Applying The Incremental Rigidity Method To Static Load Tests Performed On All-Steel Piles-Paper

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The paper evaluates the Incremental Rigidity (I.R.) method for interpreting strain-gauge measurements from static load tests on all-steel piles. Conventional analysis calculates internal force using an assumed product of Young’s modulus and pile area (EA). The I.R. method instead derives the relationship between strain and internal force from changes in applied load and strain, reducing reliance on assumed pile properties and potentially accounting for test-specific effects.<br /><br />The authors applied the method to 19 South Dakota load tests involving 17 H-piles and two pipe piles embedded in shale, glacial till, and other soils. Because steel’s stiffness is effectively strain-independent, they assumed a zero-slope relationship when interpreting incremental load-to-strain plots. Gauge levels were used only when shaft resistance above them was considered fully mobilized; engineering judgment was also applied to assess results.<br /><br />Three case studies show how the method can produce more physically plausible force profiles when conventional calculations yield impossible results, strain readings may be incorrectly calibrated, or pile-toe settlement affects measured strains. Across the database, I.R.-derived unit shaft and toe resistance generally showed lower variability than conventional estimates. Its estimates of the percentage of shaft resistance also correlated more strongly with CAPWAP dynamic-test results (R² = 0.610 versus 0.035 for conventional analysis).<br /><br />The authors conclude that the I.R. method is a valuable, generally preferable approach for interpreting strain data from static tests on all-steel piles. They recommend comparing I.R. and conventional results and validating interpretations against site-specific and complementary test data, since the method requires engineering judgment and static load tests involve multiple sources of uncertainty.

Keywords

incremental rigidity

strain-gauge measurements

static load tests

all-steel piles

internal force interpretation

shaft resistance

toe resistance

CAPWAP correlation

pile load tests

engineering judgment