Page 519 - Rock Mechanics For Underground Mining

DISCONTINUOUS SUBSIDENCE ASSOCIATED WITH CAVING METHODS OF MINING

Figure 16.15 Development of sur-
face subsidence at the San Manuel
Mine, Arizona, USA: (a) initially; (b)
in the more advanced stages of mining
(after Hatheway, 1968).

at Henderson, the crater continued to expand in volume, presumably as a result of
the compaction of caved material under continued static loading. As illustrated in
Figure 16.15, chimneying, piping or funneling was a feature of cave development
at San Manuel, presumably as a result of initial uneven draw. In this case, caving
developed in the following sequence:

1. Chimneying or piping propagated vertically above the initial drawpoints to the
contact with overlying conglomerate at the San Manuel fault which halted and
then deflected chimneying as shown in Figure 16.15a.
2. Vertical tension cracks formed at the surface marking the initial boundary of the
discontinuous subsidence zone.
3. The chimneys enlarged and coalesced into a wider subsidence zone as draw
proceeded.
4. With continuing draw, the caved mass of rock inside the subsidence crater moved
downwardsundergravityonsteeplyinclinedshearsurfacesasillustratedinFigure
16.15b.
5. As mining proceeded, new tension cracks and shear surfaces formed expanding
the subsidence zone.

16.4.2 Progressive hangingwall caving
When the orebody is not massive and is relatively steeply dipping, caving of only the
hangingwall need be considered. In such cases, progressive caving of the hangingwall
may result as mining progresses down-dip using sublevel caving methods of mining,
for example. A classic example of this form of discontinuous subsidence is that
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