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Predicting the behavior of a saturated rock with variations in pore fluid pressure during geo-energy production and storage, deep geological disposal of nuclear wastes, etc. is carried out using the isothermal theory of poroelasticity that incorporates Biot's effective stress principle. Several experimental methods for determining Biot's effective stress parameter have been documented in the literature. The original definition of Biot's effective stress is constantly being extended to account for non-linear and inelastic behavior of the rock. The objective of this study is to review the fundamentals of the original experimental approach for determining Biot's coefficient and other developments, their advantages and disadvantages, and include several case studies. Current techniques are based on different premises: jacketed and unjacketed bulk moduli or compressibility values; volume changes of the bulk and pore fluid from a drained triaxial test on a saturated sample; isotropic-isochoric compression tests on a saturated sample; matching volumetric strains or failure envelopes for dry and saturated samples; variations of rock properties, such as volumetric strain, permeability, compressional and shear wave velocities, with respect to confining stress and pore pressure; estimation of the Biot coefficient from other poroelastic parameters; and approximation of the dry bulk modulus or unjacketed bulk modulus of the rock from mineralogical compositions or ultrasonic wave velocities. This article discusses variations in Biot's effective stress coefficients produced using the different techniques and how factors such as pore geometry, test conditions, stress path, and test temperature affect the Biot's coefficient.

Understanding mechanical behavior and permeability of coal at ambient and high temperature is key in optimizing high-temperature in-situ processes such as underground coal gasification. The main objectives of this study were to characterize thermal deformation, stress-strain behavior, and gas permeability of coal samples acquired from the Genesee coal mine in Central Alberta, Canada under various temperatures and confining stresses. These measurements were conducted in a high-pressure high-temperature triaxial apparatus. Initial thermal expansion of the coal was followed by contraction in both axial and lateral directions at about 140 • C. This temperature corresponds to occurrence of pyrolysis in the coal. All specimens showed brittle behavior during shear while forming complex shear planes. The specimens exhibited compressional volumetric strain responses at all temperatures. Deformation localization initiated at various stage during shearing. Specimens sheared at 200 • C showed higher peak stresses and larger axial strains compared to those tested at room temperature (24 • C). Fluctuations of permeability were observed with confining stress and temperature. Permeability dropped at 80 • C due to thermal expansion of coal and closure of initial fractures; however, it increased at 140 and 200 • C due to a combined response of thermal expansion and pyrolysis. Small axial strain during shear was observed to reduce permeability.

Predictions of fracture displacements are required to support the safety assessments of a deep geological repository for nuclear spent fuel. Laboratory and in-situ experiments are used to estimate these properties. Despite significant contributions in the last decades, there is a knowledge gap in terms of the impact of high normal stresses on the mechanical properties of large-scale fractures under Constant Normal Stiffness (CNS) boundary conditions. Within the framework of the POST project, a cooperative effort was made by SKB (Sweden), NWMO (Canada), and Posiva from Finland (in phase 1) to study these questions. In the second phase of the POST project, a first of a kind direct shear testing machine was manufactured and calibrated that can accommodate samples up to 400 × 600 mm under normal stresses up to 10 MPa, for both CNS and Constant Normal Load (CNL) conditions, with the ability to shear the sample up to 50 mm. Several best practice procedures were developed for fracture characterization pre-, syn-, and post-shear test which utilize high resolution optical scanning, contact pressure measurements, Digital Image Correlation (DIC) measurements, and acoustic emission measurements during the shear test. Natural and tensile-induced fractures of a granitic rock as well as replicas of the hard rock fractures, at three different fracture sizes of 35×60 mm, 70×100 mm, and 300×500 mm, are now being tested. It is hoped that this program will provide a set of high-quality data which will help reduce the knowledge gap in the understanding of fracture behavior.

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