Faults and fractures are important geological features that need special attention in many engineering tasks. These discontinuities in the rock mass are key elements in seal integrity evaluations for subsurface CO2 storage. Stress changes associated with CO2 injection into reservoir formations can potentially trigger movement along pre-existing fractures. It is therefore necessary to evaluate the capability of the rock mass to withstand such movement which can lead to CO2 migration out of the confinement or unwanted microseismicity. In this study, the aim is to increase the understanding of fractured rock mass strength through the comparison between experiments performed in direct shear box and triaxial apparatus. The direct shear box has traditionally been the preferred method of quantifying the resistance of a rock mass to slip along pre-existing weakness planes. However, from subsurface sampling depths of kilometres to testing in the laboratory, a fracture will undergo stress relief that is likely to have effect on the rock mass cohesion. This is underlined by the fact that the two faces of a fractured rock mass are usually taken apart from each other during sample preparation. In triaxial tests, the purpose is often to fracture an intact sample of the rock. Further movement along this newly created fracture should therefore have many similarities with the shear box experiment, except for the beforementioned challenges with stress relief and possible fracture mismatch. Yet, to our knowledge, little work has been done trying to compare the residual strength measured in the triaxial with that measured in the direct shear box. In this experimental study, samples of the Berea sandstone are tested in both the direct shear box and triaxial apparatus. Results from direct shear box tests on pre-cut samples are compared with results from triaxial tests with shear mobilization on newly created fractures. In addition, experiments on pre-cut cylindrical samples tested using the inverted shear end caps in the triaxial apparatus are included to further bridge the gap between results obtained in the direct shear box and triaxial tests. Tests are performed under a range of normal stresses and using both air, brine and oil as pore fluid to identify potential sensitivities of the various test protocols. Results show how the residual strength in the triaxial test is connected to the slip resistance in the direct shear box, thus providing important information needed in the evaluation of typical experimental rock mechanics studies.
