Considering the stochastic characteristic of the measured deformations or stresses in rock engineering, a stochastic back analysis model to identify the initial rock stress field in the complex rock slope engineering is presented. Several forms of estimation function are studied in view of different measured information. The multinomial distributive boundary loads is developed to simulate the ground rock stress in the model. The initial rock stress and boundary loads do not necessarily follow a certain kind of distribution law and assumption. The presented model can simulate the random distributive initial rock stress field. Combined with FEM, the improving accelerated genetic algorithm is used to get the multinomial coefficients of boundary loads. The numerical results show that the model is very effective and practical.

1. INTRODUCTION

The characters of initial rock stress take on dualism. It is one of the natural characters of rock masses and during the modeling this character should be considered as sufficient as possible to reflect the natural rock quality. It also reacts on the rock engineering. And the reaction could give rise to severe influence on the rock engineering. As we know, any of these methods which measure the initial rock stress could give us a series of point stress that could not be used directly in the analysis of engineering practice (Xu, 2000). Therefore some effective ways must be found to fit the stress field according to the measured datum (Sugawara and Obara, 1999; Xu et al., 2000). But all the methods are effective to some extent. Initial stress field in rock mass mainly includes gravitational stress and tectonic stress. The numerical method of initial stress field normally begins with certain distribution or function which initial stress field is assumed to be subject, mostly line distribution or specified simple distribution is adapted to solve optimizing value or regress-analysis (Sakurai and Takeuchi, 1983; Guo, 1983; Yang, 1995) which base on the deformation or stress measured. Because the complex rock mass contains faults, joints and cracks, the initial rock stress field is very complex, its distribution cannot be comprehensively described with simple function. Sometimes, boundary load is applied to simulate tectonic stress. The method (Pan de and Chen, 1998) only solve node force on the corresponding boundary of the structure stress, regardless the distribution type of the node force, while it has a magnitude numerative quantity. Because measurement results have a indeterminate character due to the restriction of measurement technique, instruments or method and the troubling factor from the environment by chance, the study of initial stress field only adopts the knowledge of probability. Strictly, any determinate back analysis is improper. The huge Longtan hydropower engineering project is located on an environment where geologic condition is very complicated and a lot of faults are in dam area. Its area will add up to 180,000 square meters. Its outline is also very complicated. The dam site locates on the foot of the high slope and nearly closed to the slope.

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