Study On Force And Settlement Characteristics Of Long Group Piles Of High Speed Railway Bridge In Deep Soft Soil

In order to study the mechnics and settlement characteristics of thick soft soil of high-speed rail and deep piles of high-speed railway bridge, a series of studies at three site points (DK124, DK152, DK240) and eight piers of Beijing-Shanghai high-speed railway north section were carried out, which are automatic monitoring and mechanical characteristics research of deep soft soil pile foundation, theoretical calculation method research,finite element calculation method research and the prediction analysis of pile foundation settlement development process research. The main research and features are shown in the following aspects:(1) The method of "cooperating test using single point settlement gauge and level settlement gauge" is proposed, which provides a measurement of the piles foundation settlement in thick soft soil and soil settlement under pile tip. A series of compression curve for pile tip soil and the final settlement of the pile foundation have been plotted, through the compression difference of adjacent single points, sorting and analysis were carried out of the compression curve to obtain the actual thickness of compressible layer. Based on the work, a remote monitoring system of deformation is set up, the research of automatic monitoring of deep soft soil pile foundation is carried out, which provides basic data for the study of deep soft soil settlement characteristics, and the monitoring techniques provides technical support for similar projects.(2) In order to study the mechanical characteristic of thick soft soil of high-speed rail and piles of high-speed railway bridges, pile reinforcement meters are used to carry out field test. Based on monitoring data, elasticity formula is used to infer axial force distribution and variation of friction force and pile tip resistance. On this basis, according to the bearing capacity and time empirical formula, the time-bearing capacity formula was established in this paper, which provides theoretical support for the long-term prediction of bearing capacity of piles.(3) The settlement of pile foundation and lower compression layer thickness calculation of the three site points (DK124, DK152, and DK240) and eight piers in Beijing-Shanghai high-speed railway north section were carried out through domestic standards. Comparing the results with the measured value, the suitable methods are recommended to calculate the thickness of the compression and settlement of bridge group piles in thick soft soil, providing a basis for similar projects.(4) On the basis of the soil test data and the field test results, the empirical formula of compression modulus of the natural state associated with soil depth by fitting analysis was obtained. Furthermore, neural network was applied to establish the mapping relationship between soil compression modulus and pile settlement, by which the inverse analysis was carried out in the compression modulus of different soil at different depth. The inversion results and the calculated results by empirical formula are basically the same, showing that the proposed compression modulus formula can fairly reflect the relationship between compression modulus and depth, which has an engineering application and promotion.(5) Through Plaxis 3D Foundation, a finite element software of geotechnical engineering, the finite element numerical simulation of settlement were conducted, by considering the depth effect of soil compression modulus. Displacement of the pile foundation settlement and field values coincide well, which can be thought that the development law and trends of settlement through the finite element calculation were accurate and reliable. The nonlinear elastoplastic model was suitable for this kind of bridge pile foundation settlement-load-time curve’s calculations and predictions.(6) The settlement prediction of thirteen typical piers of two bridges was carried out based on the "joint forecast model". The fitting curve and measured curve have a high degree of agreement, indicating the validity and practicability of the prediction method. On the basis of previous studies, a long-term settlement prediction model of soft soil pile considering the creep deformation was established. The methods to get model parameters were studied through the fitting analysis of long-term settlement measured curve. The prediction data of pile foundation long-term settlement were in good agreement with the monitoring results, the research is of great engineering significance because the lack of monitoring data in long-term settlement prediction of soft soil pile foundation.