4. ELECTRO-OSMOSIS. This is a specialized procedure utilized in silts and
clays that are too fine-grained to be effectively drained by gravity or
vacuum methods. See DM-7.03, Chapter 2.
5. PUMPING WELLS. These wells are formed by drilling a hole of sufficient
diameter to accommodate a pipe column and filter, installing a well casing,
and placing filter material in the annular space surrounding the casing.
Pumps may be either the turbine type with a motor at the surface and pipe
column with pump bowls hung inside the well, or a submersible pump placed
within the well casing.
a. Applications. Deep pumping wells are used if (a) dewatering
installations must be kept outside the excavation area, (b) large quantities
are to be pumped for the full construction period, and (c) pumping must
commence before excavation to obtain the necessary time for drawdown. See
Figure 10 (bottom panel, Reference 9, Analysis of Groundwater Lowering
Adjacent to Open Water, by Avery) for analysis of drawdown and pumping
quantities for single wells or a group of wells in a circular pattern. Deep
wells may be used for gravels to silty fine sands, and water bearing rocks.
See Section 4 for limitations due to iron and carbonate clogging.
Bored shallow wells with suction pumps can be used to replace
wellpoints where pumping is required for several months or in silty soils
where correct filtering is critical.
b. Special Methods. Ejector or eductor pumps may be utilized within
wellpoints for lifts up to about 60 feet. The ejector pump has a nozzle
arrangement at the bottom of two small diameter riser pipes which remove
water by the Venturi principle. They are used in lieu of a multistage
wellpoint system and if the large pumping capacity of deep wells is not
required. Their primary application is for sands, but with proper control
they can also be used in silty sands and sandy silts.
6. RELIEF WELLS. These wells are sand columns used to
bleed water from
underlying strata containing artesian pressures, and to
reduce uplift forces
at critical location. Relief wells may be tapped below
ground by a
collector system to reduce back pressures acting in the
well.
a. Applications. Relief wells are frequently used as construction
expedients, and in situations where a horizontal drainage course may be
inadequate for pressure relief of deep foundations underlain by varved or
stratified soils or soils whose permeability increases with depth.
b. Analysis. See Figure 11 (Reference 10, Soil Mechanics Design,
Seepage Control, by the Corps of Engineers) for analysis of drawdown
produced by line of relief wells inboard of a long dike. To reduce uplift
pressures h+m, midway between the wells to safe values, vary the well
diameter, spacing, and penetration to obtain the best combination.
7.1-283
Change 1, September 1986
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