Experimental and analytical assessment of the pullout capacity of axially loaded under-reamed pile in cohesionless soil

To improve the resistance of the piles under pullout loads with less cost, under-reamed piles were used by enlarging the pile base. Such piles require a smaller stem diameter and length compared with conventional piles. The present investigation deals with the experimental evaluation of the ultimate pullout capacity of the under-reamed piles with spherical bulbs embedded in sand under axial loads. An experimental investigation was carried out on regular piles, single under-reamed piles, and double under-reamed piles embedded in sand with relative densities of 30%, 50%, and 80%. In addition, the bulb diameter ratios were 2 and 3. By increasing bulb diameter, number of bulbs, and sand relative density, the ultimate pullout capacity increased with a significant reduction in vertical displacement. Moreover, single under-reamed piles with a bulb diameter ratio of 3 were more significant in improving the pullout capacity. A theoretical equation was proposed to predict the ultimate pullout capacity of the under-reamed pile. The proposed equation was validated with the existing experimental results. The margin of error for the experimental results ranged from + 8.27 to − 9.29%. Also, the uplift capacity estimated using the equation correlates well with the experimental results, as R2 = 0.95.