Planning, Design and Construction of TBM Exit Cavern in Phyllite Rock Mass - A Case Study

Retrieval of Tunnel Boring Machine (TBM) is always challenging and in most cases poses significantly unique problems. Equally challenging is the planning and design of an optimized space for successful retrieval and dismantling of the machines. One such scenario was being faced in Tunnel-8 of Rishikesh Karanprayag railway line wherein the TBMs were planned to be retrieved and dismantled inside the mountain so as to achieve successful completion of project within time. A 20m wide and 19m high TBM exit cavern of 30m length was planned, design and constructed in class IV phyllitic rock mass of the Himalayan range with a rock cover of more than 150m in Tunnel-8 to achieve successful breakthrough and perform the dismantling operations. The cavern's design accounted for high overburden, weak rock mass, and proximity to parallel tunnels. This paper highlights the key challenges encountered during the planning, design, and construction of a TBM exit cavern, offering valuable insights for future infrastructure projects involving TBM launch or retrieval in complex geological settings.