Application Of Composite Soil Nailing Support For The Deep Excavation

Composite soil nailing technology was the first to be used in some soft soil of the southeast cities.In recent years,in accordance with different site conditions and geological conditions, the composite soil nailing method has found wider and wider application in deep excavation in China because of its advantages,such as:safe and reliable,easy to implement and less investment.There has not design and specification about composite soil nailing in code for design of building foundation and technical specification for retaining and protection of building foundation excavations.And theoretical research on the technique has fall behind its engineering practice recently because of complexity and uncertainty of deep pit supporting.Large-scale testing research on composite soli nailing structure has very few researchers set foot in.Comprehensive and accurate experiments or field test data are rather limited.There are many aspects need continued study and to be perfected in theories and applications.Through an elastic-plastic analyse of FLAC^3D program which based on the explicit difference method, a three-dimensional finite element model has been developed take advantage of FLAC^3D,and from the case of urban complex of the third space deep excavation project in Tangshan on composite soil nailing combined with general soil nail and prestressed anchor. Then, internal force and deformation of the supporting structure and other working principle and operational performances are studied in detail.In view of this,the thesis makes the following:(1) The development,prospects of application and research present of composite soil nailing have been comprehen sively reviewed and discussed in this paper.Further more summarizes the composite type of composite soil nailing,then its advantages,disadvantages and applicability of different type of composite soil nailing are briefly introduced in the paper.(2) The numerical computation method helps to study the mechanical mechanism of soil nailing and prestressed anchor retaining.Based on traditional active soil pressure method and unified strength theory soil pressure,the calculating model of the laternal soil pressure which is a echelon formation with the largest value in the center side is addressed.This model presents the distribution of lateral soil pressure of composite soil nailing wall more accurately and concisely. Then design calculation of soil nailing and prestressed anchor are carried out.(3) Through an elastic-plastic analyse of FLAC^3D program which based on the explicit difference method,and from the case of urban complex of the third space deep excavation project in Tangshan on composite soil nailing combined with general soil nail and prestressed anchor.Numerical simulation is carried out the horizontal displacement on the top of pit foundation,which with measured date is discussed. The results indicate as follows:①Soil nailing exhibits the tension distribution, characterized by small size in double-end and large one in the middle.②Prestressed anchor axial force in the free section and anchoring section are quite different.After the prestressing,there are a certain loss of prestress,then,axial force distribution along the cable length from large to small,and gradually stabilized.③The maximum horizontal deformation occurs at the mid-bottom part of the foundation pit,not at the top of the foundation pit.④The most dangerous failure surface appears in the deep composite soil nailing wall of excavation,and more deeper than traditional soil nailing wall.⑤The results of FLAC^3D simulated are close to pit displacement monitoring.The results prove that established numerical analysis model,parameter selection and computational method are reasonable.So,the FLAC^3Dsimulation could reflect engineering situation with the field testing, so its feasibility is proved.It is of great significance for practical engineering such as design,construction scheme,deformation monitofing.The successful application supplies a useful reference to similar deep excavation projects in future.