Analysis On The Function And Risk Of Composite Soil Nail Support In Loess Area

With the improvement of urbanization in China,the height of buildings is increasing and the depth of foundation pits is getting deeper.Driven by the actual demand,the foundation pit support system has been greatly developed.Soil nail support has been widely used in the loess area of China due to its unique advantages,and the technology of soil nail support has also become increasingly mature.However,due to the complexity of foundation pit support engineering,accidents in foundation pit projects occur from time to time,which not only cause huge economic losses,but also seriously threaten people’s lives and healthThis paper uses theoretical analysis,numerical simulation,and practical engineering examples to study the mechanism of soil nailing support,and puts forward some suggestions on how to reduce the risk of foundation pit accidents(1)At present,there are still some different opinions on the length of soil nails on the upper,middle and lower parts of the foundation pit.The main conclusion is that the support effect of soil nails at different depths of the foundation pit is different.The central soil nails can significantly improve the lateral soil pressure and effectively limit the horizontal displacement of the foundation pit,therefore,the length of soil nails in the middle of the foundation pit should not be shorter than the length of soil nails in other locations;the soil pressure at the bottom of the foundation pit is greater,so the length of the soil nails at the bottom of the foundation pit should be longer than the top of the foundation pit;The length of the upper soil nail should meet the role of restraining the development of cracks in the upper soil body.In addition,through the analysis of the working characteristics of the prestressed anchor composite soil nail support,it is concluded that there is a certain difference in the mechanism of the interaction between the anchor and the soil nail when the prestress is different,so the prestress of the anchor should be limited to a certain Range(2)The upper limit of the risk that people can bear is called the risk threshold.Based on the difference of the risk threshold and considering the difference in people’s ability to withstand the accident,the evaluation formula of the risk degree and risk level of the composite soil nail is obtained(?)In the formula:RTh is the risk threshold of composite soil nail support;Rd can describe the risk of the foundation pit,known as the degree of risk;Ai is the probability of the risk influencing factors;Bi represents the size of the loss caused by the risk factor,i takes 1,2,3,and 4 represent the design plan for supporting construction,waterproof and drainage measures,construction quality,and other influencing factors;LT represents the maximum acceptable loss.Based on the risk degree and the risk probability of the foundation pit accident,a risk assessment matrix was established,and the risk grade of the composite soil nail was obtained.In addition,the risk level is analyzed,the risk level of the foundation pit under different risk degrees is given,and the risk control measures of the composite soil nail are given.(3)Based on a variety of examples of composite soil nailing accidents,the risk of composite soil nailing in the loess area was analyzed,It is found that for foundation pits of different depths,the control factors that affect the stability of composite soil nailing are also different.Based on the collected data of composite soil nailing accidents,the evaluation formula for the risk of composite soil nailing in the loess area is given:(?)(4)The numerical simulation of a composite soil nail supporting foundation pit in Xi’an according to Midas shows that:for major foundation pit projects,when the supporting system meets a certain safety factor,continue to add supporting members to improve the supporting system The degree of redundancy significantly improves the stress of the foundation pit and controls the deformation of the foundation pit,thereby reducing the risk of the foundation pit and achieving the purpose of risk control.