Environmental Engineering Reference
In-Depth Information
Figure 2.20.
Granitic boulders or corestones showing spheroidal weathering effects. Photo courtesy of
Dr. R. Twidale.
forming A and B may be for some reason more resistant to chemical weathering than that
which occurred initially adjacent to and below them. Yet another is simply that the joints
which surrounded these rock masses were less permeable than those elsewhere, allowing
much less groundwater percolation.
Although it is not often possible to understand the reason for such anomalies in weathered
profiles in apparently “uniform” igneous rocks, it is very important to appreciate that they
occur quite commonly, and hence to expect them and allow for them in site investigations.
Figure 2.23
shows the nature of part of the weathered profile developed at the spillway of
Split Rock Dam, New South Wales. The spillway excavation below the depth of ripping
refusal was the designated quarry to supply rockfill for the dam. Alternating beds of
greywacke, greywacke breccia and siltstone dip gently into the hillslope. The greywacke is
extremely strong when fresh and relatively resistant to chemical weathering. The siltstone is
strong to very strong when fresh, but is more susceptible to chemical weathering than the
other rocks. The resulting profile, with beds of fresh greywacke (extremely strong) over-
lying variably weathered siltstone (weak rock grading to soil properties) made it difficult to
meet some rockfill requirements, with normal quarrying methods.
