a. Construction Controls. For important construction dewatering,
install piezometers below the base of excavations and behind slopes or
cofferdams to check on the performance and adequacy of drainage system.
b. Settlement Effects. Where dewatering lowers the water levels in
permeable strata adjacent to compressible soils, settlement may result. See
Chapter 5 for methods of settlement evaluation.
c. Dewatering Schemes. For construction of dewatering systems and
procedures, refer to DM-7.2, Chapter 1, and NAVFAC P-418.
2. WELLPOINT SYSTEMS. Wellpoints consist of 1-1/2 or 2-inch diameter pipes
with a perforated bottom section protected by screens. They are jetted or
placed in a prepared hole and connected by a header pipe to suction pumps.
a. Applicability. Wellpoints depend upon the water flowing by gravity
to the well screen. Pumping methods for gravity drainage generally are not
effective when the average effective grain size of a soil D+10, is less than
0.05 mm. In varved or laminated soils where silty fine sands are separated
by clayey silts or clay, gravity drainage may be effective even if the
average material has as much as 50 percent smaller than 0.05 mm.
Compressible, fine-grained materials containing an effective grain size less
than 0.01 mm can be drained by providing a vacuum seal at the ground surface
around the wellpoint, utilizing atmospheric pressure as a consolidating
force. See Section 4 for limitations due to iron and carbonate clogging.
b. Capacity. Wellpoints ordinarily produce a drawdown between 15 and
18 feet below the center of the header. For greater drawdown, install
wellpoints in successive tiers or stages as excavation proceeds. Discharge
capacity is generally 15 to 30 gpm per point. Points are spaced between 3
and 10 feet apart. In finely stratified or varved materials, use minimum
spacing of points and increase their effectiveness by placing sand in the
annular space surrounding the wellpoint.
c. Analysis. Wellpoint spacing usually is so close that the seepage
pattern is essentially two dimensional. Analyze total flow and drawdown by
flow net procedure. (See Section 2.) For fine sands and coarser material,
the quantity of water to be removed controls wellpoint layout. For silty
soils, the quantity pumped is relatively small and the number and spacing
of wellpoints will be influenced by the time available to accomplish the
necessary drawdown.
3. SUMPS. For construction convenience or to handle a large flow in
pervious soils, sumps can be excavated with soldier beam and horizontal
wood lagging. Collected seepage is removed with centrifugal pumps placed
within the sump. Analyze drawdown and flow quantities by approximating the
sump with an equivalent circular well of large diameter.
Sheeted sumps are infrequently used. Unsheeted sumps are far more
common, and are used primarily in dewatering open shallow excavations in
coarse sands, clean gravels, and rock.
7.1-282
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