The tunnel wall collapse and pothole creation on the hilly terrain surface: a case study of stabilization

An approximately 2-km-long tunnel is located in the heart of the Bailadila hills in India and is used for the transportation of iron ore excavated from nearby mines. A portion measuring 2 m height and 5 m width of the Reinforced Cement Concrete (RCC) wall of this tunnel was severely ruptured and caved in bringing a lot of debris and the slush. This caused subsidence on the hilly terrain surface exactly overlying the affected tunnel portion. Various stabilization measures consisting of cement grouting in tandem with chemicals were undertaken to strengthen the failed concrete lining as well as the rock and soil mass of tunnel walls and roof in and around the collapsed portion. Additional stabilization measures were also undertaken to back-fill the cavity/pothole formed on the hilly terrain surface. Further, monitoring of the stabilized zone was carried out by installing mechanical type convergence indicators in the tunnel walls and roof. Also, the efficacy of grout was estimated using an ultrasound sonic tester and through Schmidt hammer rebound tests. The paper presents this case study along with the conclusions and recommendations, which were drawn based on this study.