Field Test Studies On Internal Force Of Pile-anchor Supporting Structure

With the rapid development of city and the appearance of high-rise and super high-rise buildings, the evacuation and support of pit has become the key problem as the deeper and deeper pit. The pile-anchor supporting structure has been widely used in Xining, but its mechanical characteristic still need to be further studied. In this paper, through the measured internal force of the retaining pile for the deep excavation in Xining railway station, behavior and changing pattern of internal force are studied under different conditions. The following conclusions can be drawn:(1)The capping beam of retaining pile has obvious constraint to deformation, which makes the mechanical characteristics of the pile top are different from that of the free upper vertical bar. The constraint effect can not only change the state of deformation of the retaining piles but also decrease the internal force of pile body. The constraint of capping beam should be considered in the design and calculation of supporting piles.(2)The test data shows that the reinforcement stress measured value of pile-anchor supporting pile with the conventional design method is far less than the steel bar intensity design value, indicating that the standard steel strength is far into exert while the foundation pit supporting design is safe. Pile reinforcement stress is mainly changed by the pit excavation depth, the excavation exposure time and the prestressed bolt tensioning.(3)It can be seen from the experiment that the distribution and change trends of the measured bending moment and calculated bending moment are different. The maximum bending moment of pile occurs near the bottom of foundation pit in cantilever support phase. However, the maximum bending moment occurs in the range from the bottom to the one-third depth of foundation pit in anchor supporting stage.(4)If the embedded depth of supporting pile is too long, its mechanical properties can’t be improved, which indicates that the built-in depth of supporting pile can be optimized and provide reference for empirical correction.(5)From the comparison of calculated and measured bending moments, a conclusion can be drawn that the calculated results of pile-anchor supporting structure are safe using limit equilibrium method and elastic support method. The design using the limit equilibrium method is more economic over the bottom of foundation pit. while the result calculated by elastic fulcrum method is more consistent with the measured value under the bottom of foundation pit.(6)Capping beam is a flexural member and its reinforcement stress rises with the increase of excavation depth. At the end of capping beam, it has medial compression and lateral tension. At the middle part, it has medial tension and lateral compression.(7)The displacement of capping beam rises with the increase of excavation depth and obvious spatial effect can be found in pit deformation. The displacement distribution shows a shape as "︿", demonstrating that the middle displacement is large while the two tip displacements is small.(8)The internal force of pile can be effectively reduced by exerting prestressed anchor and obvious spatial effect can be found with anchor tension. In the process of anchor tensing, the pile reinforcement stress decreases slightly near the bolt elevation position, while less influence on reinforcement stress is found at other section positions of pile.