent the array of
pressure. To accomplish this, the drains must be
wells, and the drawdown at and between wells com-
spaced, have a diameter, and be filled with filter sand
puted for the actual well spacing and location. Exam-
so that
ples of the design of a deep-well system using these
(4-11)
methods and formulas are presented in figures D-2
and D-3.
where
(3) The submerged length and size of a well screen
QD = flow per drain
should be checked to ensure that the design flow per
k D = vertical permeability of sand filter
well can be achieved without excessive screen entrance
i = gradient produced by gravity = 1.0
losses or velocities. The pump intake should be set so
A D = area of drain
that adequate submergence (a minimum of 2 to 5 feet)
Generally, sand drains are spaced on 5- to 15-foot cen-
is provided when all wells are being pumped. Where
ters and have a diameter of 10 to 18 inches. The maxi-
the type of seepage (artesian and gravity) is not well
mum permeability kv of a filter that may be used to
established during the design phase, the pump intake
drain soils for which sand drains are applicable is
should be set 5 to 10 feet below the design elevation to
about 1000 to 3000 x 10-4 centimetres per second or
ensure adequate submergence. Setting the pump bowl
0.20 to 0.60 feet per minute, Thus, the maximum ca-
below the expected drawdown level will also facilitate
pacity QD of a sand drain is about 1 to 3 gallons per
drawdown measurements.
minute. An example of a dewatering design, including
sand drains, is presented in figure D-5. The capacity
e. Combined systems.
of sand drains can be significantly increased by install-
(1) Well and wellpoint systems. A dewatering sys-
ing a small (1- or 1-inch) slotted PVC pipe in the
tem composed of both deep wells and wellpoints may
drain to conduct seepage into the drain downward into
be appropriate where the groundwater table has to be
underlying more pervious strata being dewatered.
lowered appreciably and near to the top of an im-
permeable stratum. A wellpoint system alone would
f. Pressure relief systems.
require several stages of wellpoints to do the job, and a
(1) Temporary relief of artesian pressure beneath
well system alone would not be capable of lowering the
an open excavation is required during construction
4-44
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