Study On Sub-System Interaction With Evolving Settlement And Load Distribution For GRPS Embankments

Settlement control is one of the main challenges for high-speed railway projects in soft soil areas.In order to satisfy both the demand of settlement control and schedule management,Geosynthetic-Reinforced Pile-supported(GRPS)embankments are widely adopted.However,excessive settlement of foundation soil between piles are observed and it leads to unexpected detachment of the geosynthetic reinforcement(GR)with foundation support.Obvious detachment may lead to geotextile fracture and oversize settlements of the embankment,which severely threaten the railway operation safety.The generally adopted design procedure of GRPS embankments separates the calculation of tension in GR and subsoil settlement,and the interactions between the piles,reinforcement,fill and subsoil was inadequately considered.The unevenness foundation settlement could influence the stress state of GPRS embankments.The characterics of an evolving non-uniform settlement could change the load distribution in GPRS embankments,and the influences of load distribution and deformation in the embankment to the long-term service performance remains to be solved.This paper adopted laboratory experiment and multiple research methods including system engineering analysis to study the evolving feature of load transfer mechanism in GRPS embankments.A subsystem analysis is carried out to identify status in the embankments;the range of soil arching as well as the load distribution of the GR are studied considering differential settlement patterns,different stiffness of subsoil support,various GR stiffness and filling heights;a dynamic calculation method of pile-GR-foundation load and settlement is established.Main conclusions are listed as follows:(1)Model-based System Engineering(MBSE)theory is introduced to carry out a system functional logic analysis of the GPRS embankments.Firstly,the system failure mode of the embankment is studied by analyzing the overall damage modes and local damage modes,and the ultimate bearing capacity of each component of GPRS embankments in limit states are obtained.In order to establish the state transfer criterions from the normal service state to the limit state of the GPRS embankments,a four-status conceptual model of the vertical bearing evolution process of the GPRS embankments is established by using the dual equal-settlement-plane method.The GPRS embankment is divided into three subsystems according to its load-bearing function: fill-pile-grid subsystem,grid-gravel-cushion subsystem and pile-foundation soil subsystem,so as to establish a Fault Tree Analysis(FTA)of the embankment system and obtain a qualitative analysis method and quantitative analysis framework for system reliability.(2)Laboratory experiments on the interaction of sub-systems of GPRS embankment were carried out.A three-dimensional multi trapdoor model box was designed to simulate uneven settlement characteristics including different settlement sequences and foundation stiffness.Flexible pressure mapping sensor was introduced to measure the load distribution on the reinforced layer.The range of soil arch effect on the GPRS embankments of high-speed railway and the load distribution pattern on the reinforcement layer under various foundation settlement characteristics were studied.The effects of settlement in different areas of the foundation,the stiffness characteristics of shallow subsoil and the impact load on the embankment surface on the load distribution characteristics of the reinforcement layer and the evolution of the soil arch effect were studied.Load distribution formulas with and without impact load were proposed to express the pressure acting on reinforcement.(3)Discrete element method simulations were carried out to study the interaction between subsystems in GPRS embankments.Two-node cylindrical elements and threenode shell elements were introduced to achieve efficient simulation of irregular forms of geomembrane.The microscopic parameters of reinforcement layer and other materials were calibrated against material mechanical properties.The threedimensional multi trapdoor experiment process is reproduced to obtain the geogrid deformation and the contact formulation on both sides of the geomembrane,as well as the spatial distribution of soil pressure and the evolution of soil arching effect.The stability of soil arching is studied with parametric process,and the reason to “load recovery” phenomenon that GR load increase with relatively large settlement is discovered.Parametric study was introduced to analyze the coupling of soil arching and GR deflection,and an arching stability condition was discovered.(4)A dynamic calculation method of load and settlement in GRPS system is established basing on the three subsystem models,multiple-status and state transfer conditions.The soil arching model as well as interface model of the reinforcement layer are modified using the load patterns on two sides of the reinforcement,which was formulated from the laboratory experiment and discrete element method simulations.By incorporating the load-transferring model pile-foundation soil,an iterative method is used to obtain the coupled solution of the three subsystems.In the calculation process,major parameters representing the embankment system are the vertical defection of reinforcement layer,the differential settlement between subsoil surface and pile head,the load above reinforcement layer,and pressure on the subsoil surface.The feasibility of the purposed dynamic calculation method is verified against an existing soft soil embankment in coastal area,and it succeed in identifying the serviceability status of the embankment.