GROUND MOVEMENT

Nature of Problems

When too much rock is taken out of underground openings, part or all of the openings collapse. Everyone knows this, of course, and part of the layman's distaste for going underground arises from apprehension lest the mine "cave in." The miner's contempt of this fear, arising from familiarity with walls and roof that stand year after year, sometimes leads him to imagine that a rock which "stands well" will con¬tinue to do so no matter how much support he removes. The problem is to estimate what size and shape of opening may be excavated without serious danger, to foresee hazardous conditions, and to devise ways of preventing ground movement or at least of minimizing its disastrous effects. These are problems in mining engineering, but as Graton says, "inasmuch as it is the rocks that fail, it would seem that much useful light on the phenomenon . . . can in the future be shed by the geologist, the particular specialist in rocks."

The mode of failure of mine workings depends somewhat on the nature and structure of the rock—its inherent strength-characteristics and the attitude of its surfaces of weakness. But if these factors are constant, the mode of failure varies with depth. Probably much of the disagreement among writers on rock failure would be cleared up by a distinction in mechanism between failure at shallow depth and failure at great depth.

At shallow depth the environmental or confining pressure is small and the stress distribution is strongly influenced by the proximity of a free boundary (the earth's surface). At great depth, confining pressure increases the strength of the material (or so experiments would suggest) and the influence of the free boundary at the earth's surface is practically negligible.

Accordingly, failure at shallow depth takes place predominantly along joints, bedding planes and other surfaces of weakness and assumes the form of simple collapse of the overlying and adjacent material under its own weight. But at depths beginning at 3000 to 5000 feet, failure partakes more of the nature of slabbing from walls and crushing of both horizontal and vertical pillars. It is determined more by the shape of the opening than by pre-existing surfaces of weakness. As pressure effects tend to be all-sided, rather than merely vertically down-ward, failure occurs from the sides and floor as well as from the roof.

It is convenient, therefore, to describe effects of these contrasting modes of failure under the two separate headings, "Subsidence" and "Rockbursts," even though at intermediate depth they overlap and merge, and even though failure at depth is not always as violent as the term "Rockbursts" might suggest.