Page 514 - Rock Mechanics For Underground Mining

MINING-INDUCED SURFACE SUBSIDENCE

Major examples of sinkhole formation have occurredin the dolomitesand dolomitic
limestones overlying the gold-bearing Witwatersrand system of rocks in the Far West
Rand, South Africa. As discussed in section 3.2 and illustrated in Figure 3.5, the
area is divided up into a number of essentially water-tight compartments by a series
of vertical, syenite and diabase dykes. In 1960, a major de-watering program of the
Oberholzer compartment was begun. A series of sinkholes developed co-incidentally
with the lowering of the water table. As noted previously, the most disastrous case
involved the loss of 29 lives when a crusher chamber suddenly collapsed in December
1962. It was found that sinkhole development was accentuated by the presence of
excess surface water from heavy rainfall or a burst pipe.
The exact mechanism of sinkhole formation under these conditions is difficult to
establish. A plausible set of conditions that must be necessarily and sequentially
satisfied if sinkholes are to form was postulated by Jennings et al. (1965). The basic
concept is that a sinkhole will form when the equilibrium of a stable arch of material
above the void is disturbed (Figure 16.9).

(a) There must be adjacent stiff material to form the abutments for the roof of the
void. These abutments may be provided by the dolomite pinnacles or the sides of
steep-sided, subsurface canyons. The span must be appropriate to the properties
of the bridging material. If the span is too large, the arch cannot form.
(b) Arching must develop in the residuum with part or all of the self-weight being
transferred to the abutments as arching thrusts.
(c) A void must exist or develop below the arched residuum. The void may be quite
small initially, but may be enlarged by percolating water following lowering of
the water table.
(d) A reservoir must exist below the arch to accept the material which is removed as
the void is enlarged. Some means of transporting this material, such as flowing
water, is also essential.
(e) When a void of substantial size has been established in the residuum, some
disturbing agency is required to cause the roof to collapse. Water can cause
collapse of the arched material and the formation of a sinkhole by reducing the
soil strength or by washing out critical keying or binding material.
In discussing the mechanism proposed by Jennings et al. (1965), De Bruyn et al.
(2000) re-emphasised the point that the development of sinkholes is accelerated by
artificially lowering the water table. The size of a sinkhole will depend not only on
the depth of the water table but on the depth of the residuum. A large sinkhole will
not develop where the water table is high or the residuum is thin.

16.4 Discontinuous subsidence associated with caving methods of mining

16.4.1 Block caving
The mechanics of the block caving method of mining and the factors influencing the
development and propagation of the cave were discussed in Chapter 15. Obviously,
block caving results in discontinuous subsidence which influences large areas at
the surface (Figures 16.10 and 16.11). If, as is often the case in block and panel
caving mines, the orebody is vertical with a well-defined cut-off between it and
the surrounding country, rock, the cave will propagate vertically to the surface with
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