The Load Transfer Mechanism And Stability Analysis Study On The Spared Pile Embankment System

The geosynthetic reinforced piled supported embankment consists of embankment fill with single or multi-layer of geosynthetic reinforcements,spared piles,pile caps and foundation soil.This new type of highway embankment on soft soil can be referred as the spared pile embankment system.In recently,the spared pile embankment system was widely used in the highway or high speed railway on the soft soil in the region.However the spared pile embankment system as a new thing,its theoretical research lags behind engineering practice.So getting further and system researching on its working mechanism,perfecting the analysis theory and design method,the study has not only important theoretical significance but also important practical significance.The spared pile embankment system on deep soft soil was the research object.Based on the fully understand the structural characteristics of natural sedimentary clay and acSurately simulate the effects of the soil structure on the in-situ mechanical properties,the squeezing effect during pile driving was analyzed.And the working mechanism and the stability mechanism of the spared pile embankment system were foSused researched by the in-situ test,theoretical analysis and numerical analysis.The pile setting effect analysis model,the embankment load transfer model,the spared pile load transfer model and the stability analysis model were established.The setting effect was revealed in the paper the transfer characteristics of embankment load and the pile load were revealed too.And then the practical settlement calSulation and the stability analysis method were established,so the design theory was perfected.Then base on the obtained solutions and analysis models,relevant computer programs were developed and the calSulations were verified.The results mainly include the following parts:(1)Based on the undisturbed and the reconstituted soil samples contrast test results,the traditional strength sensitivity(St),the yield stress ratio(YSR),stress sensitivity(Sσ)and stability index(SI)can be employed as structural parameters to quantify the influence of the natural soil structure on the mechanical properties.An elastic-plastic constitutive model was proposed to consider the effects of soil structure and the initial anisotropy on the natural sedimentary clay.The rationality and validity of the proposed model was validated by using the trial testing results.And the structural soil compression deformation parameters a and b was introduced,the undrained shear strength calSulation formula for the KO consolidation structural soft soil was established.The rationality and validity of the proposed method was validated by using the trial testing results.(2)This paper presents an exact semi-analytical analysis for cylindrical cavity expansion in modified Cam Clay critical state soil under the undrained condition.By assuming small-strain deformation in the elastic region and largestrain deformation in the plastic region,the formulation of the problem is reduced to solving a system of firstorder ordinary differential equations for the radial,tangential and vertical effective stresses in the plastic zone.The pore pressure distribution can subsequently be determined from the radial equilibrium equation in terms of the total stresses.The semi-analytical solution derived in this paper follows the rigorous definitions for the two effective stress invariants instead of approximate ones,as usually assumed in the existing literature,and applies to the more general case where the out-of-plane(vertical)in situ stress may be different from the in-plane(horizontal)one.The results show clearly that the stress history(overconsolidation ratio)has a significant influence on the stress and pore pressure distributions around the cavity.The present solution provides an exact and realistic theoretical framework for predicting the soil behavior around the cylindrical cavity,with applications to the interpretation of pressuremeter tests and to pile installation problems.Base on the pile end failure mode,the computational formula was established between the spherical cavity expansion stress and the pile end limit resistance,The rationality and validity of the proposed method was validated by the Boston Blue Clay results.The end-beating capacity and the settlement of piles prediction model was established and the critical depth and the minimal thickness was disSussed.(3)Based on the unit cell theory and the shear displacement theory,the pile-to-pile elastic coupling principle of mirroring image method was introduced,the shear displacement cell mode was built for analyzing the load transfer of piles in large pile group.The generalized shear displacement analytical expression was deduced by introducing the generalized shear displacement concept which can be explained as a superposition of shear displacement and relative slippage of pile-soil interface.The iterative solving method was presented based on the pile tip displacement coordination condition.The example analysis reveals the effects of sparse piles space and the load level on the settlement-reducing property of sparse pile foundation.The load-displacement transfer characteristics of sparse piles and foundation are expounded.The contact mechanism of the sparse-pile foundation working property of the pile top relative displacement and the stiffness of the pile end bearing layer is explained.The validity of the model and the flexibility of the algorithm are also verified.Based on the shear displacement analysis theory and the unit cell of large pile group,a shear displacement analysis model was presented to analyze the embankment load transfer.The relationship of shear stress and relative displacement between the fill columm on pile and the surrounding fill was idealized to be perfectly elasto-plastic,then based on this assumption,the governing differential equations of load transfer of embankment unit cell were derived by introducing the generalized shear displacement concept.With the characteristics of embankment Sushion structure considered,shear displacement analytical solution for the load transfer of double-layer system was obtained.The results show that the proposed method has well applicability in embankemen supported by spared piles and its extension can be used in pile-soil load effect analysis of separated piled raft foundation.(4)Based on the base load transfer and the pile load transfer theoretical analysis,the pile settlement contained the up pierce,its compression and down injection was directly obtained.And the basal pile soil load sharing and the pile load distribution can be obtained at the same time.So the total settlement conceptual model of the pile foundation system was put forward which contained the pile relative displacement value and the pile end layer compression.Base on the load transfer characteristics of the spared pile embankment system,the additional stress field was calSulated by Mindlin-Geddes Solutions and Boussinesq Solutions.Based on the additional stress field,the stress level significantly reduced in the soil between the piles which the reason of the pile reduced the total settlement.And the additional stress level in the pile end under layer was improved which size was 1.2～1.4 times morethan Boussinesq theory.Based on the study,the practical compression deformation methods was proposed by composite the additional stresss in pile end under layer and the dformation law of natural clay.(5)Based on the serviceability limit states,the pile conpensation stability conceptual model was put forward.The subgrade base load was shared more by pile and the shear plastic deformation in the soil was suppressed.First according to the load sharing ratio of the soil between the pile cap,ignoring the existence of the pile,the distribution load pk was calSuted.Then the analytical solutions of the foundation beam under the load which axial force was constant was derived to analyzed the board diffusion effect of the mantle layer on the soft soil layer.Based on the discriminant method of lateral displacement of the soft soil foundation which used the undrained strength,the subgrade critical height load which is pk<3Su was put forward for the spared pile subgrade system.(6)Based on the ultimate limit states,the pile yield instability conceptual model was put forward which the pile subgrade system unstability caused by the pile failure when the plastic deformation load of the soft soil in passive zone acts on the pile.Based on the undraine strength of saturated soft clay theory,the elastic-plastic analysis method was improved for discriminanting the soft clay shear plastic flow theory.The dividing method of pile passive section and the passive load distribution model was put forward using the undrained strength theory and the Coulomb failre criteria.This paper derived the distribution load foundation beam analytical solution for constant axial force and approximate analytical solution for the linear change axial force,to introduce C+Mz combined foundation coefficient model,and considering the critical depth concept of the pile side soil limit resistance,based on the load transfer matrix principle,the segmented analytical method of the pile internal force was put forward.