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 passes through soft marlstones and mudstones of the Aghajari formation and strong conglomerates and sandstones of the Bakhtiari formation, which are two main widespread formations in the Zagros simply folded zone in southwest Iran. This tunnel with a 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 and geotechnical 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 (clogging potential, swelling potential, abrasive ground, unstable tunnel face and probable water inflow) associated with TBM tunneling in the given ground conditions and to examine the ability of the selected TBM to overcome the anticipated challenges. 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 and measurements in the first 1500 m of the tunnel indicate the validity of the assessments in the related engineering geological types. In addition, the study of machine specifications showed that the machine can excavate the tunnel by implementation of required modifications, especially the use of proper cutting tools and improvement in cutterhead design.