Study On The Soil Strength Response By The Excavation Unloading And Overweight-load On The Crest

With the continuous development of economy,China’s urbanization is accelerating,and the land in the city is getting tighter and tighter,so the development and utilization of underground space and the construction of super high-rise buildings become the trend of future development.In order to ensure the safety of super-deep foundation pits,it is necessary to fully understand the deformation law of various soil bodies and select suitable soil parameters during design.The stress-strain relationship of soil is related to many factors such as stress path,and the shear strength index of soil is usually obtained by conventional triaxial test in the design of deep foundation support structure,but in the actual project,the soil is in K₀ consolidation state,and the stress path of soil is very complicated in the construction process due to the different construction conditions,so the deformation law and strength index of soil under the reference of conventional triaxial test will certainly produce Certain errors will occur.A shield working shaft in Jinan City will experience lateral unloading in the active zone during the excavation of the pit,and subsequent lifting of shield mechanism parts by the pit,with loads often reaching thousands of tons,resulting in complex changes in the stress state of the soil in the active zone,which first experiences lateral unloading in the excavation and then vertical loading in the active zone,with a very special stress path of a folding line.In this paper,we take the working shaft on the south bank of Jinan tunnel through the Yellow River as the background,and study the working condition of excavation unloading-overweight loading on the top of the pit.Combined with the field monitoring data,the influence of soil parameters on the deformation of the support structure of the pit excavation is analyzed under different stress paths,and the position and size of the lifting load are changed to explore the influence of the lifting load on the support structure.The main research work of this paper is as follows:（1）The stress path during the construction of deep foundation pit is analyzed,and the triaxial test of lateral unloading and then axial loading of typical powder clay soil in the Yellow River is carried out by using advanced stress path triaxial instrument to simulate the excavation unloading-overloading condition at the top of the pit,and the results are compared with the conventional triaxial test used in the design to investigate the strength index and deformation characteristics of soil under this special stress path.From the test results,it can be seen that the soil deformation pattern and strength index are different under different stress paths,and the stress path has a great influence on the soil deformation pattern and strength index.（2）The applicability of the conventional Duncan-Tension model to the stress path corresponding to the excavation and unloading-top of pit overload condition was analyzed,and it was found that the conventional Duncan-Tension model was not applicable to the excavation and unloading-top of pit overload condition,but the subsequent analysis revealed that the derivation could be improved by referring to the derivation process of Duncan-Tension model,and finally the deformation modulus equation applicable to the stress path of excavation and unloading-top of pit overload condition was derived.（3）Numerical simulation of this deep foundation pit is carried out by large finite element software PLAXIS,and the soil body is subjected to finite element calculation by using conventional triaxial test index and lateral unloading and then axial loading test index respectively,and the monitoring data on site are combined to compare the influence of soil parameters obtained under different stress paths on the deformation of supporting structure and surface settlement,and then the position of lifting and the size of lifting load are changed to investigate the influence of lifting load on the support structure and ground settlement.It is found that the maximum horizontal deformation and the maximum surface settlement of the enclosure wall obtained by numerical simulation using the lateral unloading and then axial loading test indexes are closer to the measured values,and the influence of the lifting position and the lifting load size on the horizontal deformation of the enclosure wall is smaller and the influence on the surface settlement is larger.