Three-Dimensional Numerical Modeling of Underground Powerhouse Complex of 720 MW Mangdechhu Hydroelectric Project, Bhutan

Abstract

The underground powerhouse complex of Mangdechhu Hydroelectric Project (MHEP) comprises of a 155-m-long, 23-m-wide, and 41-m-high machine hall cavern and a 135.5-m-long, 18-m-wide, and 23-m-high transformer hall cavern. The engineering design, support system, and excavation methodology of the caverns were designed based on the geological data obtained from surface and subsurface investigations during detailed project report stage investigations, using two-dimensional elastoplastic finite element analysis by Phase2 software. However during the excavation of the caverns, a shear zone of 1–1.5 m thickness with associated fracture zone of 1.5–3 m thickness on either side was encountered. In view of the changed geological scenario, the excavation methodology and support system designed based on two-dimensional analysis appeared inadequate as such progressive three-dimensional discontinuum numerical modeling analysis using 3DEC software was undertaken to optimize the excavation methodology, sequence, and support system. The models were updated at various stages of the excavation using the encountered and anticipated geological conditions, rock mass properties, and instrumentation data to simulate the rock mass behavior during the excavation, to analyze the stress changes and associated displacements in the caverns for particular excavation stage, and to evolve a safe excavation methodology, sequence, and support system prior to benching to next level, while simultaneously revalidating the installed support system. The overall stability of the caverns was also determined after the complete excavation. By the aid of numerical modeling results, the excavation methodology, sequence, and support system were optimized and the excavation of the caverns was successfully completed within the schedule.