Research On Evaluation Method Of Collapsible Loess In Xi'an Urban Rail Transit

The collapsible loess determines the longitudinal distribution form of the line during the construction of urban tracks.With the continuous development of urban construction,subway section lines often span multiple different geomorphic units.Therefore,the analysis of the collapsibility of different geomorphic units and the reasonable evaluation of the deformation of the basement of urban rail transit structures are necessary to save investment and avoid engineering waste has great significance.At present,the current loess code is still used to evaluate the collapse of urban rail transit.The important grades of the buildings in the loess code are mainly based on the experience of multi-story house destruction while taking into account the possibility of flooding of the building.The current loess code is mainly applicable to ground structures with relatively shallow foundations.It has different stress states compared to the excavation and unloading of urban rail transit,so it cannot provide a reasonable evaluation of its collapse,and it cannot safely and effectively guide the collapse of urban rail transit sexual loess foundation treatment.In view of the above engineering problems,this article mainly uses numerical simulation and theoretical analysis to carry out a detailed analysis of the classification criteria of buildings and structures,the collapse deformation law of different geomorphic sites,the time-space characteristics of immersion in water,and the pressure of the building base.Calculation method of the amount of deformation of the collapse.The main research results obtained in this paper are as follows:(1)The classification criteria for the important levels of urban rail transit buildings are based on three factors:the important levels of construction,the ability of the structure to adapt to deformation,and the immersion environment.Methods.(2)Collecting large-scale flooding test data within the range of the Xi'an regional railway network using the landforms as the classification criteria,and analyzing the basic physical properties according to the landforms,determining the strength and weakness characteristics of different landforms and subsidence;Based on the description of the spatial-temporal variation of different geomorphic infiltration ranges,the infiltration characteristics of different geomorphic sites and different soil layers,the permeability of different geomorphologies and the reference value of the permeability coefficient of different geomorphic layers were proposed(3)By comparing and analyzing the average collapsibility coefficients of the Q3 and Q2 soil layers in different landforms,the strength of the collapsibility of the Q3 loess in different geomorphic sites is determined,and the reference values of the average collapsibility coefficient of the Q3 soil layer are proposed for different geomorphologies;Using the method of calculating the collapsing coefficient to calculate the self-gravity collapsing of different geomorphic sites and comparing it with the value obtained from laboratory tests,verifying the applicability and accuracy of the formula for collapsible deformation calculation;Based on the statistics of the accumulated deformation values of the deep punctuation points on the site and the deformation value of the site calculated by calculating the collapse coefficient,the bottom boundary of the large-scale flood test in Xi'an was determined to be within a certain range of the Q3 loess bottom boundary(4)Considering the characteristics of base stress distribution of underground structures under unloading,establish vertical compression stress calculation models for base stations of subway stations,interval tunnels and entrances and exits respectively.Two calculation methods for the amount of base collapse under actual vertical stress are put forward and applied to Hongqing Metro Station as an example.