Characterising Stiffness Properties of Bedding Parallel Fractures at High Normal Stresses for Sedimentary Rocks

Abstract The shear and normal stiffness of fractures are important parameters governing deformation behaviour of sedimentary rocks. However, shear stiffness is challenging to measure and is often inadequately applied in geotechnical design. Shear stiffness is frequently estimated without testing, instead based on assumptions and represented by a single value, despite the knowledge it should be represented as a function to capture variability. Most published stiffness data for sedimentary rocks are for joints at low normal stresses, with limited data for bedding parallel fractures at normal stresses above 1 MPa. This paper addresses this gap by presenting 55 shear stiffness results for bedding parallel fractures, tested under applied effective normal stresses ranging from 1 to 15 MPa. Results show a strong correlation between applied normal stress and shear stiffness, with shear stiffness increasing as normal stress increases. The results are then used to determine the suitability of current shear stiffness estimation methods. The findings highlight the need for significant improvement in current shear stiffness guidelines and emphasise the need for guidelines to better account for system influence with data processing protocols incorporating actual intact data and using displacement for determining the seating influence cut-off. Additionally, the common practice of using normal-to-shear stiffness ratios for estimates should be discontinued, as it is unsupported by data. Furthermore, predictions demonstrate the impact shear stiffness has on depth of fracture, supporting the necessity of representing stiffness as a function rather than a single value.