TM 5-818-5/AFM 88-5, Chap 6/NAVFAC P-418
or during periods of routine maintenance and oil
for measuring the flow from the system or wells. Pres-
change of the regular dewatering equipment. All
sure and vacuum gages should also be installed at the
standby equipment should be periodically operated to
pumps and in the header lines. For multistage well-
ensure that it is ready to function in event of a break-
point systems, the installation and operation of the
down of the regular equipment. Automatic starters,
first stage of wellpoints may offer an opportunity to
clutches, and valves may be included in the standby
check the permeability of the pervious strata, radius of
system if the dewatering requirements so dictate. Sig-
influence or distance to the source of seepage, and the
nal lights or warning buzzers may be desirable to indi-
head losses in the wellpoint system. Thus, from obser-
cate, respectively, the operation or breakdown of a
vations of the drawdown and discharge of the first
pumping unit. If control of the groundwater is critical
stage of wellpoints, the adequacy of the design for low-
to safety of the excavation or foundation, appropriate
er stages may be checked to a degree.
operating personnel should be on duty at all times.
a. Piezometers. The location of piezometers should
Where gravity flow conditions exist that allow the wa-
be selected to produce a complete and reliable picture
ter table to be lowered an appreciable amount below
of the drawdown produced by the dewatering system.
the bottom of the excavation and the recovery of the
Examples of types of piezometers and methods of in-
water table is slow, the system may be pumped only
stallation are given in paragraphs G-546) and
part time, but this procedure is rarely possible or de-
G-6h(2) of Appendix G. Piezometers should be located
sirable. Such an operating procedure should not be at-
so they will clearly indicate whether water levels re-
tempted without first carefully observing the rate of
quired by specifications are attained at significant lo-
rise of the groundwater table at critical locations in
cations. The number of piezometers depends on the
the excavations and analyzing the data with regard to
size and configuration of the excavation and the dewa-
existing soil formations and the status of the excava-
tering system. Normally, three to eight piezometers
tion.
are installed in large excavations and two or three in
smaller excavations. If the pervious strata are strati-
d. Surface water control. Ditches, dikes, sumps, and
fied and artesian pressure exists beneath the excava-
pumps for the control of surface water and the protec-
tion of dewatering pumps should be maintained
tion, piezometers should be located in each significant
stratum. Piezometers should be installed at the edge of
throughout construction of the project. Maintenance
and outside the excavation area to determine the
of ditches and sumps is of particular importance. Silt-
shape of the drawdown curve to the dewatering sys-
ing of ditches may cause overtopping of dikes and seri-
tem and the effective source of seepage to be used in
ous erosion of slopes that may clog the sumps and
evaluating the adequacy of the system. If recharge of
sump pumps. Failure of sump pumps may result in
the aquifer near the dewatering system is required to
flooding of the dewatering equipment and complete
prevent settlement of adjacent structures, control
breakdown of the system. Dikes around the top of an
piezometers should be installed in these areas. Where
excavation to prevent the entry of surface water
the groundwater is likely to cause incrustation of well
should be maintained to their design section and grade
screens, piezometers may be installed at the outer edge
at all times. Any breaks in slope protection should be
of the filter and inside the well screen to monitor the
promptly repaired.
head loss through the screen as time progresses. In
6-3. Control and evaluation of perform-
this way, if a significant increase in head loss is noted,
ance. After a dewatering or groundwater control
cleaning and reconditioning of the screens should be
system is installed, it should be pump-tested to check
undertaken to improve the efficiency of the system.
its performance and adequacy. This test should include
Provisions for measuring the drawdown in the wells or
measurement of initial groundwater or artesian water
at the line of wellpoints are desirable from both an op-
table, drawdown at critical locations in the excavation,
eration and evaluation standpoint.
flow from the system, elevation of the water level in
b. Flow measurements. Measurement of flow from a
the wells or vacuum at various points in the header,
dewatering system is desirable to evaluate the per-
and distance to the "effective" source of seepage, if
formance of the system relative to design predictions.
possible. These data should be analyzed, and if condi-
Flow measurements are also useful in recognizing any
tions at the time of test are different than those for
loss in efficiency of the system due to incrustation or
which the system was designed, the data should be ex-
clogging of the wellpoints or well screens. Appendix F
trapolated to water levels and source of seepage as-
describes the methods by which flow measurements
sumed in design. It is important to evaluate the system
can be made.
as early as possible to determine its adequacy to meet
full design requirements. Testing a dewatering system
c. Operational records. Piezometers located within
the excavated area should be observed at least once a
and monitoring its performance require the *installa-
tion of piezometers and the setting up of some means
day, or more frequently, if the situation demands, to
6-2
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