Behavior of Marble At Jinping II Project--Part 2: Rockmass

Part 2 of the paper focuses on the description of fracturing and post-peak behaviors of the rock masses of different qualities. For the good quality rockmass with GSI value over 70, the attempt was made to determine the HB parameters in order to use tranditional continuum numerical codes such as FLAC3D and 3DEC. However, for the rockmass with GSI in the range between 50 and 70, it has been found that discontinuum methods would have to be used due to the significant fracturing behavior. At the depth of 1,800m from the ground surface at the Jinping II Project, fracturing becomes a dominated ground response while advancing the tunnel in the rockmass with GSI value around 55. The tranditional continuum methods with HB constitutive law seem unable to explain such response. The discontinuum code of PFC and UDEC was thus employed to describe the fracturing-dominated behavior. As the discontinuum approach describes the rockmass behavior at micro-scale and thus is essential, the residual strength envelops are achieved with this approach for the rockmasses with different GSI values in discussion. Comparing to the peak strength envelops, the residual srength envelops also show a behavior with increasing mb but descreasing s_{cr 1. INTRODUCTION Based on the previous paper (Part 1), the understanding on marble rock scale is extended to the rock mass. It contains quantitative description on the peak strength of rock mass, residual strength and Brittle-Ductile-Plastic transformation properties, which serves for forecasting damage depth of surrounding rock of water diversion tunnels after excavation in different overburden depth and rock mass qualities. Besides using traditional continuum methods^{[1] 2. PARAMETERS OF BDP MODEL FOR ROCK MASS II A new model called BDP modeling, which can describe Brittle-Ductile-Plastic transformation properties of marble in rock scale by improving on Hoek-Brown constitutive model, is introduced in previous paper (Part 1). The determination of parameters for BDP model is quite different from that of the brittle model and perfect plastic model. And the parameters of BDP model can be divided into 2 species: conventional Hoek-Brown strength parameters and particular parameters. The particular parameters indicate the variety of the ductile properties of the post peak strength with surrounding rock. Based on the results of the field relaxation zone test on JP water diversion tunnels, the Brittle-Ductile-Plastic behavior of rock mass in huger scale are described using BDP model in the follow. In the field of hydropower engineering of China, the surrounding rock is usually divided into 5 categories (I~V) related to the rock qualities, and the corresponding supporting parameters are determined according to these 5 categories. For rock mass of Category I, GSI score is 80~100. And for rock mass of Category II, GSI score is 60~80, and so on.} , the damage of surrounding rock is described directly using two discontinuum methods: UDEC^[2] and PFC^[3], and the results of damage depth of the tunnel obtained by continuum and discontinuum method are compared.} when using HB criterion, and a cohension-weakening but friction-strengthening hehaviour when using conventional Mohr-Columb criterion.