Environmental Engineering Reference
In-Depth Information
In rock formations, the dip and bearing of the injection holes should be chosen as nor-
mal to the joint set as possible to provide for the maximum intersection of the joints, oth-
erwise many will remain ungrouted. Spacing of drill holes depends on the joint
concentrations and the effectiveness of the curtain desired.
Injection pressures vary with conditions. Care is required in rock masses to obtain a
pressure that is greater than water pressure but less than that required to cause new frac-
tures, although at times fracturing is desired, such as during stage grouting, to increase
penetrations. When grouting close to the surface of an embankment or slope, or beneath
foundations, care is required that pressures do not exceed lateral or vertical overburden
pressures or ground heave will result. In rock masses, pressures of 7 tsf are usual although
pressures as high as 40 tsf have been used in some cases (Jaeger, 1972). In fact, in recent
years pressures as high as 100 tsf have been used to compress rock masses.
8.4.3
Dewatering
General
Applications
Dewatering systems are used primarily to lower the water table to permit construction in
the dry, but they also serve to reduce seepage pressures in slopes and at the bottoms of
excavations. Dewatering can be achieved with sump pumps, wellpoints, deep wells, and
occasionally electro-osmosis. The procedure selected depends on the material to be dewa-
tered and the depth below the water table to which dewatering is desired. Systems gener-
ally applicable for various soil gradations are given in
Figure 8.35.
Selection
In the selection of the dewatering system to be employed a number of factors require con-
sideration, such as :
Depth to the water table during construction, the nature of the water (static,
perched, or artesian), and the estimated flow quantities into the excavation from
the sides and bottom (and in the case of tunnels, from the top and heading).
●
Flow quantity into the dewatering system, which depends on the permeability
and thickness of the water-bearing formations, the depth to which the water
table must be lowered to provide a dry excavation and prevent piping, and the
corresponding hydraulic gradients producing flow toward the excavation.
●
The coefficient of permeability of the various strata is the most important factor. It
can be estimated from gradation curves (see
Section 3.3.2),
or measured by full-scale
pumping tests using pyrometers to measure drawdown (see
Section 8.3.3)
.
The
probable inflow rates are estimated from well formulas, slot formulas, or flow nets.
●
The magnitude of surface subsidence to be anticipated during dewatering and
its effect on nearby structures must be evaluated, and methods selected for its
control (see
Section 10.2.5).
●
Sump Pumping
The simplest procedure for controlling water in open excavations is to provide interceptor
ditches at the slope toe, or at the bottom of a sheeted excavation, and connect them to
sumps from which water is pumped (
Figure 8.40).
The procedure is usually suitable where
flows are not too large, as in silty or clayey sands where the depth below the water table
is not great and heads are low. The ditch and sump bottom should be lined with a gravel
or crushed-stone blanket to contain the migration of fines.
