Environmental Engineering Reference
In-Depth Information
minor (i.e. typically less than 10%) reduction in seepage due to increased seepage
path length or, more commonly, holes which are much deeper than required to reach
lower permeability rock, particularly in the river section of the foundation where the dam
is highest.
For most dams, grouting will extend up the abutments to where full supply level inter-
sects the base of the permeable zone. However, where the dam abuts a spillway, the grout
curtain may be connected into the curtain under the spillway and/or if the dam abuts a rel-
atively narrow permeable ridge, the grout curtain may be extended into the ridge. (Figure
18.15a).
Extension of the grout curtain further into an abutment is only necessary if the stabil-
ity of the ridge or abutment is in question and there is a need to control piezometric pres-
sures, or if the abutment is highly permeable.
In such highly permeable rock, careful consideration must be given to the lateral extent
of grouting, or seepage may bypass the end of the grout curtain. Figure 18.15(b) shows
the grout curtain for a 20 m high water storage dam in coal measure rocks, which have
been intruded by a highly permeable dolerite sill. The grout curtain was successfully con-
structed with excellentclosure, but when the dam was filled the reservoir leaked signifi-
cantly (water level dropped 25 mm/day) because water was entering and flowing along
the dolerite sill, completely bypassing the ends of curtain. This groundwater movement
was proven conclusively by piezometers.
Figure 18.15.
(a) Example of grout curtain extended beyond dam crest; (b) Example where seepage
bypassed the end of the grout curtain.
