ISSN: 1945-7545
CODEN: GTJODJ
Published Online: 5 October 2011
Page Count: 8

Strain Gage Analysis for Nonlinear Pile Stiffness

Sinnreich, Jon
LOADTEST Inc., Gainesville, FL

(Received 25 September 2010; accepted 23 August 2011)

Abstract

Interpreting strain gage data from deep foundation (pile) axial load tests is usually assumed to be a simple calculation. The measured strain at any point in the test is multiplied by the computed Young’s modulus E of the pile to obtain stress. The stress is then multiplied by the cross-sectional area A to derive the load carried by the pile at the elevation of the strain gage. However, if the product of A and E (the axial pile stiffness) is nonlinear, the load-strain path must be considered and an incremental approach taken in order to approximate the true load value. Concrete cast-in-place piles may develop transverse tensile cracks, either due to soil restraint during curing or as a result of applied tensile loads. As such cracks open and close, the resultant axial pile stiffness will change significantly and abruptly, and the assumption of a constant stiffness can lead to significant error when computing loads from strain gages. This paper presents the mathematical derivation of the incremental load-strain calculation and case histories to illustrate the method.

Keywords:
nonlinear pile stiffness, strain gage data analysis, incremental load-strain path

Paper ID: GTJ103412
DOI: 10.1520/GTJ103412
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