Non-Linear Pile Rigidity Determined from Embedded Stressmeters in Instrumented Bi-Directional Static Load Test Piles-Paper

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The paper examines how nonlinear pile rigidity affects the conversion of embedded strain measurements into internal forces during static load testing of augered cast-in-place (ACIP) piles. Conventional approaches often assume a constant grout modulus or estimate rigidity from strain data, but these assumptions can produce unrealistic force profiles.<br /><br />In a case study, two 36-inch-diameter ACIP piles were tested using bi-directional static loading. Strain gauges were installed at several depths, and adapted Concrete Stressmeters (CSMs) were placed beside gauges at one level to measure grout stress directly. The stress and strain data allowed the authors to characterize the in-place grout response and develop a piecewise-linear rigidity model.<br /><br />One pile showed a largely linear response, while the other exhibited substantial nonlinearity. For the nonlinear pile, the stressmeter-based rigidity closely matched the piecewise-linear model. By contrast, using a constant rigidity produced calculated internal forces greater than the applied test loads—an impossible result. The incremental-rigidity method was also unsuitable because pile resistance was not fully mobilized at the instrumented levels.<br /><br />The study concludes that rigidity may vary with strain and between locations in the same pile, so a single assumed modulus may not reliably interpret all embedded strain data. Embedded stressmeters appear promising for directly measuring in-place grout properties and improving load-transfer estimates. However, their broader adoption requires further validation, including examination of the assumption that grout and reinforcing steel undergo compatible strain.

Keywords

ACIP piles

embedded strain measurements

internal forces

pile rigidity

static load testing

grout modulus

bi-directional loading

stressmeters

load transfer

nonlinear response