A Soil Quake Davisson Offset Method for Drilled Foundations-Paper

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The paper proposes modified Davisson Offset Limit methods for interpreting axial load tests on drilled foundations and helical piles, particularly when tests do not reach plunging failure. The original Davisson method was developed for driven piles and can underestimate drilled-foundation capacity because greater movement is generally needed to mobilize base resistance.<br /><br />For drilled foundations, the proposed QDOL method increases the soil-quake offset from the original B/120 to values ranging from B/60 to B/20, depending on foundation type. Recommendations include B/40–B/30 for augered cast-in-place (ACIP) and continuous-flight-auger piles, and larger offsets for some displacement piles and hollow-bar micropiles. For helical piles, the HDOL method defines the offset as a percentage of helix diameter, varying with shaft type and size.<br /><br />The methods use the initial tangent slope of the measured load–displacement curve instead of a calculated elastic slope, avoiding uncertainty in estimating pile elastic modulus. They are intended to provide straightforward capacity interpretation without graphical interpolation and to relate ultimate capacity to allowable settlement at the design service load, consistent with structural settlement limits.<br /><br />Case studies show that modified offsets can produce capacity estimates higher than the original Davisson limit and closer to other interpretation methods, while still accounting for serviceability. The appropriate factor depends on the foundation system and acceptable settlement; a higher inferred capacity should not be selected if it would violate settlement requirements. Overall, the QDOL and HDOL approaches offer practical alternatives for interpreting incomplete load tests, but their application requires judgment in selecting the offset and evaluating settlement.

Keywords

Davisson Offset Limit

axial load tests

drilled foundations

helical piles

QDOL

HDOL

settlement limits

capacity interpretation

load-displacement curve

foundation testing