Study On The Settlement Theory Of Long-short Pile Composite Foundation Under High-speed Railway Embankment In Collapsible Loess Area

The settlement of the Subgrade is controlled strictly in the normal operation of high speedrailway, the most critical one is to control foundation settlement. However, the technology ofthe long short pile composite foundation in the loess region is not only to control the totalsettlement of the foundation, but also to achieve benefits of the economic and social. Butstudies of the predecessors is not in-depth and perfect. Therefore, this paper relies on theconstruction of high-speed railway project in loess area to study systematically the settlementof the long short pile composite foundation. The research contents of the paper are mainlyembodied in the following aspects:First, after analyzing the working mechanism of long short pile composite foundationunder the load of embankment, I summed up the calculation formula of the bearing capacityof the long short pile composite foundation.At the same time, the methods of settlementcalculation of composite foundation are summarized and analysed.Second, two typical embankment sections are selected.The method of elasticembankment and Boussinesq triangular load and Boussinesq combined load are used tocalculate the additional stress in foundation.Then the composite modulus method and chordmodulus method are used to calculate the settlements of the long short pile compositefoundation,and the calculated settlements is compared with the measured settlements.Thusthe method of settlement calculation is obtained in the long short pile composite foundation inthe loess area.Third, using FLAC3D software, the long short pile composite foundation is simulated.The Mohr-Coulomb model is used as the constitutive model of the soil between piles. In thenumerical calculation, the optimum depth is3.5b, and the optimum width is4.5b, b is thewidth of the road base. Simulation results show that with the increase of the thickness of thecushion, foundation settlement increases,with the increase of the cushion modulus, foundationsettlement decreases,with the increase of the length of long pile and the modulus of the soilbetween piles over the short pile end, foundation settlement reduces significantly,with the increase of long pile modulus and the length of short pile, short pile modulus, the decrease offoundation settlement is not obvious.Fourth, according to the observation data of two subgrade settlement, hyperbolic methodparabola method and BP neural network are used to predict composite foundation settlement.The MATLAB software is used to complete model construction, sample training andprediction of the BP neural network.Then, the fit index r2and the relative errors of threeprediction methods are calculated and compared, and the BP neural network is most suitableto predict settlement in the long-short composite pile foundation of the loess area.The fittingprecision and prediction precision is very high.