The Influence Of Environment On The Brittle Failure Of Rocks

Heard, H.C. (Lawrence Radiation Laboratory, University of California)


The review by Professor J. C. Jaeger (Chapter 1) briefly summarizes and discusses a select group of papers by various workers dealing primarily with aspects of fracture in brittle rock materials. Professor Jaeger's treatment is primarily concerned with the correlation of phenomenological and theoretical failure criteria with empirical results from short duration tests at ambient temperatures and generally at low lithostatic and interstitial fluid pressures.
As engineering rock mechanics becomes more concerned with new techniques of mineral exploration and production at increasingly greater depths in the crust, it is well to be aware of the effect of the environment --that of the superposed stress field upon the lithostatic and interstitial fluid pressure, the temperature and the duration of the loading upon the failure of rocks either by fracture or flow. All results to be discussed here are from laboratory tests on small homogeneous samples, thus caution should be used in the direct extrapolation of these results to large inhomogeneous rock masses having quite different boundary conditions in the earth. The purpose of this supplement is to briefly point out the major effects of these parameters as examined singly, with all others held constant, and to cite some recent results which apply to the main theme on the brittle failure of rocks.
The total stress field imposed upon a small volume of porous and permeable rock in the earth's crust may be thought of as the sum of an applied differential stress, plus a lithostatic pressure, plus an interstitial fluid pressure. The differential stress portion may be completely specified by nine components, six shear and three normal. If the small volume element is in rotational equilibrium, by suitably choosing our coordinate axes we may resolve these nine components in to the three principal components. Since neither the lithostatic pressure nor the interstitial fluid pressure* contain shear components, the fluid pressure may be subtracted from the lithostatic to give the effective lithostatic pressure (see Terzaghi 1) and this in turn added to principal components from the differential stress to give the three principal effective stresses.
Brittle failure of rock as evidenced by either extension (tension) or shear fracture may be enhanced or suppressed in all rock types by suitably altering either the differential stress, or the lithostatic or the interstitial fluid pressure. This is reflected in the absolute values of the three principal effective stresses at failure as well. Typically, at the lower values of differential stress, low lithostatic and high interstitial pressures (equaling relatively low principal effective stresses), the rock is weak, being nearly elastic up to its rupture point and failure is by extension fracture normal to the least principal effective stress, s3. As the differential stresses and the lithostatic pressure are increased or as the interstitial pressure is decreased, the rock becomes stronger and more ductile. Failure at these conditions takes place by a combination of the extension fracture (normal to s3) and shear fracture which is inclined at an angle from a few degrees to less than 45 degrees to s1.