A displacement-based back-analysis method for rock mass modulus and horizontal in situ stress in tunneling - Illustrated with a case study

This paper presents a displacement based back-analysis method for the determination of rock mass modulus (E) and the horizontal in situ stress (P) perpendicular to the axis line of the tunnel excavation in hard and intact rock masses. The rock mass is assumed to respond in an isotropic and linear elastic manner. Essentially, the method is a best-fit solution of back-analysis by comparing the measured displacements near a tunnel face during excavation with those calculated using a three-dimensional finite element method. The method can be applied efficiently and effectively by iterative algorithms such as direct search technique and damping least square technique. In particular, a three-dimensional finite element pattern technique (3-D FEPT) is used to compute the theoretical displacements for saving computation time. The method is further illustrated with a case study. The case study deals with a test adit at Zhanghewan Pumped Storage Power Station in Hebei Province, North China. It is shown that the back-analyzed results of E and P are reliable and representative for the actual rock masses.