Environmental Engineering Reference
In-Depth Information
Figure 3.26.
Close up view of limestone pinnacles.
soil with a concave upwards layered structure. It can be inferred that this sinkhole has
developed because the residual soil has migrated slowly into the cavities in the limestone
below.
Feature (c) is a large flat area showing residual soil at the surface. It is underlain at
depth by cavities developed along joints and a fault zone. Such flat areas may show no
obvious evidence of settlement or disturbance, but there is a significant risk of a deep,
steep-sided sinkhole forming (with little or no warning) at the ground surface. This could
be initiated by migration of soil into the cavities below, causing a large void to develop near
the base of the residual soil. The sinkhole could then form by collapse of the overlying soil
into the void. A similar dangerous sinkhole could also develop within the gently sloping
sinkhole (b).
The mechanisms of sinkhole formation and their significance in dam engineering are
discussed further in Section 3.7.3.
3.7.1.2
Rock masses composed of dense fine grained rock substance containing 10%
to 90% of carbonate (usually Category O)
In these rocks the pattern of solution cavities is similar to that described in Section 3.7.1.1
and seen in
Figure 3.27
, but usually the rock substance next to some of the cavities is
weathered. Experience and logic show that in general the lower the percentage of car-
bonate in the fresh rock the higher the proportion of weathered rock formed compared to
cavities and the higher the ratio of infilled cavities to open cavities. The weathered rock is
usually much weaker and less dense than the fresh rock. However, the proportion and
properties of weathered rock formed depend also on the percentage of insoluble particles
present in the fresh rock and the degree to which these particles are bound by non-soluble
