Study On Deformation Characteristics Of Foundation Pit Of Open-cut Tunnel Of Yangtze River Floodplain Under Asymmetric Load

In the practice of foundation pit engineering,due to the complexity and inhomogeneity of soil properties,the asymmetry of buildings,roads,and vehicle loads around the foundation pit often causes many foundation pits to often bear asymmetric loads.However,in practice,the problem of asymmetry of foundation pit load is not enough,which is easy to cause waste or safety hazard.This paper relies on the foundation pit of a cut tunnel under the soft soil layer of the Yangtze River in Nanjing,and the side is a temporary road and the other side is the asymmetric load characteristics of the river.It is combined by laboratory test,field test and numerical simulation.The method is used to investigate the deformation characteristics of the open pit tunnel under asymmetric load under the floodplain of the Yangtze River,and the influence of load asymmetry on the deformation of the foundation pit is analyzed.In addition,this paper also proposes a optimization scheme for the disassembly and support problems encountered in the construction of the project,and verifies the feasibility of the scheme through the actual test results of the field test section.Finally,the optimization measures are used to effectively guide the demolition and construction of the entire foundation pit.The main research contents and results of this paper are as follows:(1)The basic soil properties and one-dimensional consolidation tests were carried out by taking the original soil samples on site,and the basic physical and compressive properties of the soil were analyzed.At the same time,the GDS triaxial consolidation undrained shear test was carried out to study the stress-strain relationship of the soil.According to the results of one-dimensional consolidation test and triaxial shear test,the reference tangent modulus and the reference secant modulus of the undisturbed soil are obtained,which provide calculation parameters for subsequent numerical simulation.(2)Based on the foundation pit of a Ming tunnel in Nanjing,the field test of the asymmetric deformation of the foundation pit under the soil conditions of the Yangtze River floodplain was carried out.Through the on-site embedding a large number of monitoring instruments,combined with the measured data,the variation law of the surrounding surface settlement,the ring beam and column settlement,the horizontal displacement of the retaining structure and the supporting axial force during the excavation of the foundation pit was analyzed,and the asymmetry was mainly discussed.The lateral displacement of the retaining structure on both sides of the foundation pit and the difference of surface settlement under load are shown.The results show that the deformation on the larger side is larger,and the influence of the asymmetric load becomes more and more obvious with the increase of the excavation depth.(3)The PLAXIS 3D numerical model was established to simulate the foundation pit excavation process.The rationality of the model was verified by comparison with the field test results,and the different effects of asymmetric load on the deformation of both sides of the foundation pit were clarified.By using this model,by changing the size of the load on one side of the foundation pit and the margin of the pit,the deformation of the foundation pit under different asymmetry degrees is compared.The results show that the increase of the degree of asymmetry not only leads to the overall structure of the enclosure from the load side.The displacement also exacerbates the settlement of the lateral surface and delays the settlement on the other side.(4)By simulating the foundation pit de-supporting process,the influence of demolition support on the stability of foundation pit is analyzed.Two optimization schemes for restricting foundation pit deformation and reinforcement structure are proposed.The feasibility of the optimization scheme is analyzed by numerical simulation.Furthermore,a 50-meter-long test section was selected at the site.Based on the above optimization scheme,concrete was poured into the gap between the front retaining structure and the side wall to increase the thickness of the retaining structure.Then the debonding construction is carried out and the deformation of the foundation pit during the disassembly process is monitored.The results show that the lateral displacement of the retaining structure and the horizontal displacement of the pile top are small,and the maximum crack width of the side wall is much smaller than the allowable crack width.The test section of the foundation pit is safe and stable,and the optimization scheme is used to effectively guide the demolition and construction of the entire foundation pit.