Stability Analysis Of Block Of Tunnel Surrounding Rock Considering Initial Stress And Deformation Process

It is a very important content to analyze the discontinuous deformation about the stability of tunnel surrounding rock in the tunnel safety evaluation. Rocks can be regarded as combination by fracture planes and blocks induced by the interaction cutting of fracture surfaces. Many engineering experience shows that the destroy form of rock mass mostly are the instability of block in the tunnel excavation process.On the basis of the "key block theory", this paper identifies the likely unstable blocks considering the tunnel excavation face, and analyzes the effect of initial geostress and the excavation secondary stress field on the stability of surrounding rock by ANSYS/ LS-DYNA finite element program. The main research contents and conclusions are as follows:(1) This paper summarizes theories and methods in recent years about the fracture plane simulation and key block identification, and develops the three dimensional network simulation systems of rock fracture based on Windows operating platform by using VC++, which is an object-oriented programming language and the graphics processing functions of OpenGL. For the certain rock fracture, this model can directly introduces the geometric parameters such as the attitude and the length; as for the uncertain fracture that can't be measured, the fractures could be generated by assuming the fracture distribution. It can be achieved that the modeling, display, zooming, rotation and movement of the rock mass, tunnel excavation face and fracture in this program, at the same time, the process that block is generated by fracture' interaction cutting can be calculated based on the key block theory, and the key block' identification and display can be achieved combining with the location of tunnel excavation face. This program can be used for modeling, display, zooming, rotation and movement of the rock mass, tunnel excavation face and fracture in this program. Meanwhile, calculations based on the key block theory could be done in the process that block is generated by fracture' interaction cutting and the key block' identification and display can be achieved with the combination of the location of tunnel excavation face.(2) The early data structure of key block can be transformed into identifiable command stream of the finite element, which can be introduced into the finite element model of tunnel built by preprocess functions of ANSYS. The process of tunnel excavation can be simulated and calculation by LS-DYNA program in which the influence of the initial geostress and the secondary stress field of surrounding rock also should be taken into consideration. Through the analysis of the specific shape block' time- displacement law in different friction coefficient, conclusions could be drawn as follows:blocks would be in a stable state when the friction coefficient is bigger because the interface can provide enough friction to stop it from sliding; with the decrease of the friction coefficient, the decline in force of block exceed the biggest friction that the interface can provide when it reaches a certain value. At this time, blocks appear instability and failure which can be used as the judgment standards of surrounding rock stability. Then we can carry on analysis of the stability of surrounding rock of safety coefficient according to the basic principles of the finite element strength reduction.(3) According to the conventional method of the key block stability analysis, we should take only the impact of likely unstable block' own gravity on its stability into consideration and leave the influence of initial geostress and the secondary stress field of tunnel surrounding rock alone. And the calculation and the comparison are proceeded in the same finite element models. As for the up-narrow and down-wide block in the research, that the traditional analysis method concluded that surrounding rock of the tunnel body must appear instability no matter how much friction coefficient is. By contrast, this paper indicates that it is more reasonable to analyze the stability of surrounding rock by considering initial geostress and secondary stress field of surrounding rock excavation.