Research On Deformation Law And Prevention And Control Measures Of Neogene Expansive Mudstone Single Track Railway Tunnel

Neogene mudstone strata have the characteristics of new formation,poor diagenesis,complex mineral composition,weak lithology,water softening,water expansion.Generally,the groundwater in such stratum is not developed.The tunnel has a certain self-stabilization ability after excavation,and will not have large deformation.In the later stage of construction or operation,tunnel lining cracking,invert floor heave and other disease are easy to occur.Once the disease occurs,its deformation is generally persistent and difficult to stabilize itself.The continuous development of deformation will cause the track uplift,lining deformation,lining cracking damage in the tunnel.At present,the research on the variation law of actual water environment around mudstone tunnel is not sufficient,and the demonstration of the treatment measures for the mudstone tunnel is not deep enough.Based on a mudstone tunnel under construction that has been damaged in Lanzhou-Hezuo Railway,the micro and macro research of the mudstone tunnel is carried out by laboratory tests,on-site detection,numerical simulation and monitoring measurement.The main research work and achievements are as follows:(1)The basic physical properties,mineral composition,chemical composition and disintegration characteristics of mudstone are obtained through laboratory tests.The microstructure and pore distribution characteristics of mudstone with different water content are obtained by scanning electron microscope.The content of free water and bound water is accurately distinguished by nuclear magnetic resonance method,The distribution characteristics of free water and bound water with water content are obtained.(2)The variation laws of lateral restrain expansion ratio and expansion force of mudstone with time under different water content are obtained,as well as the variation laws of expansion ratio with water absorption in the process of moisture absorption.Through uniaxial and triaxial tests of mudstone under different water content,the variation laws of the uniaxial compressive strength,elastic modulus,poisson’s ratio and shear strength of mudstone with water content are obtained,which laid a foundation for the establishment of mudstone expansion softening model.(3)Based on a mudstone tunnel under construction in Lanzhou-Hezuo Railway,The grey clustering method is used to comprehensively determine the mudstone expansion grade of the tunnel as medium expansive rock by taking the hydrophilic mineral composition content,dry saturated water absorption rate,which characterize the internal mechanism of mudstone expansion,and lateral constrained expansion rate,free expansion rate and ultimate expansion force,which characterize the external performance of mudstone expansion as indicators.(4)Based on the laws of expansion and mechanical properties obtained from laboratory tests,a seepage-stress coupling mechanical model of mudstone considering expansion and softening is established.Based on the expansion test in the process of mudstone moisture absorption,a deformation model of mudstone soaked in water is established and verified by numerical analysis.The simulated value is consistent with the measured value.(5)The numerical analysis of the tunnel is carried out by using the seepage-stress coupling model of mudstone considering expansion and softening.The variation of deformation and stress of lining with time can be divided into three stages: slow growth stage(0～4 years),rapid growth stage(5～7 years),and slow growth stage(after 8 years).The correctness of the model is further verified by comparing and analyzing the distribution of water field,displacement,stress,and plastic zone distribution calculated by mudstone tunnel with the variation of the field moisture content detection,displacement monitoring,and liner cracking characteristics with time.The expansion and softening laws of mudstone tunnels are analyzed.The effect of expansion is long-term and sustainable,while the softening effect remains stable after reaching a certain value.The effect of expansion on the inverted arch is much greater than the softening,and the softening effect reduces this adverse effect to a certain extent.(6)Four kinds of prevention and control measures of mudstone tunnel are comprehensively analyzed from the displacement value,maximum value of principal stress and characteristics of lining plastic zone,including optimizing the section shape,strengthening the lining structure,setting buffer layer,base anchorage grouting reinforcement.The reasonable priority of each preventive and control measures is given in according to the engineering practice.