| With the acceleration of urbanization in China,the development of underground space such as municipal pipeline,underground pipe gallery and rail transit are becoming faster and faster.Due to the different construction sequence,there are more and more new under-passing tunnels of the existing underground structure.The crossing project is the project with the highest risk in urban underground engineering construction,which needs to ensure the safety of the project and the surrounding environment at the same time.However the effect is mixed.Facing the increasingly strict requirements of deformation control,it has great practical significance to systematically study the response and control of the existing underground structure of underground excavated tunnels of the under-passing construction.Based on a large number of under-passing cases in Beijing,this paper database of under-passing projects is established by means of statistical analysis,theoretical analysis,numerical simulation and on-site monitoring.In this paper,the interaction of the projects between new tunnels-stratum-existing underground structure,key technology of under-passing construction,deformation prediction method and safety risk management and control are systematically studied.The main work and research results are as follows:(1)Based on the complex function method and Mohr Coulomb failure criterion,using the analytical solution of the ground stress and displacement of the shallow tunnel under the arbitrary distributed load on the surface of the earth,the plastic zone analysis method of the shallow tunnel construction stratum is established.The theoretical calculation model of new tunnel-stratum-existing underground structure is established,and the influence of the factors change in the under-passing construction on the plastic zone is analyzed.It is revealed that the mechanical essence of under-passing construction is the process that the new tunnel construction causes the stratum weakening,reduces the bearing capacity of the foundation under the existing underground structure,causes the existing underground structure to deform downward,and finally realizes the rebalancing.(2)Based on the case statistics and theoretical analysis,the key under-passing construction technologies of new metro section and station are systematic studied.When running the under-passing section,enough vertical spacing and the predicteddeformation can meet the standard,the horseshoe step construction method is recommended;When the interlayer soil above the arch roof can not be retained,the tratum grouting effect is not good,and there are adverse geological bodies,it is recommended to adopt the CRD method with flat roof and straight wall.When the station passes underground,when there is enough vertical space(above double the hole diameter)between the new and old tunnels and the single-layer station,it is suggested to adopt the method of multiple heading + large pipe curtain + grouting lifting;When the vertical space between the new and old tunnels is small and single-layer station,the method of multi heading + jack lifting can be used for construction,but there are strict requirements for construction quality;For single-layer or multi-layer station,hole pile underpinning method + jack lifting method can be used for construction.The deformation characteristic curve of the existing tunnel,the support characteristic curve of the new tunnel and the deformation recovery curve of the existing tunnel of the under-passing construction disturbance are drawn,and the mechanical evolution process is described.Based on the size of the cross-section excavation area of the new tunnel,the underground excavated method of the under-passing construction is divided into three levels,and the proposed construction methods of three levels are suggested.(3)Peck formula was used to fit the measured deformation of existing underground structure.After statistical analysis,it was clear that the formation loss rate could be expressed by the normal distribution.Based on an innovative prediction method of deformation probability of existing underground structure of under-passing project based on empirical method is proposed,which makes the traditional single numerical prediction method transformed into the probability problem of prediction deformation,and the prediction results are more consistent with the actual situation of parameter randomness.(4)The mechanism of jack lifting in the under-passing project is analyzed,a constitutive model which can simulate the action characteristics of lifting jack is proposed and a finite element calculation program is developed based on the nonlinear contact surface method,the cooperative analysis of the lower pile foundation-lifting jack-existing underground structure is realized.The validity of the constitutive model is proved by the example.The mechanism of jack lifting model provides the support for the accurate simulation of the deformation of existing underground structures.(5)According to the characteristics of the underground excavated tunnel of under-passing project,the whole factor of risk assessment index system is established.The method and process of risk assessment are established to realize the s afety risk assessment of all factors of under-passing project,which provides the t heoretical support for the deformation control decision-making of new project. |
