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

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Instrumented static load tests use embedded strain gauges to determine how load is transferred along cast-in-place grouted piles. Converting measured strain into internal force requires the pile’s axial rigidity (elastic modulus × cross-sectional area), but both properties can vary along the pile and are difficult to establish accurately. Geometry may be irregular, while grout modulus depends on mix, curing, installation, and strain. Consequently, conventional estimates—often based on nominal pile area and ACI correlations between compressive strength and elastic modulus—can produce physically unreasonable force profiles.<br /><br />The paper presents the Incremental Rigidity (I.R.) method, which uses changes in applied test load and measured strain at each interpretable gauge level to derive strain-dependent rigidity directly from load-test data. It therefore avoids independently assuming modulus and area when calculating internal force. With suitable engineering review, the method can also help estimate in-place grout properties.<br /><br />The authors evaluated 22 instrumented load tests, primarily on augered cast-in-place and drilled displacement piles. I.R.-derived grout moduli were typically 70–90% of values calculated using the ACI approach, although some cases differed substantially, potentially because of installation effects or anomalous zones. The comparison did not establish a universal correction factor for ACI estimates.<br /><br />The study concludes that I.R. can improve internal-force calculations by accounting for strain-dependent behavior and in-place material response. Its use should be supported by careful interpretation of gauge data and consideration of subsurface conditions, installation records, and non-destructive testing such as Thermal Integrity Profiling. The authors also recommend improving load-test practices and, where feasible, testing grout samples for both compressive strength and elastic modulus.

Keywords

instrumented static load tests

embedded strain gauges

cast-in-place grouted piles

axial rigidity

internal force

Incremental Rigidity method

strain-dependent rigidity

grout elastic modulus

ACI correlations

Thermal Integrity Profiling