Study On The Influence Of Shield Tunneling On Adjacent Stuctures In Sandy Cobble Ground

It is inevitable for metro tunnels to pass through underground pipes and adjacent structures such as pile foundation and existing tunnels.Sandy cobble ground is an obvious mechanical unstable ground and vulnerable to ground loss caused by shield tunneling.The soil disturbance will alter the ground’s stress and strain state and affect its normal use by forcing the existing structures to move and changing its stress state.As a result,it is of great academic value and social benefit to study the impact of shield tunneling on adjacent structures in controlling construction,reducing its effect on surroundings and improving the structures’ safety.Based on the metro shield tunneling in sandy cobble ground,a model EPB(Earth Pressure Balanced)machine with diameter of φ520 mm was developed.This machine consists of excavation system,propulsion system,and mucking system and it can implement a consist sequence of heading,cutting,mucking and segment erection.Combined with theoretical analysis and numerical simulation,the test was carried out mainly to study the influence of shield tunneling on adjacent pile foundations,spread foundations,and existing tunnels,and such conclusions as follows can be drawn.1.The mechanism and process of ground settlement in sandy cobble strata caused by shield tunneling are analyzed,and based on previous analysis,the impact of construction on adjacent foundations and existing tunnels are investigated.2.Three dimensional finite element method was employed to study the impact of construction on adjacent structures in sandy cobble ground and based on the numerical results,two control methods about jacking force and grouting pressure were proposed.For the twin parallel tunnels,the influence of support pressure on ground deformation was emphasized and the ground deformation curve was drawn.Besides,the stress filed and displacement field were also studied.3.Granular discrete element method is introduced to study pertinent questions of shield tunneling in sand-cobble stratum.Specifically the calibration process of microscopic calculation parameters in granular discrete element method is analyzed.The analysis concentrates on the influence from factors including particle contact stiffness,particle contact stiffness ratio of normal direction to tangential direction,particle friction coefficient,confining pressure and particle bonding strength on calibration outcome.As a consequence,the variation law of calibration parameters with those influential factors is obtained.4.The microscopic calculation parameters of sand-cobble stratum and foundation are calibrated by utilizing two-dimensional granular discrete element method.The numerical calculation outcome is compared with analytical solution to verify the propriety of calculation parameters.Based on the above,a numerical simulation on shield tunneling with adjacent spread foundation and pile foundation is conducted.It emphasizes on the variation law of spread foundation’s own stress and stress condition of strata while tunneling.The contact state of the tunnel’s circumferential soil particles is comparatively analyzed from a microscopic aspect.In terms of the effect on existing tunnel structures from tunnels under construction,the study focus on factors such as ground loss and tunnel depth to analyze shield tunneling’s impact on existing tunnel structures.5.The model experiment concentrates on adjacent spread foundation and pile foundation’s own variation condition of displacement and variation law of internal force whilst shield tunneling.The result demonstrates that the squeeze effect on the foundation’s surrounding soil,the disturbance to the spread foundation and horizontal displacement is increasingly salient with the distance from the shield machine to the foundation decreasing.The foundation also inclines because of uneven settlement residing in the foundation.Shield adjacent construction causes soil disturbance which leads to change of axial force and bending moment in piles.The bending moment exhibits such variation law:it is zero at the top and gradually increases until a summit(it appears at three quarters of the whole length),and then it declines to zero.Metro shield tunneling in urban area undoubtedly exert influence on ground and underground structure,further causing a series of environmental problems.The research achievement of this thesis may have paramount referential value and engineering significance on other researches regarding the shield tunnel construction’s influence mechanism on strata and adjacent structures,control of ground deformation and reduction of environment disturbance caused by shield tunneling.