Study On Reinforcement Mechanism Of Soil Nailing + Anchor Cable Composite Soil Nailing Wall In Loess Region

Due to the emergence of a large number of loess foundation pit projects,the composite soil nailing supporting structure with high applicability and economy is more widely used in loess foundation pit projects,and has achieved good benefits.However,the mechanism of composite soil nailing on loess foundation pit is still unclear,and the research results are mostly focused on the Macro-effect of reinforcement structure and loess foundation pit.There are not many micro-effect studies,and there is a lack of research results combining micro-effect with macro-effect.As a result,the design and construction of composite soil nails are mostly dependent on engineering experience and lack of theoretical basis.In order to explore the mechanism of composite soil nailing wall with soil nailing and anchor cable supporting loess foundation pit,based on the laboratory test of composite soil nailing wall,particle flow simulation of composite soil nailing wall and on-site monitoring of composite soil nailing wall,the micro-mechanism and macro-mechanism of composite soil nailing wall are explored.(1)In-situ sampling and preparation of undisturbed loess reinforced specimens,the stress-strain curves of different reinforced layers and different water content were analyzed under different confining pressures,and the effects of different reinforced layers,water content and confining pressure on the strength of reinforced loess were distinguished.The specimens of undisturbed loess reinforced with different moisture content after loading were tested by scanning electron microscopy.The micro-behavior of the specimens before and after loading and reinforcing were analyzed,and the shear and crack resistance mechanism of the specimens was analyzed within the framework of geotechnical damage mechanics.(2)Using the discrete element software PFC to establish the biaxial test model,calibrate the parameters of the model combined with the indoor reinforcement test;set the measuring circle to monitor the number and distribution of cracks,pore change and energy change of the model in the process of loading,and reveal its anti-shear and anti-crack mechanism from a mesoscopic point of view.(3)Based on the field practice and the parameters calibrated by biaxial test,the model of foundation pit reinforced by composite retaining structure of soil nail and anchor cable with discrete element method is established.The energy change of model after reinforcement,the movement of soil particles and the interaction between nail,anchor and soil are studied.(4)On-site monitoring is carried out to analyze the lateral displacement,settlement,stress distribution of soil nails and anchor cables during the excavation of foundation pits.Based on the cubic curve model,the expression of relative displacement between nails and soil is deduced according to the stress distribution of soil nails during excavation and unloading.Combined with the above macro and micro analysis conclusions,the mechanism of soil nailing + anchor cable composite structure strengthening loess foundation pit is summarized and revealed,and the conclusions that can guide engineering practice are drawn.