Tunnel Evaluation In Crocker Formation By Geological Strength Index (Gsi) System: A Case Study

This study was conducted to determine the value of Geological Strength Index (GSI), to predict rock mass properties, very unfavourable discontinuities combination and tunnel support pressure for rock bolts or shotcrete and to determine the suitability of GSI for a tunnel in Crocker Formation. Engineering geological mapping and discontinuity survey was done along the tunnel face as well as rock sampling. GSI values and the disturbance factor were obtained from field observation on the tunnel face. Point load and dry density test was conducted to determine the Uniaxial Compressive Strength (UCS) and unit weight, respectively. The rock mass properties, kinematic analysis and limit equilibrium analysis was used to determine the factor of safety (F.O.S) and pressure to stabilise the tunnel. The rock mass was characterised by 94.88 MPa UCS, 0.024 MN/m3 unit weight, widely space and high persistency. The GSI value is 50 with 0.8 disturbance factor. The cohesion, friction angle and tensile strength are 3.671 MPa, 25.20° and 0.056 MPa respectively. The friction angle was reduced by 5° due to lower shear strength of bedding plane. There are eight possibilities of discontinuities combinations on tunnel crown that have F.O.S lower than 2 and combination of joints 2, 4 and 6 has the maximum wedge volume of 28.37 m3. The maximum support pressure of rock bolts or shotcrete for F.O.S of 2 at the tunnel crown is 0.04 MN. The high F.O.S value may have been due to the overestimation of friction angle and cohesion of discontinuity plane. Then, this study shows that GSI system is unsuitable for the tunnel in study area which behave as anisotropic and structurally controls rock mass, but if needed, the values of rock mass properties, discontinuities combination and support pressure can be used for tunnel design.