Environmental Engineering Reference
In-Depth Information
C C
A
block 15
B
196.5m
headwater
wedge plug
block 15
block
16
block 16
02m
Scale
displaced 1 m laterally
C
C
tailwal
zoned
backfill
0
20m
filter
fault zone
Scale
drainage layer
A
A A
B B
Figure 17.22.
Clyde Dam showing design of slip joint.
movement and up to 2 m of strike-slip movement. The design is described by Hatton and
Foster (1987) and Hatton et al. (1991).
The foundation contains many other faults, mostly sheared, partly crushed zones. Some
(locally termed foliation shears) occur parallel to the gently dipping foliation, and others
cut across the foliation at steeper angles. Mapping of excavated foundations and
exploratory drives during construction showed that some of the foliation shears could
provide potential failure surfaces (Hatton et al., 1991; Paterson et al., 1983). These were
treated either by local over-excavation and deepening of concrete structures (
Figure
17.23
) or by concrete shear keys placed in adits (
Figure 17.24
). Foliation shears with
downslope components were exposed in the right abutment excavation for the dam. Some
of these contained sandy infill of alluvial origin. Dilation of the rock mass due to past
downslope creep was inferred. Four of these shears were explored by adits which were
later backfilled with concrete to act as cutoffs.
17.8.4
Treatment at sites formed by highly stressed rock
Excavation to reach foundation level in highly stressed rock should be done bearing in
mind the following:
- Exposed highly stressed rock invariably has sheet joints as its surface and a succession
of sheet joints below (see Figure 2.5);
- Any sheet joint below the base of a concrete dam has some potential to form all or part
of a downstream sliding surface;
- When excavations are made into highly stressed rock, it is possible that some existing
sheet joints may open up further and propagate, and some new sheet joints may form
(see 2.5.4). Such effects are much more likely to occur if blasting is used.
At Burdekin Falls Dam in Queensland, the approaches to selection of foundation levels
and to excavation methods were developed with understanding of the above issues. The
