Modified Strong Rock P-Y Curves for Rock-Socketed Pile Shafts-Article

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The paper proposes a modification to strong-rock (SR) p-y curves used to model the lateral resistance of rock-socketed pile shafts. Original SR curves show a sharp post-peak drop to nearly zero resistance at displacements of about 0.24% of the shaft diameter. This can cause modeled “unzipping” of rock resistance and lead to excessively long, costly rock sockets when checking lateral stability.<br /><br />The authors retain the original curves’ initial stiffness but assign a residual resistance equal to 20% of the ultimate lateral resistance, based on the assumption that fractured rock should retain resistance comparable to heavily weathered rock represented by weak-rock and Liang et al. curves. The modified curves are applicable to rock with rock quality designation (RQD) values from 25% to 100%.<br /><br />The modification was evaluated using LPILE models of a design example and three lateral load tests on 6-ft-diameter reinforced-concrete shafts socketed in siltstone. The modified curves prevented the predicted progressive loss of resistance in the example and reduced the estimated minimum socket length from 27 ft to 18 ft. Models using the original and modified curves both matched measured responses reasonably well within the test displacement ranges, supporting the curves’ initial stiffness. Back-calculated p-y curves also showed reasonable agreement in initial stiffness.<br /><br />However, the tests did not produce displacements at the rock interface large enough to reach the post-peak region. Thus, they could not verify whether the proposed residual strength accurately represents rock behavior at large deformation. The authors conclude that modified SR curves offer a simple, practical modeling option, while emphasizing the need for further large-displacement load tests.

Keywords

strong-rock p-y curves

rock-socketed piles

lateral resistance

residual strength

post-peak behavior

pile shaft stability

LPILE modeling

siltstone load tests

rock quality designation

socket length reduction