Distributed Fiber Optic Monitoring of Non-Uniform Deep Foundation Strain-Paper

Paper

Pdf Summary

The paper evaluates distributed fiber optic sensing (DFOS) for monitoring strain in deep foundations during load testing. Unlike conventional vibrating-wire gauges, which measure strain at discrete points, DFOS provides continuous profiles along a pile and can reveal variations across its cross-section.<br /><br />The field trial monitored a 3-foot-diameter cast-in-drilled-hole test shaft in California during a four-cycle top-down load test, reaching a maximum load of 1,750 kips. Four vertical strain-sensing cables and temperature-sensing cables were installed in the pile. DFOS measurements closely matched independent vibrating-wire gauge readings, with a mean difference of 2 microstrain.<br /><br />Continuous profiles exposed substantial non-uniform strain and bending within the pile. One vibrating-wire reading that might have been dismissed statistically as an outlier was confirmed by the DFOS data as valid. Removing it would have increased the calculated average strain at that depth by 30% and obscured evidence of bending. The observed bending may have resulted from a small offset between the pile’s cased and uncased sections.<br /><br />The case study demonstrates that DFOS can reduce uncertainty from interpolation between discrete measurements and help engineers interpret actual, non-ideal pile behavior. The authors conclude that it is particularly valuable when project complexity or performance requirements demand more complete strain information than point sensors alone can provide.

Keywords

distributed fiber optic sensing

DFOS

deep foundation monitoring

strain measurement

pile load testing

cast-in-drilled-hole shaft

continuous strain profile

vibrating-wire gauges

non-uniform strain

pile bending