Study On The Failure Mechanism Of Inverted Arch Of The Soft Rock Tunnel Crossed By High-speed Railway

In the existing study on the floating up of the tunnel invert,there are many problems such as unclear load mechanism,imperfect calculation theory,incomplete research on the failure mechanism and the failure criterion,which leads to the lack of correct understanding of the floating-up failure mechanism and the blindness of engineering countermeasures.Especially due to the characteristics of weak surrounding rock and strong support of the high-speed railway tunnel,the longitudinal load mechanism becomes the main controlling factor influencing the floating up of the high-speed railway tunnel invert.In this paper,the floating-up failure mechanism and control measures of the upward arch of the high-speed railway soft rock tunnel are taken as the research purpose.The main research methods such as theoretical analysis,numerical simulation,field investigation and field measurement are used to study on the deformation and force characteristics,load mechanism,calculation theory,stress state,failure mechanism and failure criterion of the tunnel invert.Based on this,two kinds of control measures are proposed,such as integral reinforcement method and local reinforcement method.And numerical model is established to compare their effect.Finally,the floating-up project case of Wuyan River Tunnel is introduced for verification.Combining with the new characteristics of high-speed railway tunnel,this paper proposes a series of new viewpoints,theories and control measures about the floating up of tunnel invert.The main work and research results are as follows.(1)Based on the Wuyan River tunnel and other invert floating up projects,the engineering characteristics,geological conditions and deformation characteristics of the tunnels are analysed.Two load mechanisms of the invert are proposed for inducing its floating up,respectively called the lateral load mechanism and the longitudinal load mechanism.The lateral load mechanism,that is,the two ends of the tunnel inverting arch structure are subjected to the vertical load transmitted by the upper stratum and the structural self-weight,and the bottom is subjected to the foundation reaction force,which makes the invert cracking point be subjected to a large lateral tensile stress.The longitudinal load mechanism,that is,the stratum changes significantly along the longitudinal direction of the tunnel,resulting in a large degree of uneven settlement.This causes the transmission of shear forces and bending moments within the tunnel structure and peaks at the hard rock section of the formation boundary.The shear force increases the lateral tensile stress of the cracking point.The bending moment causes the invert to be subjected to great longitudinal compressive stress.(2)After the determination of the load source of tunnel invert,the load structure model,loose body theory,elastic foundation model and other relevant theories and reasonable assumptions are comprehensively applied in the establishment of the mechanical model of the invert and tunnel structure.The force and deformation calculation formulas of the invert and tunnel are derived,respectively.(3)According to the calculation formulas and results of internal force and deformation of the invert and tunnel structure,the correlation and sensitivity of the main influencing factors of the invert uplift are analysed,and the failure mechanism and control principle are demonstrated.The unidirectional stress mechanism and bidirectional stress mechanism are proposed.And according to the basic mechanical properties of concrete,failure criterions are introduced for the invert under different stress states.(4)When the tunnel produces a large extent and a range of uneven settlement,the longitudinal load mechanism becomes the main factor causing the uplift of the tunnel invert.The lateral tensile stress and longitudinal compressive stress of the inverting arch at the stratum boundary are much larger than those of other sections.Under this bidirectional stress,the degree of upward collapse of the inverting arch will be much worse than that of the unidirectional stress and may cause other engineering disasters.If the tunnel does not have a large degree and range of uneven settlement,then the lateral tensile stress of the invert is mainly induced by its own surrounding rock,which rises as the controlling cause for invert uplift.And this situation generally occurs in soft caves with large depth and high ground stress.(5)Overall,the overall reinforcement method has a better control effect on tunnel deformation and stress,with the improvement degree between 50%and 70%.In particular,its control efFect on uneven settlement of surrounding rock and tunnel by is obviously stronger than that of local reinforcement.In addition,it has stronger control effect on the peak tensile stress of the invert than the local reinforcement method.The local reinforcement method with the improvement range between 20%and 30%is relatively weak in the control of uneven settlement of tunnels and strata.However,it has a prominent control effect on the longitudinal compressive stress of the invert,with the improvement range reaching 70%.