| It is proposed in the 2020 State Council government work report that the State Council should focus on supporting “the new infrastructure,new urbanization initiatives and major projects”.With the rapid development of urbanization and infrastructure construction,the utilization rate of urban underground space is getting higher and higher,and the safety of excavation and its impact on surrounding dense buildings are becoming increasingly important.Excavation causes unloading of surrounding soil,resulting in continuous changes in the direction of the principal stress on the soil,the soil along the failure surface show characteristics of strength anisotropy.The soil stress state distribution in the excavation range is not consistent with the stress generally considered.However,the research and design of excavation seldom consider the anisotropy of the soil,and it is necessary to carry out a systematic study on it.In this paper,starting from the anisotropic characteristics of clay,with the help of finite element theory,based on actual engineering,a systematic study of the response of excavation in anisotropic soil is carried out.The main research contents and results include:(1)It is proposed in the 2020 State Council government work report that the State Council should focus on supporting “the new infrastructure,new urbanization initiatives and major projects”.With the rapid development of urbanization and infrastructure construction,the utilization rate of urban underground space is getting higher and higher,and the safety of excavation and its impact on surrounding dense buildings are becoming increasingly important.Excavation causes unloading of surrounding soil,resulting in continuous changes in the direction of the principal stress on the soil,the soil along the failure surface show characteristics of strength anisotropy.The soil stress state distribution in the excavation range is not consistent with the stress generally considered.However,the research and design of excavation seldom consider the anisotropy of the soil,and it is necessary to carry out a systematic study on it.In this paper,starting from the anisotropic characteristics of clay,with the help of finite element theory,based on actual engineering,a systematic study of the response of excavation in anisotropic soil is carried out.The main research contents and results include:(1)Based on the labotary test results of the soil of the Canglang New City subway station in Suzhou,the anisotropic parameters of the soil were determined,and based on the NGI-ADP anisotropic constitutive model,considering the surrounding complex building environment,a three-dimensional finite element numerical model is built and analyzed from the perspective of soil and structural deformation response caused by the excavation,such as the lateral displacement of the retaining wall,ground settlement,and building settlement.And the model is validated based on the measured data.The results show that the three-dimensional finite element model is reasonable,and the response of excavation considering the anisotropy of clay is consistent with the measured value.(2)Based on PLAXIS and Python,a program for automatic batch calculation and efficient extraction of results was developed,and a large number of finite element calculations and parameter analysis were carried out.The effects of clay anisotropy ratio,soil strength and soil modulus strength ratio on the excavation response of excavation under different supporting stiffness and different pit geometric dimensions are studied.By analyzing the distribution of plastic points caused by excavation in different degrees of soil anisotropy,the mechanism of the response of excavation in anisotropic soil is revealed.By analyzing the deformation of the retaining wall,the safety factor of basal heave,and the ground surface settlement behind the wall,the results show that the soil anisotropy ratio has a significant influence on the excavation response.Compared with the cases without considering the anisotropy of the clay,cases with considering the anisotropy of clay show a larger lateral wall deformation,and the safety factor of the basal heave is lower.(3)Based on the axial force of struts,the apparent earth pressure of the excavation in the anisotropic clay is analyzed.The effect of clay anisotropy on apparent earth pressure of excavation under the support of rigid and flexible retaining walls is compared.The results show that,compared with rigid walls,the apparent earth pressure is significantly reduced when flexible walls are used.And the apparent earth pressure increases with the increase of the anisotropy ratio,ignoring the anisotropy of the soil will lead to underestimation of the strut axial force.In addition,based on the CIRIA's DPL(CIRIA's Distributed Prop Load)curve,a modified apparent earth pressure envelope with consideration of the anisotropy of clay is proposed for rigid and flexible retaining walls,respectively.(4)Based on the machine learning algorithm,the prediction models of the retaining wall deformation,the ground settlement behind the wall,and the safety factor of basal heave caused by the excavation in the anisotropic clay are established,and the parameter sensitivity analysis is carried out respectively.A comparative analysis of multiple adaptive spline regression(MARS),multi-layer perceptual regression(MLPR),decision tree regression(DTR),random forest regression(RFR),extreme gradient boosting(XGBoost)and other cutting-edge algorithms used to predict the response caused by excavation are carried out to verify the model accuracy and robustness.Based on 5-fold cross-validation,five algorithms used to predict the wall lateral displacement are compared.The Spearman correlation coefficient is used to analyze seven types of soil anisotropy parameters about the degree of correlation between the parameters and each with the ground settlement.The results show that the machine learning algorithm can get reasonable results on predicting the response of excavation.Compared with other algorithms,RFR and XGBoost have better accuracy and robustness.The parameter sensitivity analysis shows that the anisotropy ratio of the soil is more sensitive to the deformation of the retaining wall,the ground surface settlement behind the wall,and the safty factor of basal heave.(5)Based on the measured data of the project and the back analysis theory,the selfcompiled data sheet(Excel Spreadsheet)is used for the back analysis of the soil parameters.Given the uncertainty of the soil properties,based on the minimization of the difference between the prediction and the monitoring results,the soil parameters,and parameter combinations are reversed in stages,and then the updated parameter combinations are used for the response prediction caused by the next excavation,and gradually improve the prediction accuracy.The results show that based on the monitoring data of the lateral wall displacement of the braced excavation of the Canglang New City subway station,the soil parameters obtained from the tests are updated by back analysis,and the accuracy of the wall deformation prediction is improved. |
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