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

Instrumented static load testing is an effective means for assessing load-transfer characteristics of augered cast‑in‑place piles under static axial load. Strain gauges are typically instrumented on the steel reinforcement at specific locations along the pile’s embedded length for determining geotechnical resisting forces in side-shear and end-bearing. Conversion of strain to internal force at each gauge level can be complicated, requiring critical assessment of the pile properties and potential strain-dependent behavior. Current codes and standards provide recommendations for approximating these necessary pile properties, although these methods may not routinely yield realistic interpretation of measured strains. Non-linear rigidity may be exhibited over a static load test duration and there remains limited published methods for determining load-distribution characteristic under these conditions. Through measuring embedded strain and uniaxial stress simultaneously, the pile’s strain-dependent rigidity can be directly determined, thus minimizing uncertainty in the analytical process. While not commonly implemented in standard load test practices, stressmeters have been installed adjacent to strain gauges for this reason. A case study is presented where stressmeters were specially adapted for installation on augered cast-in-place piles on a bi‑directional static load test program. Based solely on the measured strains, it was realized that a non-linear rigidity would be required for properly converting strains to internal forces. The uniaxial stresses determined from the stressmeter measurements provided valuable insight in the pile material response to applied loads, ultimately reducing to a non-linear piecewise function of rigidity versus strain. Derivation of this non-linear piecewise function and methodology for computing internal forces are addressed. Comparisons of the non‑linear rigidity function to alternative rigidity (linear and non-linear) determination methods are presented. Recommendations for improving instrumented static load tests are summarized, primarily focusing on prospects and interpretation of embedded stressmeter measurements in top-down and bi‑directional static load tests.