Observing Friction Fatigue Using High Strain Dynamic Pile Testing-Paper

Paper

Pdf Summary

The paper proposes using high-strain dynamic pile testing to observe and quantify friction fatigue—the reduction in shaft resistance that can occur as a pile is driven. In crushable soils such as calcareous sands, repeated shearing and particle rearrangement or crushing can reduce radial stress at the pile–soil interface, lowering friction. If unaccounted for, this may lead to underestimation of required pile embedment and capacity.<br /><br />The proposed method monitors piles throughout impact driving with strain gauges and accelerometers. Changes in the measured upward-travelling stress wave provide an initial indication of resistance changes. Signal-matching analysis is then used to separate static shaft resistance from dynamic effects and compare resistance profiles at successive depths or driving stages.<br /><br />Two case studies involved steel piles driven in calcareous sands. In the first, consistent hammer energy and full impact-driving records showed a decline in shaft resistance with increasing penetration, trending toward a residual value. In the second, the wave response was harder to interpret because hammer weight, drop height and energy transfer varied; signal matching nevertheless suggested some resistance degradation. Interpretation was limited because much of the pile had been installed by vibration before monitoring began.<br /><br />Across the cases, shaft resistance appeared to relate more consistently to the ratio of distance behind the pile toe to pile diameter (h/D) than to blow count. The authors suggest h/D may be a practical design indicator, but emphasize that the limited case-study data are not suitable as a design basis.<br /><br />The paper recommends consistent hammer energy and full-drive monitoring. Where vibratory installation is used, testing before and after vibration—or an alternative method such as vibratory driving analysis—could help quantify its effects. The approach could support further research into friction fatigue in calcareous sands and other contracting soils.

Keywords

high-strain dynamic pile testing

friction fatigue

shaft resistance

calcareous sands

impact driving

signal matching analysis

stress wave

pile-soil interface

residual resistance

vibratory installation