Analysis Of Composite Soil Nailing With Pre-installed Micro-piles

The pre-installed micro piles can not only help to ensure self-stability of the soil onexcavation face, but also significantly reduce the soil deformation around the excavationpit and enhance the global stability of the supporting system. As a result, composite soilnailing with pre-installed micro piles are widely used in the deep excavationengineering in China. So it is an important topic to accurately predict the deformationand check the global stability of the supporting system. Mainly based on the finiteelement method and theoretical analysis, supplemented by literature research and fielddata, the mechanical properties of soil around the excavation pit and the deformationand stability law of the composite soil nailing with micro pile are systematically studiedin depth. And then, simplified calculation methods are proposed to check the globalstability checking and to estimate the horizontal displacement of the excavation pit.Besides, relationship between global stability checking and local stability checking forsoil nailing in design is discussed. In this thesis the following innovative conclusions orresults are made:(1)The typical stress-path in soil induced by deep excavation has been analyzed bynumerical analysis. And then the difference of the deformation and strength propertiesof the soil under various stress paths is pointed out. Several soil constitutive modelscommonly used in geotechnical numerical analysis are discussed. The suitabilities ofthese constitutive models used in numerical analysis of deep excavation are comparedthrough analyzing some typical excavation projects.(2) Three-dimensional nonlinear finite element analyses are carried out for bothsoil nailing support and composite soil nailing with pre-installed micro-piles in order toinvestigate the effect of the micro piles. The differences on horizontal displacement,settlement of excavation pit and axial force of nails between support system with andwithout micro-piles are analyzed through comparative calculations. And then severalfactors affect the deformation of support system, such as embedded length, diameter andspacing of micro pile, layout and inclination of soil nailing are studied.(3) Sliding resistance of the pre-installed micro pile in composite soil nailing isanalyzed with reference to the cutting pile force method and the elastic foundation beam method. A simplified approximate method is suggested to consider the effect of themicro pile in global stability checking for composite soil nailing. Compared with themethod suggested by the newly issued technical code on composite soil nailing andfinite element results, the proposed method is more reasonable and has a clearer idea.(4) Based on the design method of soil nailing in the current codes for excavationsupport, the need of global stability checking and local stability checking for soil nailingin design is discussed.(5)A simple and suitable method to calculate the horizontal displacement forexcavation pit of composite soil nailing support system is proposed. Compared with themeasured data, the proposed method can give good predictions of the horizontaldisplacement of composite soil nailing with pre-installed micro piles.