Research On Mechanical Behavior Of Pre-stressed Anchor Flexible Retaining Structures Based On Centrifuge Modeling Test

Prestressed anchor flexible retaining structure is a relatively new retaining technology for deep excavations,which mainly composes of prestressed anchor,bearing structure and shotcrete layer.It has been widely used in weathered rock.This paper focuses on prestressed anchor flexible retaining structure,and combines with the engineering hotspot that pressured prestressed anchor applied in the downtown areas is capable to being removed;the centrifugal physical modeling tests work as the main means of research,combining with numerical models and theoretical model for analysis,the paper takes simulation method of pressure-type prestressed anchor in centrifugal model;the automatic excavation simulation equipment in in-flight centrifuge test field;the difference and consistency of retaining effect of pressure-type and tension-type prestressd anchor;the earth pressure acting on shotcrete as research object.The main research contents are as follows:Against the characteristics that there is bearing structure existing at the far end of pressured prestressed anchor,the production and installation methods of pressured prestressed anchor in the small centrifuge model were invented;against the current situation of model test that the internal force of the small diameter steel wire cannot be monitored,the technique of precise pre-stress tension with the application of Fiber Bragg Grating sensor and secondary tension of prestressed anchor was invited.In order to better simulate practical construction sequence of prestressd anchor retaining structure,reproduce the real stress-strain condition,the indirect simulation device and technique for stepwise excavating and anchor pre-stressing in in-flight centrifuge was invited;referring to the composition of concrete and mix design,the composition of analogue material of strongly weathered slate was chosen,orthogonal test method was used to determine the proportion of similar materials in strongly weathered slate,the basic mechanical tests were carried out to determine the relevant physical and mechanical parameters,the target compound weight ratio of similar materials of strongly weathered slate was determined was gypsum:barite powder:quartz sand:mica:water =1:3.85:1,46:0.2:1;for the two types of prestressed anchor,pressured and tensioned type,the structural characteristics-different free lengths-contributes to the different anchoring modes were analyzed.By the assumption that the elastic strain of both types anchors after prestress tensioning is equal,the comparison analysis of both types of anchors was carried out with the establishment of that prestressed tension value proportional to the length of the free length.The results demonstrated that although different anchoring mechanisms exist in the two types of prestressed anchors,there is no obvious difference in failure modes;By analyzing the horizontal displacement time-history curves of different depths of excavations under different prestressing forces,it is concluded that the excavating process of each step of excavation can be basically divided into two stages:displacement release stage and displacement stabilization stage;by analyzing the time-course curve of horizontal earth pressure at different depths of excavation under the effect of distributed excavation,it is found that under different prestressing values,the redistribution of horizontal earth pressure occurred after every excavating.And with excavation proceeded,the earth pressure values at different depths of excavations decreased continuously.Among them,the fluctuation range of horizontal earth pressure in the middle of the excavation wall is larger than the top and bottom of the excavation,indicating that the redistribution of the force caused by the excavation effect to the horizontal earth pressure is greater than the top and bottom of excavations.An analytical solution was proposed in this work for limiting earth pressure on pre-stressed anchor-shotcrete retaining structures,which is in accordance with prestress diffusion model assumption,wedge theory and Mohr-Coulomb theory.Centrifuge tests were performed to investigate the feasibility of the analytical solution for the earth pressure on prestressed anchor-shotcrete retaining structures,which were further verified by centrifuge tests from other published works.It is found that the results of both centrifuge tests agreed well with the analytical solution.The tests also provide strong proof that prestress has little influence on the prestress diffuse angel based on elastic semi-infinite space theory.Aiming at the lateral load-bearing structure of rigid anchor in the prestressed anchor flexible retaining structure,the soil arching model is applied to the retaining structure.Through the static equilibrium and the Mohr-Coulomb failure criterion,the analytic solution of the horizontal earth pressure acting on the shotcrete surface layer is derived and verified by two independent centrifugal model tests.Through the comparative analysis of two analytical solutions,the analytic solution of horizontal earth pressure based on prestressed diffusion model and soil arching model is suitable for the silty clay and soft rock formation suitable for the prestressed anchor flexible supporting structure.Based on the soil arch model,the analytic solution of the horizontal earth pressure on the whole is better than that of the analytic settlement based on the prestress diffusion model,which shows that the soil arch model is more in line with the actual situation.Based on the analysis of mechanical parameter on the analytical solutions from the prestress diffusion model and the soil arch model,it is obtained under certain geological conditions,the calculated results of the two analytical solutions are consistent under the condition of the spacing of anchor l = 3m.Combined with the case of deep excavation project of Dalian Victory Plaza project retained with prestressed anchor flexible retaining structure,the mechanical behaviors of the project are compared and analyzed by three methods:numerical simulation,centrifugal model tests and analytical calculation.The horizontal deformation and vertical settlement of the side wall of excavation gradually increased with the excavating.With the increase of the excavation depth,the influence of excavating on the horizontal displacement of the excavation surface continuously reduced.Pulling cracks and local slip failure exist simultaneously:the tensile cracks exist in the rear end of the anchorage area,indicating that the anchoring effect originating from prestressed anchor was obvious to the anchoring area.Straight-line slip cracks gradually decrease from the upper surface until they disappear at the fourth row of anchors,along with the crack width decreasing.It is shown that the shearing force of anchor rod and the prestress anchorage obstruct the further penetration and slippage of the slip surface,and the pressured prestressed anchor effectively ensures the overall stability of the excavation.