ABSTRACT

The geological conditions of large underground caverns are complicated, whose support scheme affects the stability of surrounding rock, so it’s important to study on the optimum support of large underground caverns during excavations. To simulate rock mass actually and analyze its damage zones during excavations, the elasto-plastic damage model and damage evolution equation are proposed on the basis of the irreversible thermodynamics theory. The 3D elasto-plastic damage finite element code D-FEM is programmed by FORTRAN language, which has the capabilities of simulating excavations and support, computing quickly and group function. A 3D numerical model of underground caverns is established. The caverns’ excavations by different support schemes are simulated by D-FEM. The stress, displacement and damage of surrounding rock under three different schemes are analyzed. The effect of No.2 support scheme is the best and so is proposed. Considering damage evolvement, damage zones in side wall of cavern group increase remarkably compared to those obtained through FLAC3D. Therefore, it is concluded that the elasto-plastic damage finite element method can analyze the stability of underground geotechnical engineering projects.

1. INTRODUCTION

Up to now, the total installed capacity for water resources in China is approximately up to 0.172 billion kilowatt-hours (kWh). As the western development is carried out continually, more and more underground hydropower stations are being and will be built in southwestern areas, such as TGP, Ertan, Xiaolangdi, Loangtan, Xiluodu, Jinping Stage (I, II) and so on. All of the large-scale underground caverns of the hydropower stations are located in the complicated geological environment. As we know, there are numerous factors to have influences on the stability of surrounding rock masses during excavations, but the crucial factors are insitu stress, properties of rock masses, excavation and support schemes. Discontinuities just like fractures, joints and faults are inevitably existed in rock masses under long-term geological actions, which consequently results in weakening of related mechanical parameters. Under the actions of in-situ stress, a large number of micro-cracks appear in the surrounding rock masses after excavations, and they can spread with the increasing loading. Microcracks and their extension can be defined as the damage in the rock masses. When damage develops to a critical level, all kinds of failures would happen in rock masses, and even disasters. From 1970’s, the importance of damage mechanics has gradually considered. Voyiadjis et al. derived an elasto-plastic damage constitutive model, but it was not used in a practical problem [1]. Sun proposed a generalized elasto-plastic damage constitutive model for fractured rock mass [2]. Zhu et al. set up a three-dimensional elasto-plastic damage model based on fracture and damage mechanics which had been used to analyze the stability of some underground caverns and high slopes [3]. The numerical software RFPA proposed by Tang [4] also considered the elastoplastic damage. Li et al gave an elastic damage yield criterion and applied it to solve some stability problems for underground caverns [5]. Wang et al. put forward a probability damage evolutionary rule for jointed rock masses using FLAC3D [6].

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