Experimental Study on Pullout Capacity of Vertical Shallow Multi-Plate Helical Piles in Sand-Paper

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The study experimentally examined the pullout capacity of shallow, multi-plate helical piles in dense sand, comparing cylindrical piles (CMHP) with tapered piles (TMHP). Six model tests were conducted at three embedment depths, corresponding to embedment ratios of 2.25, 2.5, and 3. The piles were installed in sand at 60% relative density and pulled upward at a controlled rate; peak capacity was assessed using a displacement criterion of approximately 10% of the maximum helix diameter.<br /><br />Pullout capacity increased with embedment depth for both pile types. At an embedment ratio of 2.25, the cylindrical pile performed better, with a capacity of 51.7 kg compared with 45.4 kg for the tapered pile. At ratios of 2.5 and 3, the tapered piles had higher capacities: 60.4 versus 57.2 kg, and 73.8 versus 66.8 kg, respectively. Thus, the tapered geometry offered an advantage once embedment reached a sufficient depth. The tapered piles also showed decreasing displacement at peak load as embedment increased, whereas displacement generally increased for the cylindrical piles.<br /><br />Experimental results were compared with the theoretical model of Mitsch and Clemence. The ratios of experimental to theoretical breakout factors ranged from 0.83 to 1.01, indicating broadly reasonable agreement, though differences may reflect the model’s simplifying assumptions and small-scale testing.<br /><br />The findings highlight embedment depth and plate geometry as important design factors for shallow helical piles in sand. Further research was recommended on the effects of helix diameter and soil density.

Keywords

helical piles

pullout capacity

dense sand

tapered piles

cylindrical piles

embedment depth

embedment ratio

multi-plate

Mitsch and Clemence model

breakout factor