Design Guidelines for Characterizing the Residual Strength of p-y Curves for Rock-PDF

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The report proposes revising Strong Rock (SR) p-y curves used to analyze laterally loaded, rock-socketed piles. Original SR curves show a sharp post-peak drop to nearly zero resistance after small pile movements, a behavior the report says lacks empirical support and can produce unrealistic predictions of progressive failure.<br /><br />The study developed a three-dimensional FLAC3D model using the Hoek-Brown rock-mass criterion. The model was validated against a lateral load test of a 6-foot-diameter shaft socketed in siltstone; its predicted load-deflection response and pile demands generally agreed with measurements. The validated model was then used to estimate rock resistance for rock quality designation (RQD) values of 25%, 60%, and 100%.<br /><br />The proposed modifications account for rock quality in both ultimate and residual lateral resistance. Ultimate resistance is reduced as RQD decreases. After fracture, residual resistance is assumed to equal the ultimate resistance of heavily fractured rock with RQD of 25%, rather than approaching zero. Sensitivity analyses found that the proposed relationship was minimally affected by the depth of the rock layer.<br /><br />In an LPILE example, modified SR curves avoided the pronounced post-peak softening predicted by the original curves and maintained resistance at large displacements. The report concludes that the modified curves may offer a more realistic basis for designing rock-socketed shafts under lateral and seismic loads. However, the approach relies on modeling assumptions and requires further research and field validation.

Keywords

Strong Rock p-y curves

rock-socketed piles

lateral load analysis

Hoek-Brown criterion

FLAC3D modeling

pile load-deflection

rock quality designation

residual lateral resistance

LPILE example

seismic load design