| The pile-raft structure of high-speed railway,as a new foundation treatment technology proposed in China,can well control the post construction settlement of subgrade and ensure the safety of train operation,which has been widely used in engineering practice,such as Beijing-Tianjin intercity railway,Shanghai-Nanjing intercity railway,Beijing-Shanghai high-speed railway,Haqi high-speed railway and Daxi high-speed railway.When the raft is at different heights of embankment,the length and number of piles and the relationship between piles and soil are different,and the pile-raft structure is affected by different degrees of train load because of the different thickness of soil on the raft.At present,pile-raft structure applied in engineering adopts the structure form of raft at the bottom of embankment,which is conservative in the design of raft size and pile quantity,and lacks perfect design theory.Therefore,considering the train load and embankment load,there is still room for optimization in the design theory and structure form of pile-raft structure.In this paper,the structure characteristics of raft at different positions of embankment are compared and analyzed by the method of model test and simulation calculation,and the pile-raft structure is optimized.The main contents and conclusions of this paper are as follows:(1)Carry out indoor model test of pile-raft structure.Based on the similarity theory,a model test scheme of pile-raft structure was designed,and the test was carried out for the raft at the bottom,middle and top of embankment.The results show that the structural stress and deformation of raft in the middle or top of embankment are relatively small,so it is suggested to change the traditional construction method of raft at the bottom of embankment in engineering application,and consider placing raft inside embankment to reduce the structural stress and better control the settlement deformation of subgrade.(2)Establish the train-rail-pile-raft structure coupling model.The establishment process of vehicle track dynamic model and pile-raft subgrade model is described,and the reliability of the model in this paper is verified by comparing the data of existing literature with the vertical acceleration of rail and the raft pressure calculated by simulation.(3)Analyze the working characteristics of pile-raft structure under train load.Based on train-track-pile-raft structure coupling model,the working conditions of the raft under train load at the bottom,middle and top of the embankment are compared and analyzed,including the stress and deformation characteristics of embankment,raft and pile-soil reinforced foundation.The results show that when the raft is at the bottom of the embankment,the settlement deformation of the embankment is large and the stress of raft is mainly transverse tension,and the bearing capacity of pile and soil can be exerted properly.The part below the embankment is less affected by the train load,so the coupling effect of train and pile-raft structure can be ignored in the design.When the raft is in the middle of the embankment,its characteristics are close to those when the raft is at the bottom of the embankment,but the embankment settlement deformation and structural stress values are smaller,and it has a large safety reserve,so it is suggested to be used in practical engineering.When the raft is at the top of embankment,the subgrade surface has some differential settlement,the stress of raft is mainly longitudinal tension,the negative friction range of pile is large,and the central pile bears most of the load.Raft and pile are severely affected by train loads,so it is suggested to increase the size of raft and pile diameter to ensure the strength of the structure.(4)Optimization design of pile-raft structure.The scheme is compared and selected from aspects of settlement control effect,safety intensity and cost,and it is concluded that the structure of raft in the middle of embankment is the best choice.On this basis,it is further determined that the reasonable position of raft should be at 1/2 height of embankment.The reasonable thickness of the raft is determined to be 0.3m according to the deformation and material cost of the raft.Considering the dynamic response of pile-soil bearing characteristics and the safety of raft deformation,it is suggested that the thickness of cushion should be 0.2m,the pile should be 0.5m and the pile spacing should be 2.0m.There are 77 figures,38 tables and 71 references. |
