Direct measurement of the effective pressure law: Deformation of joints subject to pore and confining pressures

Abstract

When describing the deformation of poro‐elastic materials subject to pore pressure (PP) and confining pressure (PC), the concept of effective pressure is commonly used. In such a description the deformation is described in ternis of a single stress parameter, the effective stress (PE). Experimental studies which attempt to describe the effective pressure law are troubled by the fact that deformation of geologic materials invariably exhibits loading path dependence (hysteresis). Properties which exhibit hysteresis are not easily described by a single stress state variable and hence the concept of effective pressure becomes clouded. Therefore, the effective pressure law here is formulated to describe the relative effects of pore and confining pressure on a given property, with specific attention to the loading path. Here we develop an experimental technique for measuring the effective pressure law which is useful for many properties of interest, including those that are highly nonlinear and exhibit common types of hysteresis. Applying this method in an experimental study of the effective pressure law for joint closure, we experimentally derive an effective pressure law which describes the values of pore and confining pressure consistent with a given joint closure for a loading path of constant closure. The study can be viewed as an attempt to include both pore and confining pressure in a single constitutive law for joint closure. The constant closure loading path is such that the measurement is not affected by hysteresis caused by joint closure. The results provide insight into the microgeometrical and micromechanical properties of joints. The data are not consistent with a simple extension of commonly used linear elastic constitutive models for joint deformation which have compared favorably with experiments in the absence of pore pressure. For smooth lapped glass joints, the effective pressure relation is found to be dependent on the local joint stiffness, with the relationship between the effective pressure law and the local joint stiffness being insensitive to the measured surface topography. Similar measurements on lapped and fractured rock provide some constraints on the effective pressure behavior of jointed rock.