Field-Validated Coupled Continuum–Discrete Analysis on Stress Paths, Plastic Work and Variable-Section Pile Formation Mechanism in Downhole Dynamic Compaction

Downhole dynamic compaction (DDC) improves soft ground by delivering high-energy impacts inside boreholes, where repeated hammer blows compact soil–rock mixture backfills and exert dynamic loading on surrounding soil. However, conventional penetration records cannot reveal the internal stress evolution, deformation, and energy transfer mechanisms that govern pile formation. This study established a three-dimensional coupled continuum–discrete numerical model, in which the native ground was simulated by continuum zones, and the in-hole soil–rock mixture backfill, together with the hammer, was simulated by discrete particles. The model was validated against a field test with three staged backfilling operations and nine field blows, reproducing the measured penetration attenuation with a maximum single-blow deviation of 6 cm and a cumulative relative error of 5.8%. The results indicated that the first blow after each filling mainly caused backfill rearrangement and borehole bottom compression, while later filling and tamping increased lateral deformation around the upper backfill, with limited additional movement at depth. Vertical compression predominated beneath the borehole bottom, whereas lateral compression predominated at the off-axis monitoring point. The displacement and plastic-work distributions supported a conceptual four-zone interpretation of variable-section pile formation, including upper contact expansion, intermediate radial compression, lower backfill expansion and shallow disturbance.