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

PPT Presentation

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The presentation examines how to convert strain measurements from instrumented static load tests into internal forces in cast-in-place grouted piles, including augered cast-in-place piles, drilled displacement piles, and micropiles.

The conventional calculation uses <strong>F = EAε</strong>, where force depends on measured strain, cross-sectional area, and elastic modulus. In practice, the product <strong>EA</strong> (axial rigidity) is what is needed. Applying standard concrete or grout correlations can be unreliable: in-place properties are uncertain, modulus varies with strain, sample conditions differ from the installed pile, and aggregate content creates substantial scatter. In some cases, conventional calculations can produce implausible forces, even exceeding the applied load.

The <strong>Incremental Rigidity (I.R.) method</strong> estimates rigidity more directly from load and embedded strain measurements. It uses data recorded after shaft resistance above a strain-gauge level has been mobilized, when changes in applied load correspond to changes in internal force. The relationship between incremental load and strain is then used to estimate the material’s strain-dependent modulus and convert strain to force.

The authors compared the I.R. and ACI approaches using a database of 22 static load tests on ACIP and drilled displacement piles. In one case history, the I.R.-derived concrete modulus was 37.0 GPa, close to the ASTM C469 result of 37.4 GPa and above the ACI estimate of 34.5 GPa. Overall, I.R.-back-calculated moduli were typically 70–90% of ACI estimates for normal-weight concrete or grout samples.

The presentation concludes that the I.R. method offers a more direct way to interpret embedded strain data and address limitations in conventional strain-to-force calculations.

Keywords

strain measurements

static load tests

axial rigidity

incremental rigidity method

embedded strain gauges

cast-in-place grouted piles

augered cast-in-place piles

drilled displacement piles

micropiles

concrete modulus