| Gravel cushion set up between base and foundation is a form of isolation system. There are some practices and experimental and theoretical research projects to prove the effectiveness of isolation of Cushion. Properties of the piles-cushion-caisson composite foundation of Qiongzhou Strait Bridge Foundation were studied during the selection process. The main contents of this paper are the isolation performances of the cushion in composite foundation based on the actual situation of the Qiongzhou Strait Bridge, and the analysis of effect of cushion thickness, vertical pressure, particle size and other factors on seismic reduction ratio. In this paper, particle flow analysis software PFC3D was used to preform numerical simulation analysis to study the isolation effect of the gravel cushion in a macroscopic view, and the isolation mechanism was explained in a microscopic view by analyzing the motion of particles in cushion, the porosity and the coordination number during shaking. The main contents are as follows:1. Numerical simulation analysis of the cushion under static force. It was found that the settlement of the upper structure increased under the same vertical force with the increase of the thickness of cushion, and with the increase of vertical force, horizontal ultimate bearing capacity increased. The horizontal ultimate bearing capacity was about 0.32 to 0.44 of the vertical force, and the friction coefficient decreased with vertical stress increasing. The impact of thickness of the cushion on the ultimate bearing capacity is very small, but with the increase of the thickness of the cushion, the horizontal displacement of the upper structure at the same horizontal load increased significantly.2. The frictional sliding characteristics of particles in cushion was obtained when the ground was given a small horizontal displacement at a stable rate. With the increase of the ground displacement, horizontal frictional sliding appeared in cushion, the maximum interlayer relative displacement appeared at the contact surface between the ground and the bottom of the cushion, and the interlayer relative displacement of the lower layers in the cushion is greater than the relative displacement of the upper layers, and with the increase of the thickness of the cushion, the relative displacement between adjacent layers decreased. When the thickness of the cushion is 20mm or 40mm, the relative displacement between the test block and the top of the cushion is significant, and when the thickness of the cushion is 60mm, the relative displacement beween the test pieces and the top of the cushion is small while the deformation mainly concentrated in the lower part of the cushion.3. Numerical simulation of the cushion under earthquake showed that gravel cushion can effectively reduce the seismic acceleration response of the upper structure. The seismic reduction ratio increased with the vertical pressure decreasing or the thickness of the cushion increasing, and there was an interaction between the vertical pressure and the thickness of the cushion. The seismic reduction ratio increased with the density of the cushion decrease, when the same earthquake wave was imported twice, the seismic reduction ratio would decrease after vibration compaction, and the decreasing range is large when the cushion is well graded. The seismic reduction ratio increased with the particle size increases, and when the average particle size is the same, the seismic reduction ratio would decrease the the cushion is well graded. When particles in the cushion was stratified according to different particle diameter, cushion with large particles in the lower portion could achieve better isolation effect. When the particles in the cushion are spherical, the seismic reduction rate of the cushion is better, and when the particles are randomly made up of two overlapping spheres, the seismic reduction of the cushion is poor. The cushion could effectively reduce the seismic respond of the upper structure under El-Centro wave and the selected artificial wave in II class site, while the isolation effect was relatively poor under Taft wave. Isolation effect was more sensitive to vertical pressure under Taft wave. When the El-Centro wave was input, the seismic reduction ratio increased when the acceleration amplitude was greater.4. Micro analysis shows that during the process of ground vibration, the relative sliding in the bottom part of the cushion was big and that of the middle part of the cushion is small. When the vertical pressure increases, the relative displacement between the cushion layers is reduced, and the relative velocity between the layers is small, the acceleration of particles is generally increased. When the thickness of the cushion increases, the decay of the vibration is more obvious in the lower part of the cushion, the relative displacement between adjacent layers in the upper part of the cushion and between the test block and the top of the cushion is small. When the cushion was layered arranged according to particle size, if the size of particles in the lower part is large, the relative displacement between layers in the bottom of the cushion increases, and the motion of the ground decayed faster in the bottom of the cushion, and bottom particles suffered less friction.5. Micro analysis shows that the isolation effect of the cushion is closely related to the coordination number of particles and porosity of cushion. The coordination number of particles decreased when the vibration started. When the acceleration is large, the coordination number of particles significantly reduced, leading to decreasing of the contact between the particles, and when the acceleration is small, the coordination number of the particles corresponding increased slightly. The analysis of coordination number of different heights in cushion shows that, in general, coordination number is larger when the particle is closer to the ground, but when the ground acceleration is large, coordination number of particles in the bottom part reduced most significantly, such that the particles in the bottom were most prone to sliding. The average porosities increase first and then decrease under vibration, and the porosity of the upper portion changed most obviously.6. According to the results of numerical analysis, relatively thick cushion should be used in the Qiongzhou Strait Bridge composite foundation, the best selection of cushion thickness was 3 m considering the isolation rate and horizontal displacement according to the ratio of similitude. The cushion should not be too dense, cushion with medium dense should be used to avoid poor isolation effect and large settlement. Circular particles with large diameter is the best selection for isolation cushion, well graded cushion is less effective and cushion with graves of same diameter is not realizable, so the desirable diameter for cushion would be 3-7mm. |
