Research On Bearing Capacity Of Ultra-long Pile Foundation In Bridges Within Deep Cover Layers And On The Sloping Accumulation Bodies

With the development of the Western Development Strategy and the construction of the Chengdu-Chongqing Economic Circle,transportation construction in western regions is progressing rapidly.Long pile foundations have been widely used in bridge engineering and can meet the design requirements for foundation bearing capacity and deformation.However,the increasing number of bridge pile foundations built in deep covering layers and slope accumulation in mountainous areas of western regions has raised higher demands for the bearing capacity of long pile foundations.Currently,there is a lack of in-depth research on the bearing capacity of long pile foundations in these geological conditions,and the load transmission mechanism and force characteristics of long piles have not been fully understood.Moreover,long pile foundations located within slope accumulation are subject to more complex pile-soil interaction due to the effects of landslides and earthquakes.Therefore,studying the force and deformation characteristics of long pile foundations in deep covering layers and slope accumulation,as well as their influencing factors,is of great engineering significance.The main research content and results of this paper are as follows:(1)Field pile self-balancing method tests were carried out for an actual engineering project,and a three-dimensional numerical model was established using the Midas GTS NX finite element software.The measured data and numerical simulation results were compared and analyzed to discuss the internal forces and deformation characteristics of the ultra-long pile foundation.The reliability of the numerical simulation was demonstrated,and the finite element parameter design was optimized.Based on this,a three-dimensional numerical model was established for the ultra-long pile foundation on a deep cover layer,with the West Xi River Bridge P12 ultra-long pile foundation project as the basis of support.Under static conditions,the vertical and horizontal bearing capacity of the pile foundation was analyzed.Under vertical loads,the ultra-long pile foundation showed characteristics of a friction pile with significant compression.Under horizontal loads,the rock-soil mass had a significant anchoring effect on the pile foundation,with more obvious stress and deformation within a small range from the top of the pile.The installation of cross beams increased the horizontal shear and bending stiffness,improving the horizontal bearing capacity of the pile foundation.Therefore,the design of the pile foundation should fully consider the influence of pile compression and the advantageous design of cross beams.(2)A theoretical calculation based on a displacement-based method for the internal force and deformation of ultra-long pile groups was carried out to investigate the sliding behavior of a slope deposit under a pile group at the P6 main pier of West Xixi River Bridge.The feasibility of the theoretical method was verified by comparing the calculation results with the numerical simulation results,and the characteristics of the pile foundation stress and deformation were summarized.Furthermore,the pile foundation displacement and internal force variation under the sliding behavior of the deposit slope were discussed through numerical simulation.The influencing factors of the bearing capacity of ultra-long pile groups were studied,mainly focusing on the effects of upper structure load and anti-slide piles on the pile foundation bearing capacity.The maximum bending moment and shear force of the pile foundation under different slope strengths were found to occur at the top of the pile or near the sliding surface,indicating that the design and calculation of the pile foundation should focus on these two locations.The effect of the upper structure load on the bending moment and horizontal displacement of the pile foundation was found to be limited,while setting anti-slide piles on the rear slope was more effective than on the front,resulting in an improvement of the pile foundation stress and deformation,and an increase in the bearing capacity of ultra-long pile groups.(3)Based on the theory of time schedule analysis and combined with the actual engineering situation,a three-dimensional numerical model of the slope-pile foundationsuperstructure system was established to study the seismic response and analyze the internal forces and deformation characteristics of the pile foundation under horizontal earthquake action.Under different earthquake intensities,the displacement curve of the pile body showed an inverted triangle distribution and the characteristic of elastic embedding at the pile top.Due to the influence of the slope effect,the pile-soil interaction of the three-row pile foundation was extremely unbalanced,and the force and deformation of the pile foundation showed obvious asymmetry,leading to the largest displacement of the front row pile foundation and the rear row pile foundation being the main bending member.The superstructure load effectively reduced the shear force and displacement of the pile body,but made the force and deformation of the pile foundation more complex.Therefore,in practical seismic design,full attention should be paid to the influence of the slope effect and the superstructure load,as well as the bending resistance of the rear row pile foundation.