| Xi 'an is located in the Weihe Basin.Due to the particularity of geological structure and the exploitation of underground water,14 ground fissures have appeared successively in the urban area since 1950,which pose a serious threat to the Xi 'an metro under construction.When Xi'an metro tunnel passes through ground fissure zone,mining method is adopted and the special measures such as setting special deformation joints in sections and expanding cross-section are taken,which have proved to be effective so far.However,with the rapid development of Xi'an urban rail transits,the number of planned lines and the number of lines crossing the ground fissure sites are increasing.If the previous mining construction and special structural measures are still adopted,not only the construction period is long,the cost is high,but also the risk of waterproof pressure is significantly increased..As the mainstream construction method of urban subway,shield tunneling method has the characteristics of fast construction speed,good construction safety,controllable construction period and small impact on the surrounding environment.Whether the ground fissure sites can be passed by shield tunneling method and what countermeasures to take have become the focus of engineering field in recent years.In view of this,this paper aims at the problems faced in the construction of subway shield tunnel through ground fissures.Taking the construction of Xi 'an Metro Line 8 as the engineering background,through field investigation,the distribution and activity characteristics of ground cracks along the construction of Xi 'an Metro Line 8 are analyzed.Based on the physical model test and the finite element numerical simulation,the influence mechanism of ground fissure activity on metro shield tunnel structure under the large angle oblique(75 °)revealed,and the structural measures of shield tunnel crossing the ground fissure zone are proposed,and the numerical simulation method is used to scientifically compare and verify the proposed structural measures.The main research results are as follows:(1)On the basis of data collection and literature analysis,combined with the field survey of ground fissures along the metro line 8,the exposure situation and the impact on surrounding buildings,the relationship between Xi'an ground fissures and ground subsidence,the impact on Xi 'an Metro line 8,the feasibility of shield in ground fissure sites of Xi'an Metro Line 8 is analyzed.(2)Through physical model tests,combined with finite element numerical simulation methods,the deformation and failure mechanism of shield tunnels crossing the ground fissure at 75° oblique crossing was revealed.The results are showed that the deformation and failure of segments are mainly sheared,accompanied by torsion Deformation;The longitudinal influence range of ground fissures on the structure is roughly 5 rings on the hanging wall and4 rings on the bottom wall;The contact pressure of the surrounding rock at the top of the lining increases in the hanging wall and decreases in the footwall,and the bottom rule is opposite;The internal force of the shield lining changes at the top and bottom are basically opposite.The top of the shield lining is compressed on the hanging wall and the footwall is under tension,and while the bottom is just the opposite;The ground stress shows that the hanging wall decreases in the longitudinal direction of the structure and the bottom wall increases;While the horizontal curve is in the shape of "?" on the hanging wall and "V" on the footwall.(3)Based on physical model tests and existing research results,the countermeasures of shield tunnel passing through the ground fissures,such as surrounding rock grouting,steel plate lining,secondary lining hidden beam and steel tube segment replacing concrete segment,are proposed,and the effectiveness of the above reinforcement measures is verified by finite element numerical simulation. |
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