Environmental Engineering Reference
In-Depth Information
Thus in
Figure 18.1
primary holes are drilled first, followed by secondary and then tertiary.
The final hole spacing will commonly be 1.5 m or 3 m, but may be as close as 0.5 m. This
staged approach allows control over the amount and effectiveness of the grouting.
Grout pressures are usually (at least in Australia, USA, United Kingdom) limited to pre-
vent hydraulic fracture of the rock. The discussion in this chapter is predicated on that
assumption. In some projects particularly in continental Europe, the rock has been delib-
erately fractured to improve grout penetration.
Consolidation or “blanket” grouting for embankment dams is designed to give inten-
sive grouting of the upper layer of more fractured rock in the vicinity of the dam core, or
in regions of “high” hydraulic seepage gradient, e.g. under the plinth for a concrete face
rockfill dam. It is usually restricted to the upper 5 m to 15 m and is carried out in sequence
but commonly to a predetermined hole spacing and depth.
Figure 18.2
shows a section through a concrete gravity dam. The curtain grouting is
located near the upstream face of the dam and usually carried out from a gallery in the
dam; sometimes from the upstream heel of the dam. It is designed to reduce seepage
through the foundations and, in conjunction with the borehole drain holes, to control
uplift pressures.
18.2
GROUTING DESIGN - CEMENT GROUT
18.2.1
Staging of grouting
Grouting of holes is normally carried out in stages, the method depending on the perme-
ability and quality of the rock being grouted and the degree to which control of the grout-
methods available.
Downstage Without Packer
(Figure 18.3): This is one of the preferred methods for high
standard grouting, since each stage is drilled and grouted before the next, lower stage,
allowing progressive assessment whether the hole has reached the desired closure require-
ment. This method allows higher pressures to be used for lower stages, as it reduces the
risks of leakage from them up the top stage levels and gives better grout penetration
between holes. The grout pressures are limited by the effectiveness of the top stage grout-
ing. It does necessitate a separate set up of the drill for each stage and separate “hookups”
of the grout lines. It is, therefore, relatively expensive. This method is the one preferred by
Houlsby (1977, 1978, 1982a).
Downstage With Packer
: (Figure 18.4): This method allows use of increased grout pres-
sures for lower stages, since these pressures are not applied from the surface. However,
there may be problems with seating and leakage past the packer. Bleeding of the grout
hole (i.e. removing the “clear” water which accumulates at the top of the grout hole as the
cement settles) cannot be achieved except at the ground surface (i.e. not immediately
above the grout stage). Ewart (1985) indicates a preference for this method, because of
the potential to fracture the rock in the upper levels if downstage without packer methods
are used.
Upstage
(Figure 18.5): Does not allow progressive assessment of the depth of grout hole
needed to reach a desired closure requirement as the holes are drilled to their full depth in
one stage. The method is cheaper in principle than downstage methods since the drill rig
is only set up once, but these savings may be offset by the need for more conservative total
depths. The method is susceptible to problems with holes collapsing, or over enlarging
during drilling and grouting in poor rock conditions making seating of packer difficult or
impossible. It is also subject to the same problems as the downstage with packer method
regarding bleeding. The method is appropriate for secondary or tertiary holes, when
