Stochastic Mechanics-bayesian Method And Application Of CPT Data To Calibrate Geotechnical Parameters

Regular borehole sampling and laboratory tests have a long process,many disturbances,and large errors.Hence,it is difficult to quickly,simply and accurately obtain the geotechnical parameters in scientific calculations.Cone Penetration Test(CPT)is an in-situ testing technique with a wide range of applications and high data quality.Through theoretical research,equipment research and development,and engineering practice,many scholars have been establishing a large number of reliable theories,experiences and regression formulas for soil classification,conventional soil parameter conversion,and multi-field physical and chemical parameters.However,CPT data(cone resistance q_c,sleeve friction f_s,and pore pressure u)cannot be directly used in Geotechnical engineering,and CPT data cannot be directly converted into constitutive model parameters(e.,one such refined geotechnical parameters),the fundamental reason is insufficient research on the penetration mechanism.This paper proposes a stochastic mechanics-Bayesian method.Firstly,based on the cavity expansion theory of Mohr-Coulomb yield criterion,the macro and micro coupling numerical calculation method of the CPT penetration process is established to study penetration mechanism.Secondly,comprehensively using statistics and Bayesian theoretical knowledge to calibrate the statistical characteristics of the constitutive model parameters.Finally,the constitutive model parameters are applied to engineering practice to compare the prediction accuracy of the prior distribution and the posterior distribution to the monitoring data,and verify the rationality of the stochastic mechanics-Bayesian method of the CPT data to calibrate the geotechnical parameters.The main work and innovation are as follows:(1)Based on the Mohr-Coulomb failure criterion,the cylindrical cavity expansion theory is used to solve the limit expansion stress,and the relationship between the cone resistance,sleeve friction and the limit expansion stress are derived.Taking the(2)₁silty clay of the shallow bearing layer as the research object in Shanghai,the theoretical solution,numerical solution and true value error of cone resistance,sleeve friction and limit expansion stress are calculated from a macroscopic point of view.The results show that the errors of the limit expansion stress,cone resistance and sleeve friction calculated by the cylindrical cavity expansion theory and the true value are 1.3%,0.45%and 0.77%,respectively.(2)Firstly,through the triaxial compression test of the same scale,Which establishes the geotechnical parameter conversion formula between the macroscopic finite element and micro discrete element coupling numerical calculation.Secondly,the(2)₁ silty clay layer and(5)_1-1 gray clay soil layer in Shanghai are used as objects to verify the applicability of the macro and micro conversion formula to clay or silty clay in Shanghai.Finally,taking the(2)₁ silty clay of the shallow bearing layer as the research object in Shanghai,the particle displacement and contact force chain distribution of the soil during the CPT penetration process are analyzed from a micro scopic port of view.The results show that the errors of the limit expansion stress,cone resistance,sleeve friction and pore pressure calculated by the macro and micro coupling numerical calculations are 9.68%,2.99%,9.34%and 8.42%,respectively.The results of the macro and micro coupling numerical calculation are not only approach to the results of the cylindrical cavity expansion theory and the true value,but also the effect of pore pressure can be considered.(3)The paper establishes a stochastic mechanics-Bayesian theoretical calculation framework and calibrates the statistical characteristics of the Mohr-Coulomb constitutive model parameters.Firstly,using the relationship between cone resistance,sleeve friction and limit expansion stress,parameter sensitivity analysis is performed to determine key geotechnical parameters.Secondly,considering the key geotechnical parameters as correlated random variables,through the macro and micro numerical simulation of the dynamic penetration process of CPT,and establishing the secondary non-intersected response surface and likelihood function between the key geotechnical parameters and the statistical characteristics of the CPT data.Finally,the Markov chain Monte Carlo sampling algorithm is used to obtain the posterior distribution of the key geotechnical parameters.(4)Using the above-mentioned stochastic mechanics-Bayesian theory,the geotechnical parameters of shallow clay in Shanghai are calibrated,and the geotechnical clay parameters are applied to engineering practice.(2)₁ silty clay,(3)silty clay,(4)silty clay,(5)_1-1 clay and(5)_1-2 silty clay within a depth of 70m are regarded as a unified research object,based on the cone resistance q_c value,using the above stochastic mechanics-Bayesian method to calibrate the consolidation-undrained cohesion(c),internal friction angle(?)and shear strength(S_u).Then the calibrated geotechnical parameters are applied to the Shanghai Oasis,and the axial force and deformation values of the supporting structure are calculated and compared with the monitored values.The maximum axial force error is reduced from 1207k N to 114k N,and the maximum displacement error is reduced from 4.5mm to 2.3mm,hence the absolute error is significantly reduced.The paper integrates the interdisciplinary knowledge of macro and micro geomechanics,statistics and Bayesian theory.Based on the data of CPT,it calibrates the statistical characteristics of the key geotechnical parameters in the Mohr-Coulomb failure criterion.The innovations include the following 2 point:1)From the basic theoretical research,the relationship between the cone resistance,sleeve friction and the limit expansion stress in the cylindrical cavity expansion theory is deduced,and the macro and micro scale constitutive model parameter transformation of the CPT mechanics coupling calculation method is established.2)From the core technology research,Bayesian theory of multiple stochastic variables of geotechnical parameters and multi-source test data integration are established,the statistical characteristics of the key geotechnical parameters in the Mohr-Coulomb failure criterion are calibrated,and priciously describe the variability of the geotechnical medium.The above research provides a macro and micro coupling analysis method for understanding the CPT penetration mechanism,obtaining key geotechnical parameters in the Mohr-Coulomb failure criterion,and providing reliable geotechnical parameters for the development of large-scale underground engineering with strict deformation control.