Research On Settlement Prediction Method Of Medium Compressive Soil Foundation In Railway Subgrade Engineering

The post-construction settlement mainly comes from the long-term deformation of the foundation under the action of embankment self-weight load and superstructure self-weight,so accurate prediction of foundation settlement is essential to ensure the quality of subgrade engineering.Heavy haul railways have large capacity,high efficiency,and low cost.Especially for large railway countries,they have become an important symbol for the development of modern freight transport.However,heavy axle load and high traffic density will also make the subgrade withstand greater dynamic load and fatigue,so the heavy haul railway subgrade needs to have higher strength and stability.At present,domestic and foreign researches on prediction methods of roadbed settlement are mostly aimed at soft soil foundations with high compressibility,and are mainly targeted at high-speed railways or passenger dedicated lines.However,there are few studies on the settlement prediction of medium compressive soil foundation under heavy haul railway embankment.Therefore,it is of great scientific and practical value to explore the easy-to-operate and high-precision prediction method in combination with the measured data of heavy haul railway foundation settlement.The mathematical property equations of settlement-time curve model for foundation is built with final settlement volume,half-value time of settlement and settlement rate of half-value time.The mathematical deficiencies of the traditional two-parameter hyperbolic model and exponential model are revealed,and a three-parameter power function model for post-construction settlement-time is established.The feasibility of the three-parameter power function model was verified on the basis of on-site settlement observation data of Haolebaoji-Ji’an Railway test section,and the adaptability of the three-parameter power function model was discussed in combination with the actual settlement data of the second line of Lanzhou-Wuwei Railway.Finally,based on the three-parameter power function model,a new method for prediction of settlement of stage constructed embankment on medium compressive soil foundation is provided.The main work and results in the thesis are as following:(1)Through the mathematical characteristic equation,it is found that the two-parameter hyperbolic and exponential curve models have a fixed settlement rate of half-value time when the final settlement volume and half-value time of settlement are fixed,making the model less adaptable.The three-parameter power function model overcomes the shortcomings of the two traditional models.The three characteristic quantities of the characteristic equation are independent of each other,so the curve form is more abundant.(2)Combined with the post-construction settlement observation data of the foundation under the embankment of Yueyang-Ji’an section of Haolebaoji-Ji’an Railway,the accuracy evaluation of the prediction model was carried out with variance,correlation degree and deviation degree as the test indicators.The results show that the three-parameter power function model has better fitting accuracy and higher prediction accuracy than the two traditional models.The post-construction 256 d observation data to meet the requirements of the code predict the settlement of the foundation at 528 d.The average deviation of the three-parameter power function model is only 5.99%,while the traditional two-parameter model increases to 14.38% and 22.46%,respectively.The settlement data within 256 days after construction is used to predict the settlement value on day 528,which meets the requirements of the specification for observation time.Average deviation of the three-parameter model reaches only 5.99%,while that of the traditional models are 14.38% and 22.46%,respectively.(3)Further analysis of the settlement prediction of the roadbed of the second line of Lanzhou-Wuwei Railway and other projects found that for the section with a large post-construction settlement deformation level and certain data fluctuations,the three-parameter power function model has poor prediction accuracy when the monitoring time is short.However,when the monitoring time increases,the accuracy of its prediction is significantly improved.For the case where the post-construction settlement deformation level is small and the data is relatively fluctuating,under the same monitoring time,the three-parameter power function model performs better in the test index that characterizes the effectiveness of settlement prediction.Therefore,the three-parameter power function model has certain adaptability and can be used as a reference prediction method.(4)Based on the three-parameter power function model,a method for prediction of settlement of stage constructed embankment on medium compressive ground is presented.The core idea is to regard the cumulative settlement at a certain moment as the superposition of the settlement caused by the load increments of all levels at this moment.It not only considers the nonlinear deformation characteristics of the soil,but also analyzes the influence of the historical loads on the deformation under the step loading.This method can not only make full use of all the measured data at all levels of loading period,but also only need a small amount of data at the last stage with a shorter dead load period to get a more accurate settlement prediction value.The feasibility of this method was verified by combining the measured data of the foundation settlement of Haolebaoji-Ji’an Railway.