Overview: The Challenge of Liquefaction-Influenced Pile Systems
In many geotechnical settings, piles anchor bridges, tall buildings, and offshore platforms. When saturated sands undergo liquefaction during an earthquake, the dynamic interaction between soil, piles, and superstructures can drastically alter bearing capacity and deformation patterns. This study, a collaboration between the Department of Hydraulic Engineering at Tsinghua University and the Key Laboratory of Urban Security and Disaster Engineering at Beijing University of Technology, tackles a nuanced aspect often underrepresented in seismic design: the behavior of end-bearing friction piles with articulated pile-top connections in liquefiable soils.
Methodology: Merging Shaking-Table Experiments with 3D Numerical Modeling
The authors built a three-dimensional finite-difference