Evaluation of squeezing mechanisms in a headrace tunnel passing through phyllitic slate rock mass: A case study from the Himalaya

• Weak phyllosilicate minerals with preferred orientation reduce rock mass strength. • Foliation strike nearly parallel to the tunnel axis creates unfavorable conditions. • Foliation dip of 28–58° with high in-situ stress promotes sliding and buckling. • A highly anisotropic weak rock mass like phyllitic slate triggers progressive failure. • Delayed or insufficient support installation leads to large deformations. The construction of tunnels is essential for infrastructure development, particularly in mountainous regions; however, such regions are often characterized by complex and challenging geological conditions. Highly fractured, weathered, sheared, and tectonically disturbed rock masses significantly increase the instability during tunnel excavation. Tunnel behavior is primarily governed by rock types and in situ stress conditions. Tunnels passing through medium to high overburden with rock mass of weak and schistose character experience substantial plastic deformations (squeezing), whereas those with rock mass of strong and homogenous character experience brittle failures. This manuscript intends to quantify the extent of squeezing and elucidate the underlying mechanisms of tunnel wall instability by integrating engineering geological assessment along with on-site measurements and observations, theoretical analysis, and laboratory experiments. The investigation focuses on the headrace tunnel of the Tanahu Hydropower Project (THP) in central Nepal, where severe squeezing was encountered during excavation through highly anisotropic and schistose phyllitic slate rock. The authors emphasize that the combined effects of anisotropic lithology intersected by shear zones, weak mineralogical composition along structural fabrics, and high in situ stresses are the primary contributors to tunnel squeezing. The findings offer valuable insights into the fact that the underground openings passing through medium to high overburden in a weak and highly schistose and tectonically disturbed rock mass experience severe squeezing problems. Therefore, it is important to design effective rock support measures and excavation strategies to enhance stability, minimize delays, and improve the long-term performance of tunnels excavated in similar geotectonic conditions like in the Himalaya.