Experimental Study On Bearing Performance Of Caisson-Cushion-Pile Composite Foundation Under Low Cyclic Loading

As a new type of bridge foundation,the caisson-cushion-pile composite foundation can not only effectively improve the stress characteristics of piles and control the horizontal displacement of the cassion,but also dissipate the seismic energy during the earthquake.It has been applied in earthquake-prone areas in the world.However,there are no relevant engineering examples in China,the specific seismic performance and horizontal bearing capacity under horizontal cyclic load of the foundation are still unclear.By means of the laboratory experimental tests and numerical simulations,the seismic behavior and bearing capacity of the caisson-cushion-pile composite foundation under low-cycle cyclic loading are studied.The main work and conclusions are as follows:(1)Cyclic loading tests of six kinds of foundations were conducted.The gravel was used as cushion material of four foundations,including with and without composite foundation,different thickness of cushion and different vertical force.Another two foundations were set up with pepple and coarse sand as the main cushion material.The experimental results were analyzed,including hysteresis curves,skeleton carves,strength,stiffness,energy dissipation.It was showed that for the foundation of gravel cushion,under the same cyclic displacement,the horizontal bearing capacity,strength and stiffness of the foundation decrease with the increase of cushion thickness and the decrease of vertical force,and the energy dissipation capacity is worse under the same settlement.The use of composite foundation can effectively improve the seismic performance of foundation.The horizontal bearing capacity and the seismic performance of the pebble cushion foundation is better than the whole gravel cushion foundation.The horizontal bearing capacity and equivalent stiffness of the foundation of the coarse sand cushion are lower than gravel cushion,but the strength is more stable,and the cumulative dissipating hysteretic energy is stronger than gravel and gravel cushion foundation.(2)The settlement and overturning of the caissons in foundation and the bending moment of the pile groups in the composite foundation were analyzed.It was showed that,under low cycle reversed loading,the settlemen of the caisson with coarse sand cushion develops slowest,the gravel cushion takes second place,and the pebble is the fastest.The overturning angle of the caisson with pepple cushion is the smallest,followed by coarse sand cushion foundation and the angle of the gravel cushion foundation is the biggest.The thickness of the cushion has little effect on the settlement and inclination control of the caisson,and the decrease of vertical force leads to greater inclination of the caisson.The use of composite foundation can effectively control the settlement of the caisson,but it has little effect on the overturning angle.Middle piles endure more lateral load compared with the side piles in composite foundation.The front row piles in the composite foundation endure the most lateral load,followed by the middle row piles and the least in the back row piles.The piles in the foundation with pepple cushion endure more lateral load than coarse sand and gravel cushion foundation.The cushion could reduce the lateral load of the pile shaft.(3)Numerical models reflecting the tested foundation models in this paper were established,the analysis results were compared with the measured results to prove the accuracy of the numerical models.Based on the numerical model and combined with the experimental phenomena,the failure process of composite foundation,the movement state of cushion particles and the load transferring mechanism were analyzed.Parametric analysis was also conducted to study the influence of the different parameters on the lateral bearing capacity.The results indicated that cushion particles lateral movement presents anisotropic response since it migrates to the initial loading direction.Particles in the caisson range rotate at random,and the rotation angle and the number of particles increase gradually during the loading process.Distribution of the interparticle contact force is changed from uniformly dispersion into concentrated towards the middle of the cushion.The void ratio and coordination number of the cushion in the caisson range do not significantly change with the loading process,but the void ratio increases and the coordination number fluctuates out of the caisson range.The lateral bearing capacity of the foundation improves with the increase of the cushion compactness.The better the particle size distribution,the better the lateral bearing capacity of the foundation.Compared with the particles formed by two overlapping spheres,the lateral bearing capacity of the foundation is better when particles in the cushion are spherical.