A direct shear equipment for testing rock fractures up to 400×600 mm size, and up to 5 MN force in both normal and shear loading directions, was developed. Normal loading and direct shear tests under constant normal stiffness (CNS) and constant normal load (CNL) conditions were conducted on 300×500 mm specimens, one planar steel joint and two natural and two tensile induced rock fractures. Design targets, e.g. system to maintain undisturbed fractures up to testing and high system stiffnesses to achieve well-controlled shear tests, were verified by the experiments. A new optical system for local deformation measurements was used to accurately determine fracture displacements besides conventional non-local deformation measurements. The determined normal stiffnesses were similar previous results from the literature on smaller fractures, whereas the shear stiffness data are novel. The results provide a new insight into processes at the onset of fracture slip.
The mechanics of rock fractures are a key component for the mechanical and hydraulic properties in a rock mass. A correct prediction of the properties of a rock mass is important for several areas in engineering such as infrastructure tunnelling, mining, geological repositories for spent nuclear fuel, hydropower, geothermal energy and carbon dioxide sequestration. There is a knowledge gap on the mechanical behaviour of large fractures (at a meter scale) in a crystalline rock mass at depths of several hundred meters. That is e.g. fracture stiffnesses and shear resistance under a normal stress of 5 to 10 MPa (representing the in-situ stress state) at both constant normal load/stress (CNL) and constant normal stiffness (CNS) loading conditions. Past and present in-situ and laboratory fracture shear experiments, with a few exceptions, are either conducted on small specimens (up to 200 mm) or on larger specimens (up to 1–2 m) at low normal stresses (up to 1–2 MPa).
This lack of knowledge contributes to the prediction uncertainty of the fracture behaviour in a rock mass. The nuclear waste management companies in Finland (Posiva), Sweden (SKB, Swedish Nuclear Fuel and Waste Management Company) and Canada (NWMO, Nuclear Waste Management Organization) initiated the first phase of the cooperative POST project, carried out during 2014–2016, to address this uncertainty by focusing on the implementation of field shear testing and numerical modelling of large fractures (Siren et al. 2017). It was concluded that the chosen field testing approach has complications, such as finding representative fracture sets, difficulty in conducting the experiments, large uncertainties of results, and being cost ineffective. Among the recommendations from the project to increase the accuracy of fracture displacement predictions, was to study the mechanical behaviour of large fractures under controlled laboratory conditions, particularly at realistic CNS normal loading boundary conditions which is crucial for the post-peak (after start of slip) shear response. A study of the effect of fracture scale was also recommended to better understand scaling laws. A second phase of the POST project was initiated 2017, this time with the participation of NWMO and SKB and in cooperation with RISE (former SP Technical Research Institute of Sweden) and KTH, Royal Institute of Technology (Sweden) following several of the given recommendations, cf. Jacobsson et al (2021).
