The Tangent Stiffness Uncertainty in Strain-hardening and Strain-softening Soil

Load distribution in piles can be evaluated from strain-gage records by applying the tangent modulus method to the measured strains. The method requires tests in soil exhibiting a plastic response to the relative movement between the pile and the soil. Where the response instead is hardening or softening with increasing movement, the evaluated pile material stiffness becomes larger and smaller than true, respectively. This is demonstrated by analysis of a hypothetical test on a pile with a constant stiffness (EA/L) tested in ideally plastic, strain-hardening, and strain-softening soil.