Application of the Concrete Stressmeter for Augered Cast-in-Place (ACIP) Deep Foundation Testing

Abstract

The conventional method of computing forces from strain gage data during deep foundation load testing relies on knowing the cross-sectional area and Young’s Modulus of the pile’s concrete or grout. Errors in estimating these parameters directly affect the computed force distribution of the foundation element being tested. Since the standard formulas were derived for concrete with coarse aggregates, the problem of estimating Young’s Modulus for augered-cast-in-place (ACIP) piles or other piles constructed with grout is compounded by the lack of mathematical relationships for converting grout strength into modulus of elasticity. Concrete Stressmeters (CSMs) are instruments designed to overcome these difficulties and directly measure unidirectional stress in cementitious materials. The instrument design creates a load cell out of the in-place cementitious material itself, with the same elastic properties as the surrounding material. This study discusses the installation and data analysis of two ACIP bi-directional static load test (BDSLT) piles instrumented with CSMs, to validate the use of these instruments as an alternative to strain gages. The first test pile had a nominal diameter of 610 mm and a length of 26 m; the second test pile had a diameter of 760 mm diameter and a length of 35 m. Both piles were installed in Dade County, Florida, in alternating layers of sand and limestone. CSMs installed using the self-filling method yielded excellent comparison to force values derived from strain gages. An error analysis indicated a potential order of magnitude improvement in the accuracy of computed forces using the stressmeters as opposed to the strain gages, due to minimal reliance on an estimated Young’s Modulus. The two case histories yielded comparable and reasonable force distributions with considerably less effort and fewer assumptions. The paper aims to stimulate the discussion and development of reliable alternatives to traditional strain-gage based analyses.