Research And Application Of The Supporting Structure Of The Pile-supported Long And Short Piles In Deep Soft Soil Area

At present,China's relatively economically developed regions are mainly located in the central and eastern coastal regions.The economic development has promoted the prosperity of infrastructure construction.However,the coastal areas are also the main areas with complex geological conditions and soft soil distribution.For example,in the Pearl River Delta,there are different layers of siltation in the formation,and the deposition depth is about 3-30 m.Generally,the flow plasticity-soft plastic sludge or silt is distributed.earth.According to the main points of the rigid support design for deep foundation pits in soft soils at this stage,the deepest embedding depth that satisfies the requirements of overall stability,anti-uplift stability,and deformation control is generally adopted.This resulted in a surplus of certain safety factors for the foundation pit engineering in deep soft soil areas,the failure of the bending capacity of supporting piles to be fully utilized,the high construction cost,and the lengthy construction period.With the mature application of long and short composite piles in foundation treatment,pile foundation design,etc.,the design of foundation pit support is gradually integrated into the design concept of “long and short piles” and applied to the foundation pit project.However,the research on the theory and working properties of the composite pit supporting system for foundation pits is still in the exploratory stage,which makes the combination system of long and short piles has not been widely used in practical engineering.On the basis of summarizing previous research results,this paper uses the large-scale universal finite element software Midas GTS as the main research tool to comprehensively support and affect the supporting system of deep-grouted soft-soil foundation pit-supported long-short-piles composite support systems.Analysis and put forward its design control points,combined with three-dimensional finite element simulation analysis results and verification of monitoring data for the long-short-piles and three-supportsupport projects used in the deep foundation pit support of Jinyuan International Plaza in Hengqin Free Trade Zone,Zhuhai City.The supporting effect of long and short-piled composite support is proposed.At the same time,attention is paid to the construction and monitoring of the long-short-piled composite support structure for deep foundation pits.The main research contents and achievements are as follows:(1)Analyze systematically the analysis and calculation methods of the four types of supporting structures commonly used in foundation pit engineering(limit equilibrium method,elastic foundation reaction method,composite foundation reaction force method and finite element numerical analysis method)On the basis of sex,based on the experimental results of indoor supporting model for single-supported long-short-piles piles of foundation pits supported by Li Zhu and Zheng Gang,etc.,a multi-branch supported long-short-pile combination support theory was deduced,and a finite element numerical analysis method was also realized.The calculation and analysis of the supporting structure of deep-pile soft-soil foundation pit braced long-short-piles composite piles has greater advantages.It provides the basis for the stability study and the analysis of internal forces,etc.Theoretical basis.(2)Based on the data of a foundation pit project in Nansha District,Guangzhou,and incorporating the design concept of “long and short-pile combination support”,the short-pile pile length change and stratum were studied through the use of the three-dimensional integral analysis model established by Midas GTS software.Variations in properties,changes in ratio of piles and piles,changes in the ratio of short and long piles,changes in soil reinforcement in the passive area of the pit,and the magnitude of external loads affect the stability and force of the long-short-piles composite support system in the foundation pit.The law of the influence of deformation is an important achievement of this paper.(3)Based on the above research results on the regularity of the supporting structure of the pile-supported long-short-piled pile foundation pit,the control points of the design of the foundation pit support are proposed.The supporting long-piles need to meet the overall stability and anti-uplift stability.The requirements for short piles are to meet the requirement of "kick-off" stability,and the ratio control requirements for short columns can be divided into two cases.Moreover,for deep soft soil support structures,the control requirements for vertical bearing capacity must be met.(4)Confirming the length of the foundation pit by comparing the three-dimensional finite element simulation analysis results of the actual project of the long-short-piles and three-way internal support projects adopted in the deep foundation pit of the Jinyuan International Plaza in the Hengqin Free Trade Zone,Zhuhai City,and the monitoring data.The actual support effect of the composite support structure of piles further validates therationality and correctness of the design control points of the composite support structure for long-short-piled foundation pits proposed in this paper.(5)Combining the special engineering characteristics of soft soil and the characteristics of combined support of pile-supported long-short-piles foundation pits,the construction precautions and monitoring points of the composite support system for deep-grouted soft-soil foundation pits with long-short-piles are presented.