Meso-experiment on Vertical Characteristics of Caisson-cushion-pile Composite Foundation

In order to investigate the vertical bearing characteristics of caisson-cushion-pile composite foundation, a vertical bearing capacity test in view of piled-caisson of deep-water bridge test model was conducted in allusion to three different particle size distributions of gravel cushions. The side friction of pile, pile penetration, gravel flow trend and cushion porosity were developed by applying vertical loads on caisson, embedding strain gauges, soil pressure cells and displacement meter on the pile shaft. Continuous shooting with particle image velocimetry technique was provided during the test. And soil particle displacement field was obtained by dint of the image processing technique. The particle flow code (PFC2D) was employed to analyze the meso-characteristics such as stress transfer mode and porosity of composite foundation. The results show that the gravel particle size distribution has a great influence on the pile penetration. The pile penetration can be reduced effectively by the coarse and graded gravel. The neutral plane of the pile shaft is probably at a depth of 0.4 times pile length and then moved downwards to a depth of 0.7 times pile length. Meanwhile, the porosity of pile head area decreases from 29.2% to 11.2%. The obvious decline indicates that the particles in pile head area are compacted and formed into a core area. And it is consistent with the soil displacement field obtained by the digital image processing. The result of laboratory simulation is consistent with the numerical results from PFC2D. The porosity of the caisson corner edge tends to decrease at first and then increase. The dilatancy characteristics of soil are revealed. The porosity of pile top soil continues to decrease, forming the pile core compaction zone. The research results can provide references for the design of pile-caisson foundation of deep-water bridge. © 2017, Editorial Department of China Journal of Highway and Transport. All right reserved.