TM 5-818-5/AFM 88-5, Chap 6/NAVFAC P-418
that they can be pumped in the event pumping of the
In addition to these factors of safety being applied to
bottom stage of wellpoints does not lower the water
design features of the system, the system should be
table below the excavation slope because of stratifica-
pump-tested to verify its adequacy for the maximum
tion, and so that they can be pumped during backfill-
required groundwater lowering and maximum river or
ing operations.
groundwater table likely (normally a frequency of oc-
(a) The design of a conventional wellpoint sys-
currence of once in 5 to 10 years for the period of expo-
tem to dewater an open excavation, as discussed in
sure) to occur.
paragraph 4-2b, is outlined below.
4-9. Dewatering open excavations. An ex-
cavation can be dewatered or the artesian pressure re-
efficients (H, L, and k) of the formation to be de-
lieved by one or a combination of methods described in
watered based on investigations outlined in chapter 3.
chapter 2. The design of dewatering and groundwater
Step 2. Determine the drawdown required to
control systems for open excavations, shafts, and tun-
dewater the excavation or to dewater down to the next
nels is discussed in the following paragraphs. Ex-
stage of wellpoints, based on the maximum ground-
amples of design for various types of dewatering and
water level expected during the period of operation.
pressure relief systems are given in appendix D.
Step 3. Compute the head at the assumed slot
(he or ho) to produce the desired residual head hD in the
a. Trenching and sump pumping.
excavation.
(1) The applicability of trenches and sump pump-
Step 4. Compute the flow per lineal foot of
ing for dewatering an open excavation is discussed in
chapter 2. Where soil conditions and the depth of an
Step 5. Assume a wellpoint spacing a and com-
excavation below the water table permit trenching and
pute the flow per wellpoint, QW = aQp.
sump pumping of seepage (fig. 2-1), the rate of flow
Step 6. Calculate the required head at the well-
into the excavation can be estimated from plan and
point hw corresponding to Qw.
sectional flow analyses (fig. 4-27) or formulas pre-
sented in paragraphs 4-2 through 4-5.
Step 7. Check to see if the suction lift that can
(2) Where an excavation extends into rock and
be produced by the wellpoint pump V will lower the
there is a substantial inflow of seepage, perimeter
water level in the wellpoint to h,(p) as follows:
drains can be installed at the foundation level outside
(4-8)
of the formwork for a structure. The perimeter drain-
where
age system should be connected to a sump sealed off
V = vacuum at pump intake, feet of water
from the rest of the area to be concreted, and the seep-
M = distance from base of pervious strata to
age water pumped out. After construction, the drain-
pump intake, feet
age system should be grouted. Excessive hydrostatic
average head loss in header pipe from well-
H
C
=
pressures in the rock mass endangering the stability of
point, feet
the excavated face can be relieved by drilling 4-inch-
HW = head loss in wellpoint, riser pipe, and swing
diameter horizontal drain holes into the rock at ap-
connection to header pipe, feet
proximately lo-foot centers. For large seepage inflow,
Step 8. Set the top of the wellpoint screen at
supplementary vertical holes for deep-well pumps at
least 1 to 2 feet or more below hw-Hw to provide ade-
50- to 100-foot intervals may be desirable for tempo-
quate submergence of the wellpoint so that air will not
rary lowering of the groundwater level to provide suit-
be pulled into the system.
able conditions for concrete placement.
(b) An example of the design of a two-stage well-
point system to dewater an excavation is illustrated in
b. Wellpoint system. The design of a line or ring of
figure D-1, appendix D.
wellpoints pumped with either a conventional well-
(c) If an excavation extends below an aquifer
point pump or jet-eductors is generally based on math-
into an underlying impermeable soil or rock forma-
ematical or flow-net analysis of flow and drawdown to
tion, some seepage will pass between the wellpoints at
a continuous slot (para 4-2 through 4-5).
the lower boundary of the aquifer. This seepage may
(1) Conventional wellpoint system. The draw-
be intercepted with ditches or drains inside the excava-
down attainable per stage of wellpoints (about 15 feet)
tion and removed by sump pumps. If the underlying
is limited by the vacuum that can be developed by the
stratum is a clay, the wellpoints may be installed in
pump, the height of the pump above the header pipe,
holes drilled about 1 to 2 feet into the clay and back-
and hydraulic head losses in the wellpoint and collec-
filled with filter material. By this procedure, the water
tor system. Where two or more stages of wellpoints are
level at the wellpoints can be maintained near the bot-
required, it is customary to design each stage so that it
tom of the aquifer, and thus seepage passing between
is capable of producing the total drawdown required
the wellpoints will be minimized. Sometimes these
by that stage with none of the upper stages function-
procedures are ineffective, and a small dike in the ex-
ing. However, the upper stages are generally left in so
4-42
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