| It is of great theoretical significance and application value of engineering to carry out the study of surface subsidence and disaster reduction technology in the construction of Xi'an in saturated soft loess strata.This paper took running tunnel of Xi'an Metro Line 3 Tonghuamen-Hujiamiao-Shijiajie as background and studied with the methods of laboratory test,field experiment,theoretical analysis,numerical simulation,site practice etc.The main conclusions are as follows:The deformation mechanism of saturated soft loess was studied by in-situ tests which include cone penetration test,standard penetration test,pressuremeter test and flat dilatometer test and laboratory test.The results show that the saturated soft loess is characterized with high porosity,high water content,low strength,high compressibility,poor bearing capacity and in soft plastic or flow plastic state.Based on the data of flat dilatometer test,the empirical formulas which are coefficient of the lateral pressure at rest and the lateral coefficient of soil reaction are obtained.The main factors affecting the surface subsidence induced by the tunnel construction in the saturated soft loess of Xi'an include:the depth of tunnel,the cross section size of tunnel,the groundwater level,the physical and mechanical properties of the saturated soft loess and other soil layers,the thickness of the saturated soft loess,the location of the saturated soft loess and the tunnel construction method.The method of FLAC simulation is used to study the surface subsidence induced by different excavation methods in the construction of the four cross-section of the saturated soft loess strata in Xi'an.Based on the principle of minimum ground subsidence,a reasonable excavation method of four kinds of cross-section tunnels is put forward.The Hushi running tunnel of saturated soft loess stratum is constructed by recommended excavation method based on four kinds of cross-section tunnels.The study shows that the transverse surface subsidence is divided into three stages:the initial deformation stage,the rapid growth stage and the long-term deformation stage.The longitudinal surface subsidence can be divided into three stages:the slow subsidence stage,the subsidence acceleration stage and the stabilization stage.The surface subsidence prediction model is established by using the Peck theory,and the ground subsidence trough is forecasted.The predicted results are in agreement with the measured values.The results show that the satutared soft loess situated in the natural conditions of the subway has poor engineering properties and is in the flow plastic state.FLAC simulation results show that the influence of thickness and location on the surface subsidence of mine method construction in saturated soft loess are as follow.The thickness of saturated soft loess has an serious influence on stratum deformation.The larger the thickness of saturated soft loess is,the greater the deformation and suffering area is.When the thickness of saturated soft loess is constant,the position of saturated soft loess has a strong impact on strata deformation.The closer the saturated soft loess is to the surface,the larger the surface subsidence is.Based on the FLAC simulation,the similarities and differences of surface subsidence induced by shield construction in the case of the upper part of the tunnel are respectively saturated soft loess and new loess and in terms of reinforcement and unreinforcement of saturated soft loess in tunnel are studied.The results show that when the upper part of tunnel is saturated soft loess the extent and values of the surface subsidence of the shield tunnel are larger than when the new loess in the upper part of the tunnel.The mechanical properties of saturated soft loess in the upper part of the tunnel are improved greatly,the soil strength is enhanced,the surface subsidence value and the influence extent are dramatically reduced,and the reinforcement effect is remarkable after using grouting injection reinforcement.Based on results of surface subsidence characteristics of tunnel in saturated soft loess,the disaster type of and the corresponding control technology caused by mining method of tunnel construction and shield tunneling are obtained.The reinforcement scheme of the Longhai Railway,the reinforcement measures of the saturated soft loess strata and the control measures for the construction of the shield method are proposed.Using FLAC numerical simulation method evaluates the feasibility of the shield tunneling through the existing railway.Meanwhile,,the deformation law of the roof,the side wall and the road surface of the existing Jinhua municipal tunnel induced by the shield tunnel is studied.Engineering practice indicates that the proposed construction disaster prevention and control measures of shield tunnel underpass Longhai Railway and the side of the existing Jinhua municipal tunnel in the saturated soft soil strata are reasonable and feasible. |
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