| In recent years, inorder to relieve traffic congestion problem, lots of big cities began to plan and build the subway. In the meantime, shield construction caused a great deal of surface collapse accidents. The subway lines are generally builted under the road and residential areas below. Therefore the harmfulness of surface collapse accident is very serious and it has aroused widespread social concern.When shield crossed trough the sandy cobble stratum in Chengdu metro No.1and2line, ground subsidence accidents reached as high as dozens of times. At present, there were many research literatures on karst collapse and the ground collapse caused by underground mining and these contents have been very well researched. However, ground collapse mechanism induced by shield construction was rarely researched and it is still in initial stage. The research extent remained on level of language description and sketch map. Shield crossing trough the sandy cobble stratum in Chengdu metro No.l and2lines is taken as research background. For the characteristics of sandy pebble stratum, ground collapse mechanism induced by shield is researched and the corresponding countermeasures to avoid ground collapse are proposed. The main research contents are as follows:1. Through analysis on many lagged ground collapse cases induced by shield crossing trough the sandy cobble stratum in Chengdu metro No.1and2lines, Lagged ground collapse mechanism in sandy pebble stratum is summarized. It mainly comprises of face instability, formation of cavity due to over excavation under low support pressure and upward movement of cavity.2. In order to master mechanical properties of sandy pebble stratum, the large scale triaxial test was implemented to obtain differential stress-strain curve. According to the low cohesion and heavily discrete characteristics of sandy cobble soil, numerical computation is conducted by particle discrete element method. By numerical simulation of the large scale triaxial test, the micro parameters of the sandy pebble stratum in Chengdu were calibrated. Numerical analysis on lagged ground collapse mechanism is simulated by particle discrete element method.3. The fundamental reason of ground subsidence is that the losing stability of shield face causes excessive ground loss. Therefore face stability of shield tunneling in sandy cobble stratum is emphasis of the research. The major influencing factors of face stability in sandy pebble stratum are summarized. Though numerical calculation, influence of face stability is analysed by the factors such as friction angle, lateral pressure coefficient, shield diameters, buried depth and groundwater level. The paper analyzed the influence of support pressure on shield tunnel face deformation, surface settlement, max horizontal displacement and stress of soil. Though writing displacement display program, the regularities of deformation and collapse pattern of shield face are studied along with decrease of support pressure. The process of shield face from local instability to integral instability is proposed. Combining with practical shield face instability instance in Chengdu metro No.2lines, the reason of face losing stability is analyzed. Corresponding construction measures are proposed to avoid face losing stability in sandy cobble stratum.4. On the basis of trapezoid wedge model, limited supporting pressure formula of shield excavation face was re-derived. In the process of rededuction, the forces of triangular sides and top surface on sliding block with outside soil are considered. Numerical simulation show that shape of upside sliding block in shield excavation face is circular. Hence by adopting6meters in diameter baffle, Trapdoor model test was simulated. Terzaghi loosening earth pressure formula is modified in3-dimensin space. The paper discussed the selection parameter problem on modified trapezoid wedge model and according validating analysis was carried out.5. Owing to heavily discrete characteristic of sandy cobble, ground collapse deduced by shield belongs to the field of in-continuous medium promlem. Ground collapse deformation curve has obviously discontinuous feature. Thereby the paper adopts particle discrete element mothod for displacement prediction. In-situ measurement method is proposed to obtain ground loss value. The distribution form of ground cave is explored. Ground movement pattern of shield in sandy pebble stratum was discussed. Ground collapse deformation influenced by buried depth, ground loss value and stratum characteristic is analysed. On the basis of surface settlement, ground loss are made a classification. Ground collapse deformation influenced by many factors is difficult to solve with mathematics formulas. BP neural network and least squares support vector machine method are introduced to predict surface collapse deformation.6. According to the characteristics of shield construction in sandy pebble stratum, corresponding construction measures are proposed to avoid lagged ground collapses. Shield design parameters such as aperture ratio, aperture distribution and aperture size of shield cutter head are studied. Relation between grouting pressure and grouting amount of synchronous grouting is analyzed. The suitability of air pressure auxiliary method in sandy pebble stratum is discussed. The distribution pattern of earth pressure and air pressure is proposed for unfull of soil dregs in cabin. Air pressure value, discriminant method for producing drift sand phenomena is put forward. Auxiliary methods such as decreasing groundwater level and pipe shed of portal section was analyzed. For different ground loss, corresponding construction measures are proposed to guide the construction. |
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