Research On Mechanism And Mechanical Behavior Of Prestressed Anchor Flexible Supporting System

Prestressed anchor flexible supporting system is a new technology for retaining of deep and ultra-deep excavation. Currently, the depth of excavation of Dalian Yuan-yang Building using this technology is25.6meter, and the depth of ultra-deep excavation of Dalian International Finance Centre Building is near30meter. Prestressed anchor flexible supporting system mainly consists of shotcret, prestressed anchor, bearing structure under anchor head. For the advantage of low cost, short construction period, small covered area, construction convenient, safe and reliable, and so on, the system is wildly used in engineering, especially with construction sites contain weathered rock, medium intensity and stiff soil. According to the prestressed anchor flexible supporting system, some studies, which are focused on the anchorage mechanism of pressure-anchor, earth pressure on the shotcret, bearing capacity of bearing structure under anchor head and the mechanical behavior of foundation pit, are inverstigated by theoretical analysis, analytic calculations, comparison between experimental results and theoretical results, and numerical test. The main contents are as follows:(1) The computational formula of axial force of anchorage body and shear stress on the interface between anchorage body and soil mass are derived. Analysis of parameters, such as elastic modulus and Poisson’s ratio of anchorage body, cohesion, internal friction angle, elastic modulus and Poisson’s ratio of soil mass, adhesion and external friction angle of interface between anchorage body and soil mass, and the conditional coefficient of adhesion, is also carried out. The results show that the distributions of shear stress and axial force on the anchorage body of comparison anchor are exponential, and decrease as the distance to the bearing body. When considering the influence of adhesion on the interface between anchorage body and soil mass, the shear stress becomes small and axial force becomes large. The anchorage mechanism of the single bore multiple comparison anchor is analyzed, and the computational formula of shear stress on the interface and axial force of anchorage body are derived. Analysis results of the rule of load transferring and parameters influence show that distribution of shear stress becomes uneven and the shear stress and axial force decrease quickly when the soil mass is stiff (that is to say the values of soil mass modulus, cohesion and internal friction angle are large, and the value of Poisson’s ratio is small). Because the multiple bearing bodies bear the load together in the single bore multiple comparison anchor, the load on each bearing body reduces greatly, as well as the shear stress on the interface. Lateral friction and force on each anchorage body are developed sufficiently and the distributions are more uniform.(2) The anchorage lengths of comparison anchor under load and ultimate bearing capacity of the anchorage system are studied analytically, and the appropriate method for calculating the anchorage length is derived. Analysis results of the influence of parameters on the anchorage length show that the load transferring length is small for the condition with great modulus, small Poisson’s ratio, great cohesion and internal friction angle of soil mass. The influence of Poisson’s ration on the load transferring length is too little and can be neglected. Comparison between the theoretical calculated load transferring length and that from monitoring in the model experiment shows great agreement, as well as the comparison between the calculated anchorage length in this paper and that specified in the code. If the actual calculated length is large than specified values in the code or calculated values, the pull-out force may be less than the designed value because of the insufficient development of the lateral friction of the excessed anchorage length, duing to the assumption of uniform distribution of lateral friction. The calculated formulation, which is simple and practical, of anchorage length can provide better guidance for engineering design and construction.(3) Based on soil arching effect, the analysis model for calculating earth pressure on the shotcrete in prestressed anchor flexible supporting system is established and the relevant formula are derived. Comparative analysis of calculating results with those from silos method and code method shows that the calculated earth pressure on the shotcrete from code method, which is based on the active earth pressure, is large apparently. Calculated results from the derive formula are agreement with those from silos method. However, the values calculated from silos method are relatively imprecise because of the ignorance of soil cohesion and external friction angle. By further analysis, the simplified formula for calculating the earth pressure on the shotcrete is proposed. While considering the soil arching effect in the backfill of rigid retaining wall with a horizontal translating, the formulation for calculating the active earth pressure acting on the rigid retaining wall is derived, which show fully agreement with those from the classical analytical formula and those derived by earlier researchers. Analytical results show that the distribution of active earth pressure on the wall is non-linear because of the arching effect, and the earth pressure are larger than that from classical formula in the upper zone while smaller in the lower zone.(4) Numerical tests about the bearing structure under anchor head are carried out for prestressed anchor flexible supporting system by using three-dimensional finite element analysis software. The analysis results of bearing capacity of structure under anchor head for different types of channel steel show that thick plate and vertical ribs can increase the capacity of structure to a certain extent. Confined shotcrete can effectively control the lateral deformation and enhance the stability of the structure. Because fully confined shotcrete and channel steel can form a composite bearing structure, the capacity of the structure can be improved greatly. But half confined shotcrete can not improve the capacity of structure. Horizontal connecting rebar in the proper location can effectively control the lateral deformation of structure. The influence of confined shotcrete on the lateral deformation reduces greatly when the horizontal connecting rebar is installed. Engineering application results show that, capacities of bearing structure obtained by numerical tests can meet the project well. Results from numerical tests can provide scientific and technical guidance for the design and construction of prestressed anchor flexible supporting system.(5) Based on the actual engineering with prestressed anchor flexible supporting system, the pulling resistance of anchor and the distribution of lateral friction on the interface between anchorage body and soil mass are analyzed through pull-out tests of anchor. Results show that the distribution of lateral friction is non-linear and the pulling resistance of anchorage does not increase linearly with the anchorage length. So it may be unreasonable to improve the anchor pullout resistance just by increasing the anchorage length. An actual project with prestressed anchor flexible supporting system is simulated using FLAC software and the horizontal displacement is in great agreement with that from monitoring. The prestress influence is analyzied emphasizly on the displacement and slip field of the foundation pit in this paper. The results show that prestress of anchor can control the displacement and improve slip field of the foundation pit. However, when the prestress exceeds a certain range, the continued prestress has nearly no effect but extend schedule. Related results can provide a certain reference value for rational design and optimized construction for prestressed anchor flexible supporting system.