Application Reserch Of Composite Soil Nail Supported With Micro-steel-pipe-piles In Sandy Area

Composite soil nailing support has been widely used in China's foundation pit engineering due to its advantages of wide application scope,convenient construction,and strong support ability.However,due to its various combinations and complex geological conditions in different regions,the lack of systematic and in-depth theoretical research on composite soil nailing is currently lacking.In some foundation pit support design schemes,one or more composite soil nail support structures are used for foundation pit support.Although the foundation pit deformation can be controlled within a safe range,there is a waste of material.Caused problems such as too low engineering economic benefits.In this paper,based on an example of a foundation pit project using composite pile anchored soil nails and secondary grading + steel pipe micro-piles composite soil nails,two types of composite soil nails are supported on the foundation pit section.Pit excavation is simulated.The rational selection of soil parameters in the numerical model is mainly studied;the simulation results of the foundation pit deformation are compared with the on-site monitoring data,and the excavation of the foundation pit using the pile anchor composite soil nail and the steel pipe micro-piles composite soil nail is summarized.The deformation characteristics;and the original design of micro-piles of steel tubes are optimized,and the influence of design parameters of steel tube micro-piles on the deformation of the foundation pit is obtained,which can provide reference for the design of foundation pit support in the same area.This article has obtained the following research results:(1)Comparing the numerical simulation results with the on-site monitoring data,it is found that the two are basically the same,which verifies that the numerical simulation done in this paper is accurateand reliable.When support design software is used to support the foundation pit engineering in the same area as the foundation pit in this area,the cohesive force c of fine powder and fine sand can be taken as3 kPa.This design result is closer to the actual situation.It can ensure the safety of foundation pit engineering and also reduce the unnecessary waste caused by the design.(2)The surface settlement outside the pit increases with excavation,and the maximum settlement occurs between 0.3 and 0.7 times the excavation depth from the top of the slope.The increase in surface settlement after the addition of micro-steel-pipe piles was significantly reduced;the distribution of the axial force of each row of soil nails was in the form of large middle and small ends,and the axial force of each soil nail continued to increase before the micro-steel-pipe piles were added,and the maximum axial force was increased.All produced at 0.5 times the length of soil nails,the axial force of the row of soil nails set after the addition of micro-steel-pipe piles gradually decreased,and the position of the maximum axial force also changed to 0.3 times the length of soil nails.(3)The design scheme of the composite soil nailing support for the original steel pipe micro piles does not have the best economic benefits.Increasing the distance between the micro-pile slope and the embedded depth of the pile can not increase the stability of the pit;the pile spacing and the diameter of the pile have a great influence on the stability of the pit.For the foundation pit,the pile spacing of the steel pipe micro-piles is set to 4 to 10 times the pile diameter or the pile diameter is selected to be 100 to 240 mm.This is to ensure the safety and stability of the foundation pit and effectively reduce the slope displacement.And greatly increase the economic benefits of the project.