Performance of under-reamed piles in saturated clay examining effects of bulb configuration on compression and uplift behaviour

Recent advancements in nanotechnology have enhanced the understanding of cohesive soils and supported more efficient foundation solutions. This study experimentally examines the compression and uplift behavior of straight and under-reamed piles with varying bulb numbers to clarify their role in soil–pile interaction. Four small-scale aluminum pile models, a straight pile, and under-reamed piles with one, two, and three bulbs are tested in reconstituted saturated clay (CL) using a 450 mm embedment depth inside a rigid 600 × 600 × 600 mm steel container. Axial compression and uplift tests are conducted to assess load–load-displacement response, ultimate capacity, and changes in load-transfer mechanisms. Results show that under-reaming significantly improves pile performance. Compressive capacity relative to the straight pile increased by 42%, 108%, and 149% for the one, two, and three bulb piles, respectively. Load-transfer analysis revealed a shift from friction-dominated resistance toward end-bearing dominance. For the straight pile, shaft friction contributed 55% of total resistance, decreasing to 35%, 22%, and 18% for the one, two, and three bulb piles, respectively, indicating reduced frictional mobilization with increasing bulb number. Correspondingly, soil pressure beneath the pile tip decreased by 20–25% for single bulbs and up to 40% for double bulbs, confirming stress redistribution toward enlarged bases. Under uplift loading, performance also improved, with ultimate capacity increasing by 89%, 120%, and 186% for the one, two, and three bulb piles, respectively. These results show the strong influence of bulb geometry in modifying stress paths, enhancing end-bearing engagement, and reducing frictional reliance. Overall, the findings provide a quantitative basis for optimizing under-reamed pile configurations in saturated cohesive soils and improving foundation design efficiency.