In situ measurements of rock mass deformability

In order to examine how the mechanical properties of a rock mass vary from the centimeter to meter scale, we performed two field point-loading tests (89 kN and 890 kN) to determine the in situ modulus of deformation (Em) of a rock mass. The experimental setup is analogous to plate jacking-type tests, but, instead, using a point load. The experiments were done in the Poorman formation on the 1250 m (4100 ft.) level of the Sanford Underground Research Facility (SURF) at the site of the former Homestake goldmine in Lead, SD. For comparison with in situ values, we also conducted laboratory mechanical tests and used two geotechnical classification systems to evaluate rock stiffness. The in situ modulus of deformation increases with depth into the rock mass. This increase in stiffness is a result of the differences in mechanical properties due to the effect of excavation of the underground space. Near the surface (0 - 1.2 m depth), the rock is weakened due to damage from blasting. Beyond this damaged zone is the stress-relief zone (1.2 -1.5 m depth), where open joint sets affect rock stiffness, and beyond that lies the undisturbed zone (>1.5 m depth) where the rock is the stiffest. Laboratory measurements of Young's modulus for the Poorman formation coincide with the Em measured at ~1.2 m depth. If done properly, in situ measurements of rock stiffness are a valuable tool to fully characterize the gradient in stiffness of a rock mass, which laboratory tests and geotechical classification systems do not fully capture.