Experimental study on the reinforcement mechanism of downhole dynamic compaction pile

Transportation infrastructure built on collapsible clay ground encounters issues including large differential settlement, slope slide etc. To address these challenges, downhole dynamic compaction piles were applied. But its reinforcement mechanism was not well understood. An experimental study was conducted in this paper to characterize the dynamic responses of a fully weather red mudstone ground reinforced by downhole dynamic compaction piles, considering the ground acceleration and strain with impact number. Results showed that with increasing impacts, the hammer took longer time to reach the contact point due to ground compression. The maximum downward acceleration of soil decreased with depth but increased with impact number. Soil strain was highest near the centreline, and it decreased with distance and depth. Based on accumulative soil kinetic energy, three reinforcement zones were classified with different soil collapse potential, which were effective reinforced zone, semi-effective reinforced zone, and minor effective reinforced zone. The effective zone was small, located below the compaction core, and expanded mainly vertically with impacts. The semi-effective zone expanded vertically and slightly horizontally, while the minor effective zone expanded equally in both directions. An elliptical impact zone formed at the bottom of downhole with major axis of 6d and minor axis of 3d.