Measuring Crustal Stress: Borehole Methods

Abstract

Stresses within the Earth’s crust are “measured” indirectly by coring, slotting and loading a piece of rock with subsequent analysis of re-equilibrium deformations. This action requires assumptions about constitutive behaviour of the rock, i.e. the relationship between measured strain and inferred stress (Eq. (4.2) for anisotropic, Eq. (4.8) for isotropic rock). In addition, Eq. (4.8) includes the effect of temperature on mechanical stresses. If the location of stress measurement is chosen to be close to natural discontinuities (fracture, fault) or excavation (borehole, tunnel) boundaries, near-field stresses are determined (Sect. 4.4). The virgin or in-situ stress field can only be observed at distances of two to three times the size of the excavation discontinuity or any other stress concentrator (inclusion).