Environmental Engineering Reference
In-Depth Information
2.9.5
Rapid solution
Rocks which contain appreciable amounts of evaporite minerals (gypsum, anhydrite or
halite) as cement or matrix, may be weakened rapidly if water is allowed to pass through
them. James and Lupton (1978), James and Kirkpatrick (1980) and James (1992) provide
predictive models for solution rates, and guidelines for investigation of dam sites where
these minerals are present. These are discussed briefly in Chapter 3, Section 3.8.
2.9.6
Surface fretting due to electro-static moisture absorption
Within 3 years of the completion of Kangaroo Creek Dam, it was observed that fretting
was occurring on the undersides of some large blocks of quartz-sericite schist exposed on
the downstream face. Many of these blocks did not appear to contain sulphide minerals,
and the fretted flakes did not have the characteristic taste of sulphate salts.
West (1978) confirmed that much of the deterioration of the exposed schist blocks was
not caused by the sulphide/sulphate effects described by Trudinger (1973). He demonstrated
that the likely cause was cyclic adsorption-desorption by the near-surface few millimetres of
the schist, of moisture from the air, in a semi-confined or sheltered environment. The mois-
ture changes resulted in expansion and contraction, probably of the sericite, in this near-
surface layer.
2.10
LANDSLIDING AT DAM SITES
At the site of any dam located across a river it is quite common to find evidence of past
landsliding on at least one side of the valley. This is not surprising because most river val-
leys will have developed by some combination of the following processes:
(a) erosive downcutting by the stream, which unloads the materials underlying the valley
floor and slides and causes the valley sides to be steepened;
(b) mechanical weathering processes (resulting from a) which weaken and usually
increase the permeability of the materials under the valley floor and sides;
(c) chemical weathering processes which further weaken the materials forming the valley
sides, converting near-surface rocks partly or wholly to soils and generally lowering
their mass permeability, and
(d) soil creep, erosion, deposition, and rockfalls on the valley sides, giving rise to deposits
of slopewash and scree, and to overall flattenning of the valley slopes.
The overall effect of (a), (b) and (c) is to reduce the stability of the valley sides.
The valley-bottom profile on
Figure 2.27
was developed by these processes, in granitic
rocks and in a mediterranean or semi-arid climate. Because of the relatively dry conditions
and a pattern of joints which favours stability, no landsliding has occurred or appears
likely to occur.
However, if the joints were in less favourable orientations and the climate wetter, result-
ing in a higher water table, landsliding would be more likely to occur.
Figure 2.28
, taken
from Patton and Hendron (1973), shows such a situation.
Wedge A B D, bounded by relict joints in the low permeability zone of residual soil and
extremely weathered rock, may be subject to excessive water pressures from the underly-
ing more permeable zone, causing it to slide.
A much larger, deeper slide would be possible if a continuous thin seam of extremely
weathered granite parallel to surface AB was present within the more rocky Zone IIA, day-
lighting at the lower line of springs(S).
