Analysis of tunnel considering Modified Mohr-Coulomb criterions

摘要

Analysis of tunnels and underground excavations requires a realistic prediction of strength and deformation characteristics of rockmass. The strength characteristics of rockmass must be determined by laboratory testing such as triaxial test but generally, classification approach is used to predict the triaxial strength of rockmass. A number of empirical criterion had been developed in past to simulate the triaxial behavior of rockmass. The Generalized Hoek-Brown (GHB) criterion (2002) is most widely and generally used. A new triaxial strength criterion was presented recently known as Modified Mohr-Coulomb (MMC) criterion. Probabilistic approach combined with various classification approaches is used to predict the triaxial strength of rockmass giving much higher sense of confidence. The GHB and MMC criterion are nonlinear in nature. However, the continuum numerical codes and analysis methods generally do not accept such nonlinear criterion and may allow only Mohr-Coulomb linear criteria. The researcher generally fit a linear equation on nonlinear data point with in certain range of minor principal stress to calculate the equivalent Mohr-Coulomb criterion parameter i.e. cohesion and angle of internal friction. This approach retains the nonlinear behavior of the rockmass. The present article discusses in detail the calculation of triaxial data point for five different rockmass data from tunnel cases published in literature. The triaxial data points are calculated based on GHB and MMC criterions. Equivalent Mohr-Coulomb parameters are calculated by fitting a linear equation on the non-linear data point. Analysis of unsupported tunnel case is done with the help of convergence confinement method. A quantitative comparison between MMC and GHB criterion is done in light of the analysis results. MMC criterion predicts a lower value of equivalent cohesion while a higher value of angle of internal friction. The response of tunnel predicted by MMC criterion are on lower side but comparable.