In situ measurements of rock mass deformability using fiber Bragg grating strain gauges

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 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 4100 level (~1250 m underground) of the Sanford Underground Research Facility (SURF) at the site of the former Homestake gold mine 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 softest due to induced fractures and 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. 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 or geotechnical classification systems do not fully capture.