Study On Spatial Variability Of Loess Soil Parameters In Lanzhou And Stratum Deformation Of Subway Shield Tunnel Excavation

After a long period of geology and sedimentation,loess has a large spatial variability,which is characterized by vertical joint development and large void ratio in space.It is of great significance to carry out the spatial variability of loess soil parameters in Lanzhou for the construction of infrastructure in Lanzhou area.In this paper,the spatial variability of physical and mechanical parameters of loess in Lanzhou Metro Line 1 is analyzed by random field theory and geostatistics theory.The spatial variability of soil parameters is introduced into shield tunnel excavation engineering.FLAC^3D software is used to realize the numerical simulation of shield tunnel excavation considering the spatial variability of soil parameters.Combined with Monte Carlo strategy,the law of surface settlement and horizontal displacement under random calculation is calculated,and the confidence interval and reliability index of settlement are calculated.The main contents are as follows:（1）Based on the physical and mechanical parameters of loess in Lanzhou Metro Line 1,the optimal probability distribution of loess soil parameters is analyzed by K-S finite element comparison method.It is concluded that most of the physical and mechanical parameters of loess obey normal distribution,lognormal distribution and Weibull distribution.Pearson correlation coefficient was used to characterize the correlation between physical and mechanical parameters.Six pairs of parameters showed complete correlation,and six pairs of parameters showed moderate correlation/strong correlation.（2）The fluctuation range of cone tip resistance is calculated by correlation function method,fluctuation function method,semi-variation function method and weighted linear programming method.The mean value and fluctuation range of the fluctuation range calculated by the four methods are not much different.The calculation results can provide reference value for foundation and foundation reliability design in Lanzhou area.（3）The optimal function model of loess cone tip resistance calculated by the correlation function method is in the form of exponential cosine correlation function.The optimal function model of the semi-variogram method has 18.5%exponential model,18.5%spherical model and 63%Gaussian model.When the sampling interval is 0.1～1.0 m,the fluctuation range gradually increases with the increase of sampling interval.When the sampling interval is greater than 0.5 m,the calculation results of the three methods have little difference.When the number of fitting points is 1/12～1/2 of the total data,with the increase of the number of fitting points,the fitting coefficient increases first and then decreases.When the fitting data points are 1/10,the fitting coefficient reaches the maximum.（4）The spatial variability of physical and mechanical parameters of loess is calculated by combining the fluctuation range with the variance reduction function,and the spatial variability evaluation of soil parameters is realized.The results show that the overall coefficient of variation of soil parameters>point coefficient of variation>spatial coefficient of variation.The water content,compression coefficient,compression modulus and saturation of loess soil parameters are all in high variability,and the remaining parameters are in low variability.The covariance matrix decomposition method is used to simulate the random field of soil parameters.The anisotropy coefficient n is taken as the ratio of horizontal fluctuation range to vertical fluctuation range.With the increase of n,the random field of soil compression modulus is gradually layered in the depth direction,and the variability is gradually reduced.When n increases to 1000,the random field model is approximately a random variable model.（5）The numerical calculation model of one-to-one mapping between the random field of soil parameters and the three-dimensional model of shield tunnel is established.Combined with Monte Carlo strategy,the surface deformation of shield tunnel excavation under different anisotropy coefficients n is studied.The calculation results show that as n increases,it gradually decreases,and the calculation results are closer to the deterministic analysis results.The surface settlement and surface horizontal displacement considering spatial variability are greater than the deterministic analysis results.Monte Carlo random simulation and reliability index calculation were carried out for 8 random calculation conditions.Lognormal fitting was performed on the calculated maximum surface deformation and maximum surface horizontal displacement,and it was found that the fitting effect was good.Under the condition of 95%confidence and given allowable settlement of 20 mm,the reliability of all working conditions is greater than 3,and the surface settlement of all working conditions is in a stable and safe state.