ABSTRACT
This review is focused on the influence of porosity f and permeability k on the mechanical behavior of rocks and rock masses and on ways in which rock mechanicists are contributing to the prediction of f and k at depth in the Earth's Crust. Major conclusions are as follows: (1) The law of effective stress utilizing 100% of the pore fluid pressure holds for most practical purposes, provided the porosity is communicated and the permeability, relative to loading-rate, permits equilibration of the pore pressure. (2) The decrease in strength with increasing porosity is well documented for sandstones, siltstones, and a few limestones. This trend follows the general form: ss = a fb, where ss is ultimate strength and a and b are constants. For granular aggregates, this trend is predictable through use of Hertzian theory. (3) Interpretation of hydrofracturing records requires knowledge of both f and k, especially in light of recent experiments on the rate of pressurization. (4) Strategies for earthquake control through adjustment of fluid pressures at depth rely on the permeability of the fault-gouge-host-rock-system. Pertinent data are meager, but suggest that communication rates are rapid when wells are connected by fractures. On the other hand, it may take several months to achieve desired increments of pore pressure throughout a rock mass on the scale of km3. (5) Analytical and numerical methods for quantatively predicting fluid flow through fractures have reached a high degree of sophistication, but still the measured flow rates often differ significantly from predicted ones. The influence of effective confining pressure on fracture f and k is just now emerging from experimental work. (6) At least six empirical, theoretical, and experimental approaches to the prediction of f and k at depth are being pursued. Porosity reduction with depth is better documented than that for permeability. Purely mechanical compaction sets maximum limits on f and k for a given depth, but precise prediction requires a better understanding of physico-chemical processes. 7) Study of the mechanical and physicochemical mechanisms of compaction through experimentation and optical and scanning electron microscopy have and will probably prove rewarding.
INTRODUCTION
The mechanical behavior of a rock or rock mass results from interaction between its intrinsic properties and the physicochemical environment at the time of deformation. Two of the most important intrinsic properties are porosity and permeability. In this review, two aspects of their relation to rock mechanics are discussed, namely: (a) how do porosity and permeability influence the mechanical behavior of rocks and rock masses, and (b) what is being done within the discipline of rock mechanics to help predict the porosity and permeability of the rock mass. After a brief introduction to porosity (f) and permeability (k), their influence on rock behavior will be summarized through discussion of the effective- pressure concept, the variation of strength with f, hydraulic fracturing, and faulting and earthquake control. Then ways to predict f and k will be considered with emphasis on the results of triaxial testing, compaction experiments, associated analytical models, and the mechanisms of porosity and permeability reduction.
