Analysis of Strain-Gage Records from a Static Loading Test on a CFA Pile-Article

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The study analyzes strain-gage data from a static load test on a 610-mm-diameter, 10-m-long continuous-flight-auger (CFA) pile installed in sand and lacustrine clay. The pile had strain-gage pairs at three depths and was loaded in nominal 40-kN increments.<br /><br />The test procedure included an hour-long hold at 800 kN, unloading and reloading, and then further loading. The authors show that this interruption complicated interpretation: unloading and reloading reverse shaft shear and create hysteresis in strain records. Unequal holding times and imperfect load control also limited the usefulness of the measurements.<br /><br />Using secant- and tangent-stiffness methods, the authors estimated the pile’s axial stiffness at about 13 GN. They converted measured strains into axial loads at the gage levels, derived load distributions, and used effective-stress back-analysis to estimate shaft resistance. These results were then used to model the pile’s load–movement response. The analysis indicated that shaft resistance continued to increase with movement, consistent with slight strain-hardening, rather than becoming fully plastic.<br /><br />The calibrated response provides more useful information than estimating capacity from the pile-head load–movement curve alone. It can help predict settlement and assess similar piles at the site.<br /><br />The authors recommend testing instrumented piles using consistent load increments and equal holding durations, maintaining accurate load control, and avoiding unloading–reloading sequences. Such procedures improve strain-data interpretation and make test results more reliable for foundation design.

Keywords

strain-gage data

static load test

continuous-flight-auger pile

pile settlement

axial stiffness

shaft resistance

load distribution

effective-stress back-analysis

unloading-reloading hysteresis

foundation design