Study On The Mechanical Behaviour Of Asymmetric Closely Adjacent Tunnels Construction Process

By fully considering the different structure types of two tunnels with double asymmetric closely adjacent holes,the research mainly focuses on the calculation method of asymmetric closely adjacent tunnel surrounding rock pressure,the structure characteristics,the influence partition of the proximity construction,the optimization of construction scheme and the time-space effect of the construction mechanics by comprehensive use of theoretical analysis,model test,numerical simulation and field monitoring method.Through a series of studies,both theoretical basis and technical support can be provided for the design and construction of the asymmetric closely adjacent tunnel.The main research contents are as follows:(1)On the basis of the existing tunnel surrounding rock pressure calculation theory,this paper takes the geometric asymmetric of the closely adjacent tunnel and the influence of excavation sequence on the surrounding rock load into account.The action mode of the surrounding rock load on the asymmetric closely adjacent tunnel is clarified and the corresponding calculation method of surrounding rock pressure is proposed.Then,the distribution law and characteristics of surrounding rock pressure are discussed.At last,the calculation formula is validated by comparing with the measured data.(2)According to the mechanical characteristics of the sand and gravel stratum,the surrounding rock model material is prepared by considering the particle gradation and the internal friction angle as the main similar indicators.The model tests are performed by the independently developing large planar model test platform.By using the jack loading system to simulate different buried depth,this paper performs a systematic study of asymmetric close adjacent tunnel to investigate the surrounding rock stress distributions,structure mechanic characteristics,crack evolution laws and structure damage processes.Besides.the results of this research are compared with that of the non-symmetrical multi-arch tunnel,which provides a reference for the structure selection of the asymmetric tunnel project.(3)In this paper,a composite influence zone partition criterion is proposed by considering the additional deformation,additional stress and plastic zone as the indexes and the corresponding partition methods of the influence zone are also suggested.Taking the typical asymmetric closely adjacent tunnel project in Beijing urban area as an example,95 different numerical models are built considering the different tunnel section size and spatial distribution location.The influences of the new tunnel excavation on the existing tunnel are emphatically analyzed through the stress and deformation of retaining structure and the effect of plastic zone of surrounding rock.Moreover,the results of the influence zone partition and the corresponding construction strategies are provided.The conclusions are reliable and intuitionistic,which provides valuable references for the similar projects.(4)Refered to Beijing metro line 16 suzhou street station interval project,the large-scale three-dimensional numerical models are modeled to analyze the deformation of surround rocks and the stress of the supporting structures during the whole construction process of the asymmetric closely adjacent tunnel with variable cross-section in sandy pebble stratum.Optimized analysis is conducted,which includes the tunnel excavation methods,the reinforcement plans of middle wall and the layout schemes of lock foot anchor pipe.According to the numerical simulation results,the key points and countermeasures of the asymmetrical closely adjacent tunnelling in sand and gravel stratum are proposed.(5)Based on the field test,the construction mechanical behaviors and the space-time characteristic of variable cross-section asymmetric closely adjacent tunnels in the sandy pebble stratum are summarized by analyzing the ground deformation.contact pressure distribution between surrounding rock and lining,and stress characteristics of supporting structure during excavation.The main conclusions are significant guidance for the construction safety,quality control and progress management of other similar projects.