Page 513 - Rock Mechanics For Underground Mining

SINKHOLES IN CARBONATE ROCKS

caving takes place through a very weak rock mass, a residual soil, or rock previously
disturbed by a caving method of mining. Even though a limiting equilibrium analysis
may indicate that chimney caving to surface is possible, the development of the cave
can be influenced by draw control in the stope. Goel and Page (1982) use draw density
and geometrical parameters in an empirical method for predicting the probability of
chimney cave occurrence over a mining area. Numerical methods using the finite
difference code, FLAC, or distinct element methods, for example, may also be used
to model chimney caving.

16.3 Sinkholes in carbonate rocks

Aformofdiscontinuoussubsidencethathasasimilarsurfaceexpressionbutadifferent
cause from the various types of chimney caving, can occur in carbonate rocks such
as dolomites and limestones. These rocks are susceptible to solution by slightly acid
waters percolating initially through discontinuities in the rock mass. With time, quite
large volumes of rock can be dissolved leading to what are known as karst features.
These carbonate rocks are also susceptible to deep and irregular surface weathering
which produces a highly variable depth to rock head (Figure 16.9). The mantle of
unconsolidated materials may include residual clay or residuum resulting from the
weathering of the rock.
Cavities develop in carbonate rocks, generally above the water table, where surface
water containing small amounts of dissolved gases which render it slightly acidic,
flows downwards towards the water table. Experience in mining and civil engineer-
ing has been that sinkholes develop from these cavities when the rock mass is de-
watered and the water table is lowered. Lowering the water table has several effects.
It permits existing caverns to enlarge and may cause new ones to be developed, it
removes buoyancy support forces, and it can increase the velocity of downward water
movements which can then become sufficient to wash out unconsolidated surface
materials.

Figure 16.9 Section showing gro-
und susceptible to solution-induced
subsidence (after Jennings et al.,
1965).

495