A Discussion of Back-Calculated Rigidity Methods for Deep Foundation Load Testing-Video

Video Transcription

Video Summary

The presentation explains how to interpret strain-gauge data from deep-foundation load tests to determine pile load distribution and soil resistance. The speaker distinguishes “rigidity” (cross-sectional area times Young’s modulus) from “stiffness,” which relates force to displacement. Because concrete and grout may behave nonlinearly at the strains reached during testing, the presentation models their stress–strain relationship with a quadratic function.<br /><br />Two back-calculation approaches are discussed: the secant method, typically suited to top-load tests, and the tangent method, often used in bidirectional tests when soil resistance affects measurements between the jack and gauges. Both derive from the same quadratic assumption and can produce equivalent rigidity estimates, although tangent-method results require appropriate interpretation and may involve greater subjectivity. The speaker warns that using a fixed ACI concrete modulus can distort inferred resistance across soil layers and lead to misleading design conclusions. Known cross-sectional-area measurements can help adjust results for piles with variable geometry.

Keywords

strain-gauge data

deep-foundation load tests

pile load distribution

soil resistance

secant method

tangent method