Engineering geological studies used for redesigning and employing a hard rock TBM in soft rock formations of Chamshir water conveyance tunnel

The Chamshir water conveyance tunnel (CWCT) with a length of 7.4 km and boring diameter of 5.3 m is excavated using a refurbished single shield TBM, initially designed to work in hard rock formations. This study focuses on engineering geological issues, which are important for checking the suitability of this machine and proposing required modifications for improving its performance in soft rock. The main objective of the investigation was to predict geological problems and risks associated with TBM tunneling in the given ground conditions and to examine the ability of the selected TBM to overcome the anticipated challenges. Geotechnical parameters that could affect TBM operation are described in this paper. These parameters include the engineering geological and hydrogeological properties of the rock units, rock mass boreability and abrasivity, and assessment of geological problems (clogging potential, swelling, abrasive ground, unstable tunnel face and water inflow). Estimated tool wear in this project is also discussed using a new special prediction model, and the results are compared with actual disc cutter consumption in the excavated section of the tunnel. The actual observations in the first 1500 m of the tunnel indicate the validity of the assessments in the related engineering geological units.