Risk management by geotechnical monitoring for safe excavation of a large rock cavern: a case study

The present report highlights the methods used to monitor the rock mass behavior and performance of a cavern during excavation in order to verify and, if necessary, adjust the rock support to ensure safe construction at each stage of excavation. The present geotechnical monitoring practice for underground structures involves convergence monitoring with the help of optical targets and rock mass displacement with the help of borehole extensometers. More importantly, it involves detailed planning to finalize the position of each monitoring instrument, based on the location and orientation of geological features. It is equally important to ensure proper recording of monitoring data in order to analyze and take immediate action in case of any adverse situation. This requires dedicated high-end automated software, which can record, analyse and produce significant results from a large quantity of recorded data, to prevent what could otherwise be a catastrophic failure. Initially, geotechnical monitoring was carried out in the recently excavated zone of the cavern on a daily basis. Based on continuous monitoring data for at least 1 week, the frequency of subsequent monitoring can be decided. In most cases, the deformation of rock mass was considerably less than the alarming values which were calculated based on detailed design for different rock classes.