Geological and geomechanical heterogeneity in deep hydropower tunnels: A rock burst failure case study

Tunneling in the Himalayas is full of surprises due to the active state of stresses, along with hostile geological and geomechanical heterogeneities. These sometimes go unnoticed during the design stage, which can result in rock burst type failures, which is the case with a recently completed hydroelectric project in Pakistan. In this project, twin headrace tunnels are excavated by tunnel boring machines (TBMs). Several rock bursts have occurred in the tunnels of the case study project, which are influenced by many factors like in-situ and excavation induced stresses, rock type and its brittleness, bedding orientation, geological anomalies, and geological structures. The Himalayas lay on the most active plate margin zone, and excavation here is more difficult than in the Andes, Alps, or any other mountain belt in the world. The project area is surrounded by the Main Boundary Thrust (MBT), the most active fault in the region, along with local faults that pass through the headrace tunnels. Due to this active stress condition, thrust faulting is common, and a complex structural geological regime is prevalent. Abnormal in-situ stress conditions at a deep depth have caused unique geological anomalies in the unique sedimentary geological settings of the project area. The TBM, which has little flexibility during excavation, has done little to disperse the in-situ stresses near the boundary of the tunnel and has exaggerated the situation. The two rock burst events of January 13, 2016, and May 31, 2015, are selected for this study; they are classified as strain and fault-slip bursts, respectively. Empirical approaches have been used to evaluate the proneness of rock burst occurrence. Numerical simulation has also predicted the actual failure zone well, in such deep excavation. The details documented for these events not only provide a basis for understanding the process of rock burst in the Himalayas but also provide a good reference regarding the occurrence of rock burst in deep civil tunnels excavated in the hard rock.