Study On The Settlement Characteristics Of High Speed Railway Rigid Pile Net Composite Foundation

As an effective way of soft soil foundation improvement to control settlement, rigid pile-net composite foundation has been widely used in the construction of high-speed railway subgrade and has achieved good engineering benefits. However, mechanism of rigid pile composite foundation improving soft soil foundation and theory of the settlement is not complete, the theory research is far behind from the engineering practice. So, a thorough study on rigid pile composite foundation reinforced soft soil foundation technique is made in this thesis through the method of model experiment, field test and numerical analysis.The force deformation characteristics and load transfer law of railway rigid pile-net composite foundation is analyzed and the common settlement calculation methods are summarized. Based on the investigation of existing rigid pile composite foundation model test experience, the similarity criterion of rigid pile composite foundation model test is obtained by using dimensional analysis method. According to the similarity criterion, four groups of comparable model tests are designed. Through the model tests, a large amount of valuable experimental data is obtained:the ultimate bearing capacity of soft soil foundation before and after reinforcement, the load-settlement curves and load-stress curves. The rule of substratum settlement and quantity of the pile stabbing change with the pile length and layer modulus is revealed.The ABAQUS software is used to establish the elastic-plastic 3D model based on the model experiment. The range of the friction parameters of the foundation soil and the pile soil is determined. Numerical simulation results show that at the bottom of the pile stress of the pile and the soil stress between the pile had a great difference in the range of 2 times pile diameter, which presents the shape of’W. The additional stress attenuation is very fast, in the position of 5 times pile diameter times the pile diameter below the pile bottom, the additional stress is similar to the uniform load distribution.Through the field test, embedding the settlement deformation and stress monitoring at test section, analysis of surface settlement, layered settlement of foundation soil and the pile-soil stress. The field test results show that the settlement and deformation of foundation are increasing with the increase of load. In the period of dead load, the test shows that the amount of compression of the underlying layer is larger than that in the reinforced area, the settlement development tends to convergence in five months. Using the existing settlement calculation method to calculate the settlement of the monitoring section, the calculation results are close to the measured values by using the partition calculation method and L/3 method.Based on the finite element analysis software PLAXIS, numerical simulation of rigid pile composite foundation stratified on the subgrade fill construction process and one year preloading period, rigid pile composite foundation settlement characteristics were analyzed and compared with the field test results. The numerical simulation results show that the development trend of settlement deformation is basically consistent with the measured data. The model of composite foundation of different pile spacing is simulated. The results show the model with pile spacing equivalent to 6 times of pile diameter is economical and reasonable.