►

Total stresses

These describe the external loading on an element of rock. The most obvious example is the vertical total stress resulting from the weight of the overlying strata. This stress can be determined directly from the depth and the average density.

Another (directly measurable) value is the closure pressure in a hydraulic frac. This corresponds to the magnitude of the minimum component of total in situ stress.

►

Pore pressures

These are the pressures exerted by the fluids which most rocks contain. Because of the real rock structure the pore pressure is mechanically only partially effective, and the degree to which it is effective generally varies with direction. This reduction in its mechanical effectiveness is described by a directional parameter, the pore pressure effectiveness.

►

Effective stresses

These stresses always describe the relevant loadings for the evaluation of deformations and of stability. They are defined as the (direction dependent) difference of the total stress and the effective pore pressure.

►

Changes of the in situ stresses

Changes in the in situ stresses can occur following changes made in the pore pressure, and independent of the local secondary stresses forming around underground openings. Knowledge of the changes of in situ stress is essential for prior assessments to be made of in situ deformations and of the related modifications of porosity and permeability.