Environmental Engineering Reference
In-Depth Information
Standpipes will always be required where the rock near the surface is fractured or weak,
precluding use of a packer. Standpipes have the advantages of preventing clogging of the
hole by debris from adjacent areas and providing easy hook-up for grouting. Note that
the part of the standpipe projecting from the surface has to be removed before placing
earthfill.
18.3.3
Grout caps
Concrete grout caps (Figure 18.18) are required when grouting closely fractured or low
strength rock in the situation where a standpipe cannot be sealed into the foundation
and/or leakage of grout to the surface will be excessive. The grout cap also provides a bet-
ter cutoff through the upper part of the foundation than is practicable with grouting from
standpipes.
Grout caps should be excavated into the foundation rock without explosives and if
practicable should be of square or rectangular section as shown in Figure 18.18(a). This
gives good resistance to lifting of the cap under the pressure of grouting.
The wider/shallow shapes shown in Figure 18.18(b) and (c) are prone to displacement
during grouting and may need to be anchored into the foundation with steel dowel bars
grouted say 2 m into the rock.
Some organisations, e.g. Rural Water Commission of Victoria, have favoured construc-
tion of a grout cap in all foundations, constructed above the cutoff level as shown in
Figure 18.18(c). This arrangement requires anchor dowels in all cases. The arrangement
is justified on the basis of allowing grouting of all the rock, i.e. right to the surface, and
increasing the resistance to erosion along the cutoff surface by providing a more tortuous
path for water, but has the disadvantage that it interferes with compaction of earthfill in
the cutoff trench. Such grout caps are not recommended by the authors.
In closely fractured rock there is often some advantage in applying a layer of slush con-
crete or shotcrete to the cutoff surface before grouting. This allows use of slightly higher
grout pressures in some rock types, prevents excessive leakage to the surface and gener-
ally facilitates grouting. It also prevents damage to the cutoff surface by construction equip-
ment during the grouting operation. Increasing grout pressures even with a surface cover
Figure 18.18.
Grout caps (a) ideal shape (b) poor shape (c) grout cap above cutoff level.
