Environmental Engineering Reference
In-Depth Information
necessary to collect the seepage water downstream of the dam, either in a catch dam or in
seepage collector wells and pump it back into the storage.
A minimum grout hole spacing of 1.5 m is recommended because, if closer spacing is
required to achieve a Lugeon value standard, the width of the resulting grout curtain is too
narrow to significantly reduce seepage. Generally it would be better to control slope pore
pressures by drainage rather than grouting. Clearly there may be exceptions where large
takes are encountered in these holes, but as a general rule grouting with holes at closer than
1.5 m spacing is a waste of time and money. It may also be a pointless exercise, given the
potential for percussion-drilled holes to deviate from their planned position. At Thomson
dam (Victoria), one 90 m deep hole was shown to be 27 m off line, which does pose the ques-
tion as to the value of a deep single line curtain of closely-spaced holes.
The authors are of the opinion that it may be justified to reduce hole spacings below
1.5 m in some deeply weathered and closely fractured saddle dam foundations, where
erodibility and control of seepage pore pressures may be critical for slope stability.
It should be noted that adoption of larger Lugeon value closure criteria at depth and/or
higher grout pressures at depth may lead to secondary and tertiary holes not penetrating
to the same depth as the primary holes.
A more detailed discussion on the selection of closure criteria is given in Fell et al.
(1992).
18.2.5
The depth and lateral extent of grouting
So far as is practicable grout holes should be taken to the depth at which the Lugeon clo-
sure criteria are achieved. In near horizontally layered rock this might be clearly identifi-
able as a lower permeability zone of rock beneath the valley floor, e.g. siltstone within
interbedded siltstone and sandstone, but may be at different depths around the dam abut-
ments due to the influence of stress relief, weathering and rock types. Figure 18.14 gives
an example where the stratigraphy is simple and clearly known.
The depth of higher Lugeon value rock is seldom known with any accuracy and the
grout holes must penetrate to at least one stage below the estimated base level to prove
that the necessary conditions have been met.
The use of “rules of thumb” to determine the depth of grouting is not recommended as
there is no logical basis for them.
If such rules are applied they will either yield holes which are too shallow, resulting in
a grout curtain which only partially penetrates the permeable foundation, giving only
Figure 18.14.
Example of grout curtain in a simple sedimentary rock environment.
