| In recent years,with more and more underground excavations such as expressway tunnel,high-speed railway tunnel and metros being built. The engineering geological conditions are becoming more and more complex. Because of the complexity of the jointed rock mass, the widely used analysis method which based on the continuum mechanics to the analysis of underground engineering have been difficult to meet the underground engineering rock mass problem analysis and construction requirements. The design of all kinds of underground supporting structure is based on the calculation of surrounding rock pressure, and the accurate calculation of surrounding rock pressure depends on the accurate determination of surrounding rock failure mode,and the rock is a nature geologic material that is through a variety of physical and chemical changes and the mechanical properties is much more complex than general material.and there are also many influence factors of surrounding rock failure mode and because of the poor stability and complicated deformation failure of the large span underground excavations, therefore, it is very important to study the failure mode of the surrounding rock mass of the underground excavation in the jointed rock mass. Study on the failure mode of the surrounding rock mass of the flat large span underground excavation in jointed rock masses that have three joint sets existence.Under the conditions of II (hard rock)and III(mid-hard rock) class rock the failure mode,failure process of surrounding rock,the generation conditions of failure modes,influence factors of failure model and the engineering control strategies to the typical failure are analyzed after excavation of underground cavern by using combining methods of discrete element numerical simulation and theoretical analyses. The main research conclusions are as follows.(1) Through the research of domestic and foreign literature, induction summary abroad and research achievement about the failure mode, and puts forward the problems existing in the present studies, and there are many influence factors of failure mode through analyze the results of the previous research, and determine the main research content and technical route.(2) Design test plan by orthogonal test method and discrete element method (UDEC) was used to simulate the excavation of the flat large span underground excavation in jointed rock mass which have three joint sets existence.Given four failure modes under the conditions of II class rock:crown fall block, wedge, the crown collapse, vault bent inside the drum, vault straight tube failure. Given nine failure modes under the conditions of Ⅲclass rock:crown fall block,crown fall block- side wall extrusion,crown fall block-floor heave, vault straight tube type (chimney type) failure, side wall extrusion,wedge, the crown collapse, vault bend- floor heave, crown bending inner drum.Through analysis the failure process of various types of failure mode under the conditions of Ⅱ and Ⅲclass rock that failure process can be divided into four phases: the stage of fracture, the stage of crack propagation, the duration of the crack, the stage of the failure of the fracture. Qualitative analysis is made on the condition of each failure mode of Ⅱ and Ⅲclass rock, and the main control factors of each failure mode are obtained.(3) Through single factor experiments, the influence of joint spacing, joint dip angle, cavern span, depth and lateral pressure coefficient on the failure mode of surrounding rock is studied. The joint space, joint angle, tunnel span, depth, lateral pressure coefficient of surrounding rock failure mode, failure zone, shear zone, joints and plastic zones have a certain impact.(4) For the typical failure mode in this article, and combined with the practical engineering the basic method of dynamic identification of surrounding rock failure mode is presented, than put forward corresponding engineering control strategy for the typical failure mode. |
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