Research On Unloading Rebound Characteristics Of Clay Stratum Above Existing Tunnel

Transportation is the key to the development of the country and the foundation of a strong country.The report of the 20 th Party Congress once again emphasizes accelerating the construction of a strong transportation country and striving to realize the rail transit network of city clusters and metropolitan areas,which fully reflects the high importance and ardent expectation of the Party Central Committee to transportation work with Comrade Xi Jinping as the core.With the rapid development of urban rail transit,the number of tunnels in the underground space is increasing,and it is inevitable that there will be engineering conditions that require foundation excavation above the existing shield tunnel.In order to effectively prevent the adverse effects caused by the soil rebound and eliminate the safety hazards brought by the pit construction activities,it is necessary to study the soil rebound deformation characteristics under this working condition and the development law of rebound characteristics under the influence of various factors.In this paper,the following work is carried out with the project of underground space between Guangxi University Station-Xiuling Road Station Guangcai Area of Nanning Rail Transit Line 5 as the engineering background:(1)The parameters of the Modified Cambridge(MCC)principal structure model,the Hardened Soil(HS)principal structure model and the Hardened Soil(HSS)principal structure model considering small strain were determined for a typical pulverized clay stratum in Nanning by combining indoor tests on in-situ soils with in-situ tests;(2)Numerical simulations were carried out using Plaxis 2D finite element simulation software to systematically study the rebound deformation characteristics of the soil during the unloading excavation of the powder clay stratum above the existing shield tunnel,and detailed analyses of the internal force distribution,soil deformation and displacement,and deformation characteristics of the structure were carried out using three different intrinsic models to describe the mechanical properties of the soil;(3)Using a combination of manual monitoring and automated monitoring to obtain field monitoring data on the deformation of the foundation pit and the underpass during the project,the development process and deformation characteristics of the soil deformation during the excavation of the foundation pit were counted and analyzed,and the results were compared with the results of finite element numerical simulation to clarify the applicability of different intrinsic structure models in the study;(4)By changing the burial depth and horizontal position of the tunnel under the pit and the reinforcement measures at the bottom of the pit,the influence law of such factors on the rebound deformation characteristics of the soil after excavation of the pit was studied and obtained.After the above research work,this paper obtained the stress field and displacement field distribution of the soil under the excavation of the pit above the existing tunnel using different principal structure models,and concluded that the HSS model is more consistent with the measured deformation when analyzing the soil deformation in the finite element numerical simulation under this condition,and has a more in-depth understanding of the influence of three factors on the soil rebound deformation characteristics,including the burial depth of the tunnel under the pit,the horizontal position and the reinforcement measures at the bottom of the pit,then analyzed and proposed the corresponding soil rebound estimation formula,so as to play a certain degree of guidance for the development of similar projects.