TM 5-818-5/AFM 88-5, Chap 6/NAVFAC P-418
screen length to aquifer thickness may result in rela-
ing required to dewater an excavation may vary from
tively little drawdown within the excavation, even
5 to 50,000 gallons per minute or more. Thus, flow to a
though the water table is lowered 15 to 20 feet at the
drainage system will have an important effect on the
design and selection of the wells, pumps, and piping
line of wellpoints. For deep aquifers, a deep-well sys-
system. Turbine or submersible pumps for pumping
tem will generally be more applicable, or the length of
deep wells are available in sizes from 3 to 14 inches
the wellpoints should be increased and the wellpoints
set deep and surrounded with a high-capacity filter.
with capacities ranging from 5 to 5000 gallons per
On the other hand, if the aquifer is relatively thin or
minute at heads up to 500 feet. Wellpoint pumps are
available in sizes from 6 to 12 inches with capacities
stratified wellpoints may be best suited to the situa-
ranging from 500 to 5000 gallons per minute depend-
tion.
(b) The perviousness and drainability of a soil or
ing upon vacuum and discharge heads. Jet-eductor
pumps are available that will pump from 3 to 20 gal-
rock may dictate the general type of a dewatering sys-
lons per minute for lifts up to 100 feet. Where soil con-
tem to be used for a project. A guide for the selection
ditions dictate the use of vacuum or electroosmotic
of a dewatering system related to the grain size of soils
is presented in figure 2-12. Some gravels and rock for-
wellpoint systems, the rate of pumpage will be very
mations may be so permeable that a barrier to flow,
small. The rate of pumpage will depend largely on the
such as a slurry trench, grout curtain, sheet pile cutoff,
distance to the effective source of seepage, amount of
drawdown or pressure relief required, and thickness
or freezing, may be necessary to reduce the quantity of
and perviousness of the aquifer through which the
flow to the dewatering system to reasonable propor-
tions. Clean, free-draining sands can be effectively de-
flow is occurring.
watered by wells or wellpoints. Drainage of sandy silts
(6) Intermittent pumping. Pumping labor costs can
and silts will usually require the application of addi-
occasionally be materially reduced by pumping a dewa-
tional vacuum to well or wellpoint dewatering sys-
tering system only one or two shifts per day. While
tems, or possibly the use of the electroosmotic method
this operation is not generally possible, nor advan-
of dewatering where soils are silty or clayey. However,
tageous, it can be economical where the dewatered
area is large; subsoils below subgrade elevation are
where thin sand layers are present, special require-
deep, pervious, and homogeneous; and the pumping
ments may be unnecessary. Electroosmosis should nev-
plant is oversize. Where these conditions exist, the
er be used until a test of a conventional system of well-
points, wells with vacuum, or jet-eductor wellpoints
pumping system can be operated to produce an abnor-
mally large drawdown during one or two shifts. The
has been attempted.
recovery during nonpumping shifts raises the ground-
water level, but not sufficiently to approach subgrade
tude of the drawdown required is an important con-
sideration in selecting a dewatering system. If the
elevation. This type of pumping plant operation
drawdown required is large, deep wells or jet-eductor
should be permitted only where adequate piezometers
wellpoints may be the best because of their ability to
have been installed and are read frequently.
achieve large drawdowns from the top of an excava-
(7) Effect of groundwater lowering on adjacent
tion, whereas many stages of wellpoints would be re-
increases the load on foundation soils below the ori-
quired to accomplish the same drawdown. Deep wells
can be used for a wide range of flows by selecting
ginal groundwater table. As most soils consolidate
pumps of appropriate size, but jet-eductor wellpoints
upon application of additional load, structures located
are not as flexible. Since jet-eductor pumps are rela-
within the radius of influence of a dewatering system
tively inefficient, they are most applicable where well
may settle. The possibility of such settlement should
flows are small as in silty to fine sand formations.
be investigated before a dewatering system is de-
signed. Establishing reference hubs on adjacent struc-
groundwater control required for a project will have a
tures prior to the start of dewatering operations will
significant bearing on the design of the dewatering
permit measuring any settlement that occurs during
pumps, power supply, and standby power and equip-
dewatering, and provides a warning of possible dis-
ment. If the dewatering problem is one involving the
tress or failure of a structure that might be affected.
relief of artesian pressure to prevent a "blowup" of the
Recharge of the groundwater, as illustrated in figure.
bottom of an excavation, the rate of water table re-
2-13, may be necessary to reduce or eliminate distress
bound, in event of failure of the system, may be ex-
to adjacent structures, or it may be necessary to use
tremely rapid. Such a situation may influence the type
positive cutoffs to avoid lowering the groundwater
of pressure relief system selected and require inclusion
level outside of an excavation. Positive cutoffs include
of standby equipment with automatic power transfer
soil freezing and slurry cutoff techniques. Observa-
and starting equipment.
tions should be made of the water level in nearby wells
(5) Required rate of pumping. The rate of pump
before and during dewatering to determine any effect
2-12
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