Geology Reference
In-Depth Information
Caverns are large-span underground openings and these are used for
many purposes, including sports halls, power stations and oil and gas
storage (Sterling, 1993). Hydroelectric power caverns and large three-
lane road tunnels are typically 20 to 25m span, but caverns have been
constructed successfully in good-quality rock with spans in excess of 60m
(Broch
et al
., 1996), and natural caves are found with much larger spans.
There is considerable guidance in the literature on approaches to
their design and construction (e.g. Hoek&Brown, 1980; GEO, 1992).
Many design issues are similar to tunnels but because they are at
xed
locations, ground investigation decisions are more straightforward.
The other major difference is scale. Whereas many tunnel walls lock
up as the rock dilates, and need little support to ensure stability, in a
cavern there is more potential for large-scale strain and failure mechan-
isms to develop. For example, large caverns were required for a pro-
posed high-speed rail station at Taegu, Korea, in strong mudstone.
Preliminary numerical analyses were carried out to design permanent
concrete liners and bolting support, assuming essentially isotropic rock
mass parameters. The design had to be revised when it was realised
that the rock structure was strongly anisotropic with bedding mostly
horizontal and many near-vertical tensile joints in
lled with calcite
(Figure 6.20)
. These joints could allow discrete failure into the crown
of the openings, as illustrated by Maury (1993) for mine workings.
Figure 6.20
Rock core from vertical borehole in strong mudstone, Taegu,
South Korea. Note near-vertical persistent joint in
lled with calcite. This
network of joints (two sets orthogonal to bedding) were encountered
frequently in preliminary boreholes, and appreciation of their signi
cance led
b) additional ground investigation using inclined rather than vertical holes to
characterise the rock mass better.
