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

PPT Presentation

Pdf Summary

The presentation examines how to determine the nonlinear rigidity of grouted cast-in-place piles during instrumented static load tests. Converting measured strain into internal force requires knowing the pile’s composite stiffness (elastic modulus multiplied by cross-sectional area). In practice, grout modulus is often estimated from compressive strength using empirical correlations, but these correlations vary widely and may not represent in-place material behavior. Modulus also changes with strain and depends on aggregate content.<br /><br />The presenter discusses using a piecewise-linear rigidity function to convert strain to force, rather than assuming a single constant modulus. The approach is illustrated with two 36-inch-diameter augered cast-in-place piles bearing on rock and tested using bi-directional jacks. Concrete stressmeters (CSMs)—load cells in series with concrete cylinders made from the same mix as the pile—were installed alongside strainmeters to measure stress directly and help determine in-place grout properties. The piles had average measured grout strengths of 5,800 and 6,560 psi.<br /><br />Comparisons among CSM measurements, strainmeter-derived rigidity, and conventional estimates show the potential for substantial differences; conventional calculations can produce internal-force estimates exceeding the applied test load. The results support piecewise-linear rigidity as a reasonable method for interpreting strain and indicate that embedded stressmeters can directly inform estimates of in-place grout stiffness.<br /><br />The presentation concludes that no routine method currently exists for directly measuring in-place grout modulus. Further research is needed to improve strength–modulus correlations, understand the causes of nonlinear rigidity, and refine stressmeters for cast-in-place foundations.

Keywords

nonlinear rigidity

grouted cast-in-place piles

instrumented static load tests

piecewise-linear rigidity

strain-to-force conversion

composite stiffness

grout modulus

concrete stressmeters

bi-directional jacks

in-place grout properties