Research On Bearing Mechanism Of Rigid Long-short-pile Composite Foundation Under Horizontal Load

Along with the growth of the population and the development of urbanization,high-rise buildings have been widely used in the current era.At the same time,the long-short-pile composite foundation has been widely promoted in the foundation treatment of modern high-rise buildings thanks to its unique advantages.In addition to high-rise buildings need to withstand huge vertical loads,special areas must also withstand various horizontal loads that cannot be underestimated due to strong winds or earthquakes.The existing national codes and standards have not yet provided relevant regulations on the calculation method of the bearing capacity of composite foundations under horizontal loads.In view of the existing problems,this paper takes rigid long-short-pile composite foundation as the main research object,and conducts research on the horizontal bearing mechanism of composite foundation.In this paper,a set of relatively composite model test system of composite foundation under horizontal load is formed by making test scheme and developing test equipment.Carry out indoor model test of four-pile composite foundation with four long and short piles under four different loads.The horizontal displacement of the pile top and the cap and the strain of the pile were obtained in the test.The law of displacement distribution and the internal force of the pile are analyzed.In addition,firstly,by verifying the equal-size numerical simulation results established by the ABAQUS software with the experimental results.Furthermore,a nine-pile composite foundation model is established on the basis of four piles and the single factor analysis is carried out.The influence of factors such as load size,cushion layer thickness,pile length and other factors on the bearing mechanism of rigid long-short-pile composite foundation is deeply studied.The main conclusions are as follows:(1)When the horizontal load acts on the long-short-pile composite foundation,the distribution of the horizontal displacement of the long and short piles along the pile is different,and the horizontal displacement of the pile top is related to its location.The short pile rotates as a whole along a certain point of the pile body,and the long pile has multiple points where the displacement is zero,which shows the characteristics of the elastic long pile.(2)When the horizontal load acts on the composite foundation with four long and short piles,the bending moment of each pile decreases with the increase of the vertical load,and the maximum bending moment is located at 3D(D is the diameter of the pile);For the composite foundation with nine long and short piles,the bending moment of the back row of piles increases with the increase of the load,while the bending moment of the front row of piles and the middle row of piles gradually decreases with the increase of the vertical load,and the maximum point of the bending moment tends to move down with the pile depth.The maximum value is within the range of 5D ～ 8D.It is considered that increasing the vertical load can coordinate the bending moment of the piles in different areas.(3)In the multi-pile composite foundation with long and short piles,piles in different positions perpendicular to the direction of horizontal load bear little difference in horizontal load;Along the horizontal load direction,the first row of piles plays a role in the initial stage of horizontal loading,and as the horizontal force continues to be applied,settlement difference occurs on both sides of the cap.Each row of piles enters the working state in sequence along the loading direction,and finally the front row of piles bears more horizontal loads.(4)Increasing the vertical load can increase the horizontal bearing capacity of the composite foundation;With the increase of the cushion thickness,the settlement difference between the two sides of the cap away from and near the horizontal load increases,the horizontal displacement of the pile top decreases,and the horizontal load borne by the piles in the composite foundation is reduced.When the load is reduced,the pile body is less likely to be damaged;The length of the long pile remains the same,and the increase of the length of the short pile can effectively control the settlement of the cap and the difference in settlement between the two sides of the cap,and improve the ability of piles to bear horizontal loads.