Bearing Characteristics And Settlement Rules Of Pile-net Composite Foundation(CFG Long Pile + CSC Short Pile)in Collapsible Loess Area

In recent years,with the rapid development of China’s railway construction,the role of high-speed railway is becoming more and more important.The Yinxi high-speed railway project connects Yinchuan city and Xi’an city,which plays an important role in the construction development of the entire railway line and even the western part of China.However,there are many collapsible loess areas on the railway line,and its collapsibility has many inconveniences for the construction of high-speed railway.So it is necessary to adopt a reasonable foundation treatment.In this paper,through theoretical analysis and numerical simulation,the mechanical characteristics and settlement rules of pile-net composite foundation with cement fly-ash gravel(CFG)long pile and cement-soil compacted(CSC)short pile in collapsible loess area are studied;the conclusions are drawn and the parameters are analyzed,and on-site monitoring for the Yinchuan-Xi’an high-speed railway(YXZQ-5section of Gansu-Ningxia roadbed)are carried out for engineering verification;finally,the settlement prediction is made through the monitoring data.The main study contents are as follows:Firstly,the stress characteristics and settlement rules of pile-net composite foundation with long-short piles in collapsible loess area are summarized through the multi-plan numerical simulation method.The pile-soil settlement,pile-soil stress ratio and internal stress distribution of embankment are used to analyze the influence of the parameters on settlement and bearing characteristics of the composite foundation,and the parameters are such as the embankment filling height,the cushion thickness and the geogrid setting;through the numerical model established in this paper,the parameters are proposed to a reasonable range of values;in order to better determine the design optimization direction,the sensitivity analysis of the stress characteristics and settlement is carried out by the grey correlation analysis method.These jobs provide theoretical guidance for the design and construction of composite foundation in collapsible loess areas such as Yinxi high speed railway project.Secondly,relying on the on-site test of Yinchuan-Xi’an high-speed railway project(YXZQ-5 section of the Gansu-Ningxia section),the field test data are compared with the analytical model results and the rationality of the model is verified;the loading period is divided into three stages,the stress characteristics of the composite foundation,the variation law of the degree of soil arching effect and the settlement during the filling stage and post-filling stage of the pile-net composite foundation with CFG long pile and CSC short pile in the collapsible loess area are studied through the pile-soil stress ratio,the load sharing ratio,the variation law of stress distribution in the interior of the embankment and the pile settlement;furthermore,the relationships between settlement deformation,stress growth rate and loading rate of composite foundation are analyzed.And the role of CFG piles and CSC piles in different construction stages are discussed;finally,the degree of consolidation of the composite foundation is judged by the pore pressure curve,which is helpful to determine the preloading time and judging the foundation treatment effect.Finally,relying on the the preloading test of the Yinchuan-Xi’an high-speed railway project(YXZQ-5 section of the Gansu-Ningxia roadbed),the settlement prediction methods of pile-net composite foundation with long-short piles in collapsible loess area are studied.Then the different methods are comprehensively evaluated,and the appropriate method is selected to predict the settlement of the Gansu-Ningxia section of Yinxi Railway.It provides a reference for the selection of settlement prediction methods of the composite foundation in collapsible loess areas such as Yinxi high-speed railway.