Bearing Deformation Behavior And Stability Analysis Method Of High Strength Pile Composite Ground For High Speed Railway

With the rapid development of society and economy,the improvement of railway construction standard and the shortening of construction period,especially the large number of high-speed railway construction in recent years,the post-construction settlement control standard of railway subgrade becomes more and more strict.The traditional treatment methods,which mainly include drainage consolidation,surcharge preloading,and composite ground treatment methods,which mainly include compacted sand pile and gravel pile,are difficult to meet the needs of engineering.The high strength pile composite ground with the strength of C15 or above has become the main method of ground treatment for high-speed railway subgrade engineering in China.According to the current calculation theory and design method of high-strength pile composite ground in railway subgrade engineering,the composite ground theory that the strain of pile equals to that of soil under rigid foundation is basically followed,which cannot reflect the basic behavior of "non-compatible deformation" of pile and soil under flexible embankment cushion layer.In addition,the high st rength characteristics of the pile can naturally meet the stability requirements.As a result,the stability checking method of high strength pile composite ground is missing.The aim of this paper is to establish the design and calculation method of bearing deformation which can reflect the non-compatible deformation characteristics of various structural components in the high-speed railway ground system and to put forward the stability analysis and calculation method which matches the failure mode of pile shaft.Focusing on the key issues of pile-soil interaction,load sharing,settlement calculation and stability analysis of high-strength pile composite ground,the theoretical research on bearing deformation characteristics and stability analysis method of high-strength pile composite ground of high-speed railway was carried out.Therefore,A set of design and calculation method of high strength pile composite ground with distinct characteristics of railway subgrade engineering is formed.The main work and conclusions are as follows:(1)Unified hardening/softening model for pile-soil interfacePile-soil interaction model is the basis of bearing deformation calculation and stability analysis of pile composite ground.There are many traditional linear discontinuous hardening or softening pile-soil interaction models,but they only simulate the characteristics of hardening/softening behavior of side-friction.Based on the characteristics of hardening/softening,which reflect the relationship between lateral resistance and displacement at the interface of pile and soil,the characteristic parameters such as nonlinear degree of curve of hardening section,attenuation coefficient of softening section,lateral resistance and displacement corresponding to peak point and residual state are adopted.The unified nonlinear continuous pile-soil interaction model and 6-parameter hyperbolic exponential combination function(H-E model)are established to describe the whole process of hardening/softening with lateral resistance and displacement.Through the statistical analysis of the related test literature,the range of model parameters under different soil properties is defined.According to the vertical force balance condition and deformation coordination relation of each micro section of pile,the calculation method of bearing deformation of single pile under vertical compression is established.The results show that the H-E model can suitably simulate the nonlinear relationship between lateral resistance and displacement.The single pile bearing deformation calculation method can reasonably predict the uniform lateral resistance or softening,the load-settlement relationship of single pile in layered ground and the distribution law of pile stress.(2)Stress concentration ratio calculation of high strength pile composite ground underembankment based on non-compatible deformationStress concentration ratio describing the load transfer efficiency between piles and surrounding soil,is a key parameter for predicting the bearing and deformation behavior of rigid pile-supported high-speed railway embankments.Based on the results of shear tests that ultimate shear displacement is less effected by stress levels but greatly influenced by specimen sizes,an ideal elastic-plastic load transfer model considering the variation of pile-soil interface shear stiffness with stress levels is developed.According to the three-section distribution pattern of pile-soil interaction that contains the upper plastic zone of negative friction,the central elastic zone of coordinate deformation and the lower plastic zone of friction bearing,a distribution model for skin friction on pile-soil interface describing the nonlinear elastic-zone and inhomogeneous plastic-zone is established.Taking the equivalent reinforcement of single pile with equal area in rigid pile composite ground under uniformly distributed embankment load as investigative object,and according to the force equilibriums differential equations of a sim plified analytical unit cell and the compression deformation compatibility equations of pile-soil-cushion,the formulas of the depth of the neutral plane and stress concentration ratio are derived.The analysis indicates that the depth of the neutral plane descends with the increasing of the flexibility coefficient of cushion,and descends firstly then ascends with the increase of embankment load.The stress concentration ratio decreases with the increasing of the flexibility coefficient of cushion,and increases at the beginning and decreases afterwards with the increase of embankment load.Hereby,the technical ideas including the pile arrangement and the cushion design using the maximum stress concentration ratio are put forward.