Results and Lessons Learned from Converting Strain to Internal Force in Instrumented Static Loading Tests Using the Incremental Rigidity Method

摘要

A valuable objective of performing static loading tests on deep foundations is to determine load-transfer response. Integral to this objective is determining internal forces at various locations within the deep-foundation element. These internal forces are usually obtained from strain measurements. Using applied test loads and measured strains, the incremental rigidity ("IR") method determines the relationship between axial rigidity, EA, and strain at each strain gage level. This paper presents results from a number of instrumented static loading tests where the IR method was used to convert measured strains to internal forces. Various illustrations, conclusions, and lessons learned from application of the IR method to these case histories are presented. Topics presented include the strain dependence of rigidity, interpretable versus non-interpretable IR results, the strain magnitudes required to yield interpretable IR results, the use of at- or above-grade strain gages, the benefit of averaging test load and strain values, determining variable rigidities within an apparently uniform pile, determining rigidities at non-interpretable strain gage levels, and applying the IR method to complex geometries. Results from a closed-end concrete-filled steel pipe pile are presented which indicate that the elastic modulus of concrete may increase linearly with depth in the pile.