Study On Prestressed Anchor Cable Composite Soil Nailing Supporting Technology Of Foundation Pit In Sand And Gravel Strata

The prestressed anchor cable composite soil nailing supporting technology is developedfrom the soil nailing supporting technology fully combines the advantages of the soil nailingsupporting technology and the prestressed anchor cable supporting technology, which is widelyused in foundation pit supporting engineering. But now it is often used in cohesion strata such assilt and clay, and its application in sand and gravel strata is rather little, in which the theoryevidently lags behind the practice. Therefore, further study is fouced on this subject, and themain works undertoken and the research results are as follows.(1) On basis of analyzing the typical engineering examples, the overall and further researchon the design theory and method of the prestressed anchor cable composite soil nailingsupporting technology in sand and graval strata is done by means of theoretical analysis, fieldtest and monitoring and numerical simulation. It is concluded that the non-zero cohesion orstructure force should be given to the sand graval strata in foundation pit prestressed anchorcable composite soil nailing supporting design, which can not only greatly reduce the supportingquantities, but also ensure the security of the construction.(2) The shear strength of the sand and graval strata is discussed based on Mohr-Coulombstrength theory, and it is regarded that the inherent structure force (interlocking force) andfriction angle are the important parameters in describing its shear strength to the sand and gravalstrata, which should be considered in the design. The strength parameters of the sand and gravalstrata is tested in situ through the direct shear test. It is regarded that the cohesion of the sand andgraval strata is much related to the its density and structure characteristic, and it is suggested tobe3~10kPa.(3) Based on the quasi-cohesion theory in the soil nailing supporting design, it is regardedthat the cohesion of the composite soil formed by the soil nailing supporting will increase, andthe calculation formula of the quasi-cohesion is deduced.(4) The prestressed anchor cable composite soil nailing supporting mechanism is studied bya numerical code——FLAC3D. It is concluded that the cohesion of the sand and graval strata hasmuch effect on the displacement of the foundation pit slope. With increase in its cohesion, thedisplacement of the foundation pit slope rapidly decreases. The existence of the prestressedanchor cable can evidently improve the soil nailing’s force, reduce the displacement of thefoundation pit slope.(5) Through the field monitoring of the force of the nails and anchor cables of thefoundation pit in sand and graval strata, the results show that the force of the nails and anchorcables firstly increase and then decrease along with its length, and the same phenomenon also occurs with the excavation of the foundation pit. By comparing the design and calculation resultsof these two conditions with the cohesion of the sand and graval being zero and non-zero, itshows that the actual monitoring results is in more agreement with the calculation result whenthe cohesion is non-zero. It shows the rationality of the non-zero cohesion of the sand and graval.(6) Taking the actual foundation pit engineering as an example, the effect of cohesion of thesand and graval on the displacement of the foundation pit slope, the force of the anchor cablesand soil nailings and the soil plastic zone is discussed by numerical method. The results showthat the displacement of the foundation pit slope and the force of the supporting structureevidently decreases with the increase in cohesion of the sand and graval strata, while its decreaseextent gradually becomes flat.