TM 5-815-5/AFM 88-5, Chap 6/NAVFAC P-418
vacuum capacity to ensure efficient operations of the
system.
2-6. Vertical sand drains. Where a stratified
semipervious stratum with a low vertical permeability
overlies a pervious stratum and the groundwater table
has to be lowered in both strata, the water table in the
upper stratum can be lowered by means of sand drains
as shown in figures 2-9. If properly designed and in-
stalled, sand drains will intercept seepage in the upper
stratum and conduct it into the lower, more permeable
stratum being dewatered with wells or wellpoints.
Sand drains consist of a column of pervious sand
placed in a cased hole, either driven or drilled through
the soil, with the casing subsequently removed. The ca-
pacity of sand drains can be significantly increased by
installation of a slotted 1% or 2-inch pipe inside the
sand drain to conduct the water down to the more per-
vious stratum.
Note: Vacuum in header = 25 ft; vacuum
in filter and soil in vicinity of well-
2-7. Electra-osmosis. Some soils, such as silts,
point = approximately 10 ft.
clayey silts, and clayey silty sands, at times cannot be
(From "Foundation Engineering," G. A. Leonards, ed.,
dewatered by pumping from wellpoints or wells. How-
1962, McGraw-Hill Book Company. Used with permission
ever, such soils can be drained by wells or wellpoints
of McGraw-Hill Book Company.)
combined with a flow of direct electric current
through the soil toward the wells. Creation of a hy-
Figure 2-5. Vacuum wellpoint system.
draulic gradient by pumping from the wells or well-
points with the passage of direct electrical current
neath an excavation. They are particularly suited for
through the soil causes the water contained in the soil
dewatering large excavations requiring high rates of
voids to migrate from the positive electrode (anode) to
pumping, and for dewatering deep excavations for
the negative electrode (cathode). By making the cath-
dams, tunnels, locks, powerhouses, and shafts. Excava-
ode a wellpoint, the water that migrates to the cathode
tions and shafts as deep as 300 feet can be dewatered
can be removed by either vacuum or eductor pumping
by pumping from deep wells with turbine or submersi-
(fig. 2-10).
ble pumps. The principal advantages of deep wells are
that they can be installed around the periphery of an
2-8. Cutoffs. Cutoff curtains can be used to stop or
excavation and thus leave the construction area unem-
minimize seepage into an excavation where the cutoff
cumbered by dewatering equipment, as shown in fig-
can be installed down to an impervious formation.
ure 2-7, and the excavation can be predrained for its
Such cutoffs can be constructed by driving steel sheet
full depth.
piling, grouting existing soil with cement or chemical
b. Deep wells for dewatering are similar in type and
grout, excavating by means of a slurry trench and
construction to commercial water wells. They com-
backfilling with a plastic mix of bentonite and`soil, in-
monly have a screen with a diameter of 6 to 24 inches
stalling a concrete wall, possibly consisting of overlap-
with lengths up to 300 feet and are generally installed
ping shafts, or freezing. However, groundwater within
with a filter around the screen to prevent the infiltra-
the area enclosed by a cutoff curtain, or leakage
tion of foundation materials into the well and to im-
through or under such a curtain, will have to be
prove the yield of the well.
pumped out with a well or wellpoint system as shown
in figure 2-11.
c. Deep wells may be used in conjunction with a vac-
uum system to dewater small, deep excavations for
a. Cement and chemical grout curtains. A cutoff
tunnels, shafts, or caissons sunk in relatively fine-
around an excavation in coarse sand and gravel or por-
grained or stratified pervious soils or rock below the
ous rock can be created by injecting cement or chem-
groundwater table. The addition of a vacuum to the
ical grout into the voids of the soil. For grouting to be
well screen and filter will increase the hydraulic grad-
effective, the voids in the rock or soil must be large
ient to the well and will create a vacuum within the
enough to accept the grout, and the holes must be close
surrounding soil that will prevent or minimize seepage
enough together so that a continuous grout curtain is
from perched water into the excavation. Installations
obtained. The type of grout that can be used depends
of this type, as shown in figure 2-8, require adequate
upon the size of voids in the sand and gravel or rock to
2-5
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