Analysis of Tunnel Support Requirements Using Deterministic and Probabilistic Approaches in Average Quality Rock Mass

Abstract

The support design for rock tunnels involves a complex problem because of the various types of uncertainties present in rock mass properties; therefore, probabilistic approaches are used to consider these uncertainties systematically in the presented analysis. In earlier studies, uncertainties in peak strength parameters and deformation modulus were considered while uncertainty in residual strength parameters was neglected. Another important factor is the assumption regarding the postpeak behavior of the rock mass, which depends on the rock mass quality in the field. An average quality rock mass generally shows strain-softening behavior in a range from peak to residual strength, which generally has been neglected because of the assumption that the rock mass is elastic-perfectly plastic. However, it was observed that the yield zone depth and displacements around the tunnel are highly sensitive to residual strength parameters, and residual strength parameters can considerably influence the estimation of the support requirements of a tunnel. In this paper, a new computational approach, based on the geological strength index (GSI) and the use of deterministic and probabilistic methods, is described for the reinforcement design of a tunnel in an average quality rock mass. This approach considers the variability in residual strength parameters along with peak strength parameters and the deformation modulus. Moreover, the strength drop in the stress-strain behavior of the rock mass is also considered in the analysis. An underground powerhouse cavern from the Himalayan region of India is taken as the case study to show the methodology used in the research. The study brings out the advantages of the probabilistic over the deterministic approach for the support design of a tunnel. Another important conclusion from the study is that the support requirements for the tunnel are greatly influenced by residual strength parameters, and therefore, uncertainty and strength drop in the residual strength parameters should be properly considered when designing support for tunnels.