Assessing the deformability of rock at the bottom of a vertical bore

Abstract

Sedimentary rocks, associated with mineral extraction, come into a state of geostatic equilibrium following their deposition. Owing to stress relaxation, the lateral pressure coefficient X can be taken as unity, and the state of stress of the rock regarded as hydrostatic. In hydrostatic phenomena liquids and solids behave as if they were ideally elastic [1]. Thus, undisturbed native rock can be regarded as elastic, and the components of the stress at any point are governed by the weight of the superincumbent rock. The cutting of a working will cause changes in these stresses in the rock of the face.Let us consider a vertical shaft, of depth z, with a circular cross section of radius a, assuming that we know the mechanical properties of the rock and the nature of the circulating liquids filling it. We will take the lateral pressure coefficient X as unity [2]. (Note that this method can be used for any lateral pressure coefficient.) We have to determine the state of stress of the rocks at the bottom, and also the conditions of formation and propagation of the plastic zone in relation to depth and the mechanical properties of the rocks and the circulating fluids, allowing for the geological conditions.

How high pressures affect the deformation properties of sedimentary rocks

“…As a result of the hydrostatic pressure, an additional load is exerted on the face of the mine working, preventing formation and development of this zone; this increases the breaking strength, and prevents formation of craters by flaking off [?]. Thus the plastic properties of rocks increase with depth, which increases the energy expended on fracture [8][9][10] and reduces the drilling efficiency.…”