Environmental Engineering Reference
In-Depth Information
Figure 2.19.
Idealised weathered profile in granitic rocks. (Ruxton & Berry, 1957).
The cracks which isolate the shells are like small scale sheet joints and are probably caused by
relief of residual stresses and other stresses set up by capillary, osmotic and other chemical
weathering processes.
Figure 2.21
shows what initially appears to be an excellent example of the idealised
weathered granitic profile of Ruxton and Berry (1957), exposed in elevation view, by ero-
sion. The upper slopes show mainly extremely weathered granite (soil properties). In the
lower, rock slope this material occurs between corestones and becomes progressively less
abundant, and is eventually absent in the outcrops close to water level.
The actual situation on this hillside is not so simple. Near the centre of the photograph,
on the skyline and elsewhere at intermediate levels on the slope, hidden by trees, there are
granite boulders and areas of what appear to be outcrops of essentially fresh granite.
Excavations made into slopes showing similar surface evidence have shown subsurface
profiles as on
Figure 2.22
. At its right hand edge this figure shows a weathered profile
similar to that of Ruxton and Berry (1957) except that the extremely weathered material
extends locally much deeper along and next to the fault. Near the centre the profile at
depth is also similar to that of Ruxton and Berry (1957). However, outcrop area A and
the boulders B occur at the ground surface and are underlain by 7-10 m of extremely
weathered material. On the left side, outcrop area C looks similar to area A but is con-
tinuous downwards into mainly fresh bedrock.
The differences between the profiles A, B and C raise the following questions:
-Are outcrop A and boulders B really
in situ
?
-
If they are
in situ
, then why don't they continue downwards into progressively less
weathered rock mass as is the case at outcrop area C?
It can usually be shown from the continuity of the joint and fault pattern within the
extremely weathered rock, whether or not masses of rock such as A and B are
in situ
. In
Figure 2.22 the continuity of coated joints is clear evidence that both are
in situ
. It is often
not possible to provide an unequivocal answer to the second question. One possibility is
that the extremely weathered rock under A and B was partly decomposed by chemical
alteration (Section 2.7) before becoming weathered. Another is that the rock substance
