Environmental Engineering Reference
In-Depth Information
were initiated as shear fractures by the moving ice (i.e. by “glacitectonic thrusting”) and
then opened up by ice wedging. The authors have found similarly fractured rock with
open and infilled joints at the bases of many valleys which have not been subjected to
glaciation (see Chapter 2, Section 2.5.4) and suggest that the effects seen by Knill may
have been formed largely by stress relief.
Regardless of their origin, the presence of such features at many sites means that the rock
next to the base of glacial deposits is likely to be of poor quality e.g. in terms of compress-
ibility, permeability and erodibility. Where the existence of such poor quality rock would be
of significance to the stability and/or watertightness of the dam it is important that this “rock-
head” zone be investigated thoroughly.
Money (1985) draws attention to the difficulties often encountered while doing this by core
drilling and mentions cases where large boulders have been mistaken for bedrock. He refers to
UK practice at that time which was to recommend that the in situ rock be proven by a mini-
mum of 3 m of cored rock. He states that this figure is likely to be inadequate and that it is cer-
tainly not enough to allow for adequate permeability testing of the upper part of the bedrock.
The authors agree and suggest that the actual depth of coring needed will depend upon:
-The inherent fabric of the bedrock (i.e. is it massive or too well-cleaved or closely
jointed for it to have formed large boulders?);
- The quality of the core samples and the extent to which core orientation can assist in
assessing whether it is in situ or not. Core orientation may be determined either by
impression packer or orientation device or by the presence of bedding or foliation with
known, consistent orientation;
- The actual depth of the disturbed zone.
Difficulties in delineation of the top of in situ rock can occur also where the rock in this
zone has been chemically weathered. This situation exists at the site for Kosciusko Dam
in New South Wales ( Figure 3.52 ).
It appears that intense weathering has occurred in both the bedrock and the till.
Geological surface mapping, track exposures and boreholes on the right bank show that
most of the upper 5-20 m of the bedrock comprises residual “boulders” of fresh to
slightly weathered granite set in a matrix of highly to extremely weathered granite which
is mainly a very compact silty, clayey sand.
A shaft close to the creek on the left bank ( Figure 3.53 ) showed similarly weathered
granite boulders set partly in a matrix of gravelly clay (till) and partly in glaciofluvial
sands and clays. It was clear that without very good recovery of little-disturbed core, these
materials could not be readily distinguished from the in situ weathered “bouldery”
sequence. At 8.6 m these materials rested on extremely weathered rock whose mineral
content and foliation attitude matched the known bedrock on the right bank. This weath-
ered rock was therefore inferred to be in situ .
Holes drilled elsewhere on the left bank recovered about 20% of fresh or slightly
weathered granite in “boulder” lengths, but little or no matrix. Hence the upper surface
of the in situ rock (assuming that it was more than slightly weathered) could not be deter-
mined here from the drilling results. The top of mainly fresh granite was inferred from the
drill cores together with the results of refraction seismic traverses (Figure 3.52).
Glastonbury (2002), Glastonbury and Fell (2002c) note that many of the large rapid land-
slides which have occurred in historic times have occurred in valleys which have been
glaciated, and attribute this to the stress relief effects on the valley sides as the glaciers retreat.
3.12.3
Glaciofluvial deposits
As well as depositing some glaciofluvial materials in their immediate vicinity (as discussed
in Section 3.12.2) glaciers release very large meltwater flows giving rise to deposition of
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