(3)Settlement calculation of high strength pile composite ground considering pile-soilrelative slip soil and non-uniform deformation of soil between pilesBecause of the unequal compressing deformation of reinforced soil layer for embankment composite ground,the settlement has great difference between calculation value designed by the composite ground theory for rigid foundation and measured value.To establish the settlement calculation method of rigid pile composite ground considering pile-soil relative slip and unequal compression deformation of soil bet ween piles under embankment load,on the basis of the calculation model which can reflect the nonlinear and non-uniform side resistance of the upper negative friction plastic zone the middle compatible deformation elastic zone and the lower friction bearing pl astic zone under the embankment,the single pile equivalent reinforcement element in the composite ground with equal pile length under uniform load is taken as the research object.The vertical displacement model of soil between piles is established to reflect the characteristics of soil slippage and non-uniform compression deformation between piles.According to the vertical force balance equation of micro-element body and the coordination relationship between pile-soil-cushion stress and deformation,the settlement analytical expressions are derived to describe the characteristics of high-strength pile under embankment,such as the top and bottom thrust,non-compatible pile-soil interaction,pile side negative friction resistance and cushion adjustment.Hereby,the settlement calculation of high strength pile composite ground under embankment has good adaptability.(4)Comprehensive equivalent shear strength method for stability analysis of highstrength pile composite ground under embankmentIn view of the lack of stability checking method of high strength pile composite ground under embankment in Chinese code and the existing analysis method does not consider the influence of pile axial force and cushion effect on anti-sliding contribution.Taking CFG pile composite ground of high-speed railway,where pile body is prone to bending failure,as the research object,the determination method of pile anti-sliding force reflecting non-uniform bending failure of CFG pile in composite ground is formed by introducing the coefficient representing the bending ability of pile body at different positions of ground section.Based on the principle of equivalent moment of pile to prevent ground sliding,the equivalent shear strength expression of pile based on circular slip surface position is derived.Hence,a comprehensive equivalent shear strength method for stability analysis of composite ground of high-strength piles is established,which takes into account the progressive failure of piles and the influences of axial force and tensile force.A typical engineering case of CFG pile composite ground of a high-speed railway is taken as the standard for verification.Through comparison and analysis with existing methods of stability analysis of high strength pile composite ground,namely composite shear strength method,equivalent sand pile method,equivalent load method,British BS8006 code method,equivalent shear strength method,the comprehensive equivalent shear strength method is proved to be more reliable.(5)Non-uniform strength design and optimal control strategy of high strength pilecomposite ground under embankmentAccording to the difference of anti-sliding mechanism of pile at different positions of ground section,the section of composite ground is divided into four sections,which are characterized by extension-bending,bending-shear,compression-bending and compression failure modes.Based on the comprehensive equivalent shear strength method for stability analysis of high-strength pile composite ground,the key piles that affect the stability of high-strength pile composite ground and the technical countermeasures of zone strengthening and ground beam strengthening measures are discussed.The results turn out,the reinforcement effect of the ground beam installed on the pile top in the bending-shear zone is the best,the compression zone is the worst,the extension-bending zone and the compression-bending zone take the second place.When the ground beams are simultaneously set in the extension-bending zone and the bending-shear zone or the bending-shear zone and the compression-bending zone,high ground stability can be obtained.On the premise of ensuring the stability of composite ground,based on economic considerations,the suggestion that ground beams can be preferentially installed in the extension-bending zone and the bending-shear zone or the bending-shear zone and the compression-bending zone is proposed.