TM 5-818-5/AFM 88-5, Chap 6/NAVFAC P-418
cavation just inside the toe of the excavation may be
of the jet nozzle in the pump. Generally, a jet-eductor
required to prevent seepage from entering the work
pump requires an input flow of about 2 to 2 times
the flow to be pumped depending on the operating
area. Sump pumping can be used to remove water
pressure and design of the nozzle. Consequently, if
from within the diked area.
flow from the wells or wellpoints is large, a deep-well
(2) Jet-eductor (well or) wellpoint systems. Flow
system will be more appropriate. The pressure header
and drawdown to a jet-eductor (well or) wellpoint sys-
supplying a system of jet eductors must be of such size
tem can be computed or analyzed as discussed in para-
that a fairly uniform pressure is applied to all of the
graph 4-2b. Jet-eductor dewatering systems can be de-
eductors.
signed as follows:
(3) Vacuum wellpoint system. Vacuum wellpoint
Step 1. Assume the line or ring of wells or well-
systems for dewatering fine-grained soils are similar
points to be a drainage slot.
to conventional wellpoint systems except the wellpoint
Step 2. Compute the total flow to the system for
and riser are surrounded with filter sand that is sealed
the required drawdown and penetration of the well
at the top, and additional vacuum pump capacity is
screens.
Step 3. Assume a well or wellpoint spacing that
provided to ensure development of the maximum vac-
uum in the wellpoint and filter regardless of air loss.
will result in a reasonable flow for the well or well-
In order to obtain 8 feet of vacuum in a wellpoint and
point and jet-eductor pump.
filter column, with a pump capable of maintaining a
Step 4. Compute the head at the well or well-
25 foot vacuum in the header, the maximum lift is
point hw required to achieve the desired drawdown.
25-8 or 17 feet. Where a vacuum type of wellpoint
Step 5. Set eductor pump at M = hw-Hw with
system is required, the pump capacity is small. The ca-
some allowance for future loss of well efficiency.
pacity of the vacuum pump will depend on the air per-
The wells or wellpoints and filters should be selected
meability of the soil, the vacuum to be maintained in
and designed in accordance with the criteria set forth
the filter, the proximity of the wellpoints to the exca-
under paragraph 4-6.
vation, the effectiveness of the seal at the top of the
(a) If the soil formation being drained is strati-
filter, and the number of wellpoints being pumped. In
fied and an appreciable flow of water must be drained
very fine-g-rained soils, pumping must be continuous.
down through the filter around the riser pipe to the
The flow may be so small that water must be added to
wellpoint, the spacing of the wellpoints and the perme-
the system to cool the pump properly.
ability of the filter must be such that the flow from
formations above the wellpoints does not exceed
c. Electroosmosis
Q w = kvA i
(4-9)
(1) An electroosmotic dewatering system consists
where
of anodes (positive electrodes, usually a pipe or rod)
Qw = flow from formation above wellpoint
and cathodes (negative electrodes, usually wellpoints
K v = vertical permeability of filter
or small wells installed with a surrounding filter),
A = horizontal area of filter
across which a d-c voltage is applied. The depth of the
i = gradient produced by gravity = 1.0
electrodes should be at least 5 feet below the bottom of
Substitution of small diameter well screens for well-
the slope to be stabilized. The spacing and arrange-
points may be indicated for stratified formations.
ment of the electrodes may vary, depending on the
Where a formation is stratified or there is little avail-
dimensions of the slope to be stabilized and the voltage
able submergence for the wellpoints, jet-eductor well-
available at the site. Cathode spacings of 25 to 40 feet
points and risers should be provided with a pervious
have been used, with the anodes installed midway be-
filter, and the wellpoints set at least 10 feet back from
tween the cathodes. Electrical gradients of 1.5- to 4-
the edge of a vertical excavation.
volts-per-foot distance between electrodes have been
(b) Jet-eductor pumps may be powered with
successful in electroosmotic stabilization. The electri-
individual small high-pressure centrifugal pumps or
cal gradient should be less than about 15 volts per foot
with one or two large pumps pumping into a single
of distance between electrodes for long-term installa-
pressure pipe furnishing water to each eductor with a
tions to prevent loss in efficiency due to heating the
single return header. With a single-pump setup, the
ground. Applied voltages of 30 to 100 volts are usually
water is usually circulated through a stilling tank with
satisfactory; a low voltage is usually sufficient if the
an overflow for the flow from wells or wellpoints (fig.
groundwater has a high mineral content.
2-6). Design of jet eductors must consider the static
(2) The discharge of a cathode wellpoint may be
lift from the wells or wellpoints to the water level in
estimated from the equation
the recirculation tank; head loss in the return riser
(4-10)
pipe; head loss in the return header; and flow from the
Qe = keieaz
wellpoint. The (net) capacity of a jet-eductor pump de-
where
k e = coefficient of electroosmotic permeability
pends on the pressure head, input flow, and diameter
4-43
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