The unanticipated performance of a weak massive rock mass at depth and the added value of observational engineering

摘要

A site investigation for the Fresnillo shaft expansion program revealed that mylonitic shale, characterised as massive but weak, would be encountered near the planned shaft bottom and in the vicinity of large excavations required for the proposed crusher station and ore handling infrastructure. The rock mass is sparsely jointed, however, fails readily along graphitic planes of weakness in the mylonitic fabric. At roughly 950 m depth, excavations within this weak rock mass were expected to squeeze and excavation support was designed accordingly. The mylonitic shale was encountered within a ramp in a nearby region of the mine prior to shaft sinking and was observed to stand-up well with minimal ground support at 790 m below surface. This paper demonstrates the value of observational engineering where observed ground behaviour was back-analysed to improve the site geomechanical model, thus, providing an opportunity for design optimisation and risk mitigation for a major shaft expansion project.