Study On The Mechanical Behaviour Of Subway Construction In Cobble Stratum Based On Granular Discrete Element Method

In recent years, with China's social and economic development, the urban population increasing rapidly, so more and more cities began to plan and build the subway. However, because China has a vast territory, the geological conditions are great difference in different regions, some new metro encountered to the very complexed geological conditions during the construction, such as the cobble stratum which is widely distributed along Chengdu Subway line 1 and 2. Generally speaking, the cobble stratum consisting of the cohesive soil, pebble soil, coarse sand, gravel and so on, and occasional consisting of the giant stones. Most the structure of cobble stratum are loose, and theirs distribution were not even. The engineering properties of the cobble stratum is essential different from the sand or the cohesive soil, it has strongly discrete character. At present, the underground engineering which are builded by the open cut method, shield-driven and mining excavation method in cobble stratum is seldom, and there is no exsisting experience can be referenced. With the comprehensively launches of Chengdu subway project, building the subway safely and economically in cobble stratum is the key problem which is needed to be solved. Hereby, according to the several key problems needed to be solved which encountered during building Chengdu Subway line 1 and 2, the earth pressure distribution of cobble stratum,the tunnel face stability and the construction projects were studied in this dissertation. which embodies in 4 ways as follows:1. Through establishing the triaxial numerical test model, studied the parameters of the particles by many samples test and analysed the influence law of meso parameter to the characteristics of materials macromechanics. And according to the law, calibrated the meso parameters and macro parameters of the dense, medium dense and slightly dense sandy pebble soil in chengdu subway line 1 and 2. And the feasibility of which using the granular discrete element to simulate the foundation pit enclosure piles was discussed by the theoretical analysis and numerical method, and then calibrated the macro physical and mechanical parameters and meso parameters of the supporting structure.2. According to the discrete character of cobble stratum, using granular discrete element numerical method, studied the earth pressure distribution of cobble stratum foundation pit when the wall move with basic modes-translation(T), rotation about base(RB), rotation about top(RT), and the correction calculation method of earth pressure of cobble stratum foundation pit based on the translational wall-movement modes'has been determined by compared the compute results with the classical theory. And then,according to the tianfu-square foundation pit which was the most depth(28.5 m) and the largest scale (area is about 48000m2) in chengdu history, the failure and collapse evolution process were proved. And finally, using the correction calculation method of earth pressure of cobble stratum foundation pit which was proposed in this paper, analysed the construction safety of foundation in several different support condition and dicided the reasonable construction and supporting scheme of tianfu-square foundation pit.3. According to the construction situation of shield tunnel in Chengdu subway line 1, the face collapse and failure process of cobble stratum shield and the stress distribution evolution of the soil during the excavation surface collapsing were simulated and analysed, and he deformation and failure mechanism of the shield excavation surface was made clear, the collapse and failure modes of cobble stratum shield tunnel excavation face was put forward.And then the influence law of surrounding rock deformation and stress by the varying support pressure of excavation surface in cobble stratum shield tunnel was discussed, The relation between face support pressure and soil deformation indicates that the influence of support pressure can be figured as three stages,that is insensitive stage, sensitive stage and failure stage. Hereby, the 2-dimension mode of shield tunnel face limit support pressure was improved and the 3-dimension calculation model of limit support pressure which is suitable for cobble stratum shield tunnel face was established. In the process, By the numerical simulation test of baffle move, discussed the relationship between loosening earth pressure and baffle displacement. According to the large difference of results by terzaghi theory and numerical computation, modified the terzaghi calculation formula. Finally, the limit pressure in shield module at ZDK13+630 in chengdu Subway line 1 was analysed by the 3-dimension limit support pressure calculation mode which is established in this dissertation.4. According to the construction condition of the shallow bury subsurface excavation test section in chengdu subway line 1, the failure and collapse mechanism of shallow bury subsurface excavation in cobble stratum were analysed by particle discrete element method. The action effect of the advance support such as pipe roof, small pipe and front anchor in cobble stratum shallow bury subsurface excavation tunnel was proved, and the reasonable pre-supporting method and its parameters were given. And then the effection of working security by CD method and bench method in the cobble stratum shallow bury subsurface excavation tunnel was analysed, the results showed that the bench method have some advantages in the cobble stratum shallow bury subsurface excavation tunnel at construction dewatering condition.