Understanding the frictional behavior of rock discontinuities is essential for the proper design of geotechnical structures in or on rock. While there has been extensive work on the shear behavior of joints and faults under dry conditions, comparatively less research has been devoted to their behavior under saturated conditions. Yet, in the field, the rock may be below the water table and thus, it may be saturated. The objective of this study is to find the magnitude of the pore pressure needed for full saturation of a rock specimen. It is part of a larger project to determine the geophysical response of a saturated jointed specimen undergoing shear. All of the experiments were performed on specimens of Indiana limestone. Saturation was achieved by: (1) vacuum saturation; and (2) increasing the back pressure of the rock until saturation is achieved. Vacuum saturation was accomplished by placing the rock specimen under vacuum to eliminate air bubbles trapped inside the rock. Afterwards, the specimen was placed in a custom-built device that enabled both the chamber pressure and the back pressure to be gradually increased until a specimen is saturated. The degree of saturation was monitored by measuring the B-value of the specimen as the chamber and back pressures were increased. Once the B-value did not change with additional increases in back pressure, saturation was assumed to be achieved. The evolution of the B-value with back pressure suggests that a back pressure of 3.2 ∼ 3.6 MPa is needed to fully saturate Indiana limestone.

1. Introduction

A longstanding goal in rock engineering is to improve current abilities to detect and/or predict imminent failure along or from discontinuities, such as fractures, joints, and faults. It is important to develop continuous, accurate and non-destructive methods to probe inside rock to detect discontinuities and to monitor their evolution with changes in stress over time. Recent laboratory experiments have shown that active seismic monitoring may be used to detect precursors to shear failure of rock discontinuities (Hedayat et al., 2014; Hedayat et al., 2018; El Fil et al., 2019). Changes in transmitted and reflected compressional (P) and shear (S) wave amplitudes can also provide insight into the changes of mechanical properties of the rock matrix and of the discontinuities (Pyrak-Nolte et al., 1990; Chen et al., 1993; Nakagawa et al., 2000; Hedayat et al., 2014; Modiriasari et al., 2017). However, all previous experiments have been conducted on dry rock discontinuities. In the field, rock discontinuities are often found below the water table. This situation poses additional challenges, since the presence of water affects the shear behavior of a rock discontinuity (Barton, 1976), and there seems to be some experimental evidence that it also affects the joint's geophysical signature. For example, Choi (2013) reported, in jointed porous rocks, a large drop in transmitted wave amplitude with fluid invasion, while others (Pyrak-Nolte et al., 1990; Pyrak-Nolte et al., 2006) observed an increase in transmission amplitude in nonporous rocks. A possible explanation for such contradictory results might be the presence of air bubbles entrapped in the pores of the porous rocks. Thus, it is necessary to remove all the air bubbles and ensure full saturation of the rock matrix to be able to determine the mechanical and geophysical response of a saturated rock joint during shearing.

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