Critical strain method incorporating calculation of equivalent H-B strength parameters for back analysis during tunnel excavation

Abstract

A comprehensive method is proposed for the continuous update of the design rockmass parameters during tunnel excavation coupled to novel calculation of the Hoek-Brown strength parameters for given Mohr-Coulomb ones. The method starts with the strength parameters of Mohr-Coulomb or Hoek-Brown criterion used at the design stage of tunnels, and then updates them by using the displacements measured during the excavation of tunnels. The update is performed based on a well established back analysis procedure that determines both cohesion C and internal friction angle φ of the Mohr-Coulomb criterion. Then, these parameters either directly update or are converted to equivalent Hoek-Brown parameters that update the original design parameters of the tunnel. The need arising for conversion of parameters during the back analysis is thus addressed: A method is developed for the evaluation of the equivalent Hoek-Brown parameters for an axisymmetric structure subjected to major principal initial stress p0 and supported by internal pressure pi. Four characteristic cases are analysed theoretically and numerically with a finite differences code and found to be in excellent agreement. The equivalent parameters solution is developed in Matlab source code under a creative commons license and greatly enables the automation of the update procedure.Based on the updated parameters, it is possible to continuously modify the original design of underground support structures, according to the results of back analysis of measured displacements. The modified strength parameters of the H-B criterion can also be used for assessing the stability of the tunnels for the next excavation stage of the tunnels.