ABSTRACT
Construction of underground tunnel portal in shallow rock cover in fragile geological formations possess many geotechnical challenges. Managing these challenges often requires support of numerical and analytical approaches. This paper presents a case study of Main Access Tunnel (MAT) of 9 m finished D shape, for a pumped storage project in India, which is under construction. The tunnel was to be made in extremely poor rock class with shallow rock cover which is about 4.5 m. The geological strata consisted of highly weathered shale, characterized by its heterogenous and anisotropic nature. Rock mass properties of shale also adversely affected under saturated condition. Numerical analysis of the tunnel was carried out, with regular site mapping and field test results of the rock mass. Numerical modelling techniques are effectively used for understanding the rock mass behaviour. The tunnel has been excavated using conventional method of tunnel with short round lengths and cautiously moving ahead. The conventional support system in the form of pipe roofing, SFRS, rock bolts and steel ribs have been provided. The support system was reviewed timely, based on strata encountered ahead of the tunnel face and modified as per site conditions. The tunnel was also provided with adequate instrumentation for monitoring purpose which comprises of MPBX, Inclinometer, targets and load cells. This paper highlights the case study of design and successful completion of the initial reach of 40 m length tunnel in poor geology and shallow overburden, with conventional tunnelling techniques.
