Converting Strain to Internal Force in Instrumented Static Load Tests on Cast-In-Place Grouted Piles-Video

Video Transcription

Video Summary

The presentation examines how grout properties affect interpretation of instrumented static load tests on auger-cast, displacement, and other grouted piles. Strain gauges measure deformation, but calculating internal forces also requires the pile’s axial rigidity—the combined effect of grout modulus and cross-sectional area. Common semi-empirical methods, such as ACI correlations between compressive strength and elastic modulus, can produce substantial uncertainty because grout behavior depends on mix composition and strain.<br /><br />The speaker presents an incremental-rigidity method that uses changes in applied loads and measured strains to estimate strain-dependent grout properties. Applied to 22 load tests, it showed significant variation from conventional predictions, potentially leading to non-conservative estimates of load transfer. The talk emphasizes careful instrumentation, quality assurance, and engineering judgment, and recommends more direct grout modulus testing and further research. Additional examples suggest that better measurements can identify construction irregularities and produce more physically realistic interpretations of pile resistance.

Keywords

grout properties

instrumented static load tests

auger-cast piles

strain gauges

grout modulus

incremental-rigidity method