Research On Mechanical Performance Of Prestressed Anchor Flexible Supporting System In Deep Foundation Pit

As a safe and reliable, economical and feasible, and easily operated technique, the prestressed anchor flexible supporting system has been widely used in the deep foundation pit. However, the current theory research on prestressed anchor flexible supporting technique is far lagged behind the engineering practice. There are still accidences, related to deep foundation pit occurring, because of poor design; hence it is necessary to conduct deep research about the theory principle about this technique.This paper conducts comprehensive and in-depth research on the deformation and stability of pre-stressed anchor flexible supporting with respect to the aspect of experiment research, theory research and numerical computation. The research is reduced as the following:(1) The basic structure and characteristics of prestressed anchor flexible supporting system is comprehensively summarized. The components of bearing structure under the anchor is analyzed using finite element numerical method, and the optimal matching principle is obtained. The practical computation method for this technique is generalized in use of traditional limit balance method. The prestressed anchor flexible supporting system has advantage in foundation pit deformation control and stability improvement, compared with soil nail supporting system.(2) Based on digital image measurement system, the experiment about unloading stress path of the soil mass during excavation is conducted. Lab experiment shows that stress deviator of soil mass at damage state during excavation is obviously smaller than shear stress of general tri-axial experiment. After foundation pit excavation unloads, soil shear strength is lowered, and the variation of internal friction angle should be taken into account for the foundation pit design.(3) Based on Kelvin's displacement solution, the distribution function of anchor sector shear stress and axial stress for a single prestressed anchor bar. The research displays that, different prestressed anchor bar damage mode corresponds to different soil condition, and the anchor bar design criteria should be set to the weakest point. The location of maximum shear stress is not the connection point of free sector and anchor sector, but is somewhere else beneath that point, and the shear stress increases to the maximum and then decreases to zero along the anchor sector. The shear stress distribution is related to soil mass mechanical characteristics, and there will be stress concentration for hard soil and it will be uniform for soft soil.The single prestressed anchor bar mechanism is numerically analyzed through CABLE element in finite difference method FLAC2D. Numerical simulation and engineering practice both show that grout injection pressure has great influence on prestressed anchor bar limit tension capacity, which can be improved through high pressure grout injection technique. The prestressed anchor bar limit tension capacity can be improved by increasing the length of anchor sector, but the relationship of anchor bar limit capacity and anchor sector length is nonlinear. When the anchor sector length arrives to the critical value, it will be useless to increase the limit tension capacity. Hence it is infeasible to simply increase the anchor sector bar length to improve the limit tension capacity of anchor bar.(4) Based on finite difference method FLAC2D and slip surface stress method, the foundation pit with prestressed anchor bar excavation process, along with the factors related to foundation pit deformation and stability, is numerically analyzed. The result illustrates that: (1) The finite difference method, combined with slip surface stress method, can show the deformation and stability safety factor of the foundation pit, which can reflects the stress state and deformation of the soil mass, and can also simulate the excavation process, then the foundation pit stability can objectively evaluated. (2) The horizontal displacement of foundation pit with prestressed anchor bar flexible supporting corresponds to the vertical deformation, both of which are linear distribution. The maximum horizontal displacement usually takes place on the top of the foundation pit, and it decreases with the increasing of the depth. The maximum vertical deformation occurs on the pit wall, which decreases with the increasing of the distance from pit wall. Increasing prestressed value can obviously decrease foundation pit displacement. (3) The anchor bar prestress has great influence on foundation pit slip field. With the increase of prestress, the slip zone shrinks and even disappears when the prestress equals to critical value. (4) The main damage style is tension damage on the top of the foundation pit and shear damage at the bottom, and the plastic zone shrinks when the prestress increases. (5) The finite difference method along with slip surface stress method, can obtain potential slip shape and safety factor for foundation pit with different supporting condition. Taking the stress field during foundation pit excavation into consideration, the safety factor increases with the increase of anchor bar prestress, but the increase amplitude is gradually decreasing, so it is infeasible to guarantee the foundation pit stability simply through increasing the prestress of the anchor bar. (6) The soil mass gravity, internal friction angle and cohesive, and the parameter of anchor bar, prestress, free sector length and inclination angle are of great significance for stability and deformation of the foundation pit.