Research On Reliability Of Pile-net Composite Foundation For Serviceability Limit State In High-speed Railway

In recent years,new requirements for railway subgrade project have been put forward due to the leapfrog development of high-speed railway,so the control of residual settlement has become an important guarantee for the construction and safe operation of high-speed railway.The allowable stress design method(ASDM)has always been mainly used in the subgrade design of Chinese railway construction,however,with the development and application of reliability theory of geotechnical engineering,ASDM could not fully meet the requirements of current railway subgrade design with internationalization trend.Therefore,combined with characteristics of railway subgrade engineering,it is highly necessary to study the design method of serviceability limit state of high-speed railway subgrade based on reliability theory.In this paper,the serviceability limit state design method for pile-net composite foundation of high-speed railway is determined as the research topic,and the following high-speed railways in soft and loose soil areas,Wuhan to Guangzhou,Beijing to Shanghai,Shanghai to Nanjing and Beijing to Shanghai(southern section)is selected as the research objects.Combined with test results on site and long-term observation data,this article adopts a comprehensive research method,including probability statistics theory,field in-situ testing,numerical simulation,calibration calculation combined with inversion analysis,to systematically study the subgrade settlement characteristics,stress-strain rule and settlement determinacy calculation method for pile-net composite foundation of high-speed railway.Based on different reliability calculation methods and random field theory,a practicability limit state equation was determined for pile-net composite subgrade on the condition of autocorrelation geotechnical parameters.Combined with existing typical cases of high-speed railway subgrade and long-term settlement monitoring data,a settlement deviation coefficient and recommended value of target reliability has been put forward,also a design expression and partial coefficient in limit state were established,which provides reference for further study on the design of normal service limit state of high-speed railway subgrade.To summarize,the main research content and achievement are as follows:1.The deterministic calculation method of pile-net composite subgrade are summarized and analyzed,it proposes a simplified model for settlement calculation of pile-net composite foundation considering creep effect,and compares and analysis the quantitative results of various calculation methods,and discusses the settlement characteristics,load distribution and stress transfer rule of pile-net composite subgrade.The analysis results show that the code method can more quickly calculate the total settlement in the construction process,but the calculation model can not reflect well the interaction mechanism between pile and soil,and the calculation of additional stress on the foundation is not accurate,resulting in the calculation results of settlement disperses widely,which is difficult to meet the millimeter level of settlement after construction of high-speed railway.The M-B united solution for additional stress shows the additional stress in the soil near the pile tip increases obviously,with a relatively big difference compared with the normative calculation results.The stress variation rule basically matches with the numerical analysis results of rigid pile composite foundation and the variation trend of actual field stress data.Combined with M-B united solution,it analysis the calculation method of residual settlement and Koppejan creep model.The settlement calculation of pile-net composite foundation of high-speed railway based on Koppejian creep model basically matches with the stress variety law and actual measuring data change rule.Compared with the previous two methods,Koppejian creep model has more advantages,it can not only consider the pre-compression load,time effect and other factors during the construction,but also can calculate the settlement after acceptance of subgroup,thus it's convenient to make comparative analysis on the allowable settlement of subgrade required by relevant specification.2.Abaqus finite element software is used to simulate the pile-net composite subgrade,thus to analyze the variation characteristics and rules of settlement,stress-strain and pore water pressure in the process of progressive loading-unloading.The software can also verify the accuracy of various deterministic calculation methods of subgrade,and provide the calculation basis for reliability analysis of serviceability limit state of pile-netThe simulation results show that:(1)The settlement varies greatly in the filling stage,and increases continuously in the preloading stage.The settlement rebounds after unloading,then the settlement changes smoothly,the calculated settlement after acceptance meets the design specification requirements.(2)Preloading accelerates the dissipation of pore water pressure,in which the pore water pressure varies greatly during the filling stage.After the preloading stage,the pore water pressure decreases by 56%compared with the filling stage.After unloading,pore water pressure rebounds slightly,then decreases gradually and tends to be stable.(3)The results of Abaqus's numerical model basically match with subgrade settlement and stress variable rule,which indicates that Koppejian's creep model can be used as a calculation model for settlement design and reliability analysis of high-speed railway subgrade.3.Related theories and methods of reliability analysis of geotechnical engineering are introduced systematically.Based on the theory of random field,the reliability calculation formula for serviceability limit state with consideration of self-correlation of soil layer was deduced.According to the theoretical derivation,JC method,JC method considering the self-correlation of parameters and stochastic response surface method(SRSM)considering the self-correlation are used to calculate the reliability of the pipe network.The reliability calculation and orthogonal analysis of typical cases of composite subgrade under normal service limit state were conducted,also a sensitivity analysis on the variability of geotechnical parameters has been conducted.The results show that compared with JC method calculation,the improved stochastic response surface method has comprehensively considered the influence of various conditions,which makes the results more stable and reliable.Compared with embankment friction angle,the coefficient variation of secondary consolidation of composite subgrade is more sensitive to reliability of serviceability service limit state.4.To eliminate the influence of dimension factors and model uncertainty,this paper puts forward the settlement deviation coefficient?d(the ratio of control standard value to calculation value of settlement after acceptance)based on previous research results.This coefficient can greatly reduce the adverse effects on serviceability limit state study of railway subgrade caused by the following uncertainties,such as system error,spatial variability of soil parameters,statistical error and field test.Meanwhile,it makes the settlement calculation value of composite foundation is comparable under different allowable settlement control conditions for railways of different levels.The design level and actual operation situation of railway subgrade can be more intuitively reflected through the settlement deviation coefficient.This paper makes statistical analysis on the distribution law,mean value and variability of settlement deviation coefficient,and uses the distribution test to calculate the recommended value of the best settlement deviation coefficient.5.Taking 3 high-speed railways(Wuhan to Guangzhou,Shanghai to Nanjing intercity and Beijing to Shanghai)as the objects of study,and based on algorithm theory of Koppejan creep model and spatial variability of geotechnical parameters,the functional relationship between the reliability under serviceability limit state of pile-net composite subgrade of high-speed railway and settlement deviation coefficient is established by calibration calculation,then the target reliability index can be obtained by using the optimal settlement deviation coefficient.Through the equation of the serviceability limit state,the partial coefficient is calculated for analysis and optimization,then,a design expression in the limit state is established.The calculation results show that settlement deviation coefficient?d is basically linearly related to reliability?.When the geotechnical parameters present normal distribution,the recommended value of the target reliability index under normal service limit state of railway composite subgrade is 2.2,and the design expression in the limit state is as follows:(?)6.Due to the complexity of reliability calculation method and process,based on C#language and following the characteristics of expansibility,integration and maintainability,the paper compiles the calculation program for settlement and reliable calculation of serviceability limit state of railway pile-net composite subgrade.Also calculation software has been developed for batch processing of data and graphics with a friendly interface for easy operation,which provides a new method for settlement calculation and reliability research of pile-net composite subgrade.