Effect of Under-Ream Angle and Bulb Spacing on Capacity of Under-Reamed Piles in Marine Clay-Paper

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The study examined how under-ream angle and bulb spacing affect the load capacity of under-reamed piles in marine clay, where such piles are commonly used in practice. Nine timber pile models were tested: four single-bulb piles with angles of 15°, 30°, 45° and 60°; four double-bulb piles with spacings of 1.0, 1.25, 1.5 and 1.75 times the bulb diameter (Du); and one uniform-diameter pile for comparison.<br /><br />Adding an under-ream bulb substantially increased capacity: single under-reamed piles carried at least twice the load of the normal pile. Among the single-bulb models, the 45° pile had the greatest capacity at the settlement criteria examined, with the 60° model performing similarly. This supports the standard’s recommendation of an approximately 45° angle, though the results suggest angles between 45° and 60° may be effective in marine clay.<br /><br />Double-bulb capacity varied relatively little with spacing. The 1.25Du spacing generally performed best, particularly at the smaller settlement limits, consistent with the standard’s recommended range of 1.25–1.5Du. However, double-bulb piles did not consistently outperform the best single-bulb pile, contrary to expectations based on soil-cylinder action between bulbs.<br /><br />Overall, the model tests indicate that bulb geometry influences pile capacity in marine clay, while also highlighting that recommendations and expected benefits may not hold uniformly across settlement criteria.

Keywords

under-reamed piles

marine clay

load capacity

under-ream angle

bulb spacing

single-bulb pile

double-bulb pile

timber pile models

settlement criteria

pile geometry