Research On Composite Soil Nailing Support Technology Of Double-row Steel Pipe Piles In Deep Miscellaneous Fill Sites

In recent years,with the continuous development of infrastructure construction in China,urban land is becoming increasingly tense.In order to alleviate the contradiction between population increase and land shortage,a large number of high-rise buildings have been built in dense urban areas,and a large number of deep and large foundation pits have emerged.At the same time,with the continuous outward expansion of the city,the disadvantages of the extensive construction waste landfill in the early stage gradually appear.More and more buildings are built in the deep fill area,which brings difficulties to the foundation pit support and excavation of new projects,resulting in the problem of foundation pit support in the deep fill area;As one of the dangerous and large projects,the foundation pit support should meet the three index requirements of "safety,economy and applicability".However,as a temporary project,the investment cost of foundation pit support is too high,which is not conducive to the economy and the promotion of green construction.Composite soil nailing wall support is gradually recognized by the engineering community because of its advantages such as less impact,good applicability to the surrounding environment,economic cost and short construction period.However,the composite soil nailing wall with single row of steel pipe piles has certain restrictions on its application in the site with thick fill and poor safety due to its small stiffness.This thesis focuses on the engineering problem of foundation pit support of deep miscellaneous fill.Based on the foundation pit support project of nanbeikang B-7 plot in Jinan,this thesis discusses the applicability of double row micro steel pipe pile composite soil nailing support system in deep miscellaneous fill area,and puts forward a simplified calculation method of double row micropile composite soil nailing wall,so as to carry out field test,indoor numerical simulation and comparative analysis of third-party monitoring data,The research contents are as follows:(1)The characteristics of double row steel pipe pile composite soil nailing support system are discussed.Based on the calculation formula of foundation pit specification and the calculation software of deep foundation pit,the simplified calculation and comparative analysis of double row steel pipe pile composite soil nailing wall support are carried out,and the rationality of the simplified calculation is verified in combination with the actual engineering effect.(2)Based on the optical fiber sensor,the field test and research work is carried out,the stress test of steel pipe pile and anchor rod under the condition of foundation pit excavation and slope top surcharge is carried out,the stress variation law of steel pipe pile and anchor rod is analyzed,and the stress characteristics of each support member are summarized,so as to provide a reference basis for the support design of similar projects.(3)Midas/GTS software is used for numerical simulation calculation,and the pile bending moment,deep horizontal displacement of pile body and stress of anchor bolt are analyzed under each excavation condition of foundation pit,which are compared with the field test results.(4)The feasibility study of the third row of steel pipe pile and composite foundation pit is carried out,and the numerical monitoring results of the third row of soil nailing field are collected and compared with those of the third row of soil nailing field.The research shows that double row micropile composite soil nailing support is suitable for foundation pit engineering in deep filling site.Through the test and numerical analysis results,in this support type,the stress of the front row pile is more important than the rear row pile,and the effect of the middle and upper anchor rod on the stability of the foundation pit is higher than the lower anchor rod.The design of the front row pile and the middle and upper anchor rod should be strengthened;At present,the simplified calculation method of Lizheng software can be used as a reference for design,but its deformation analysis ability needs to be further improved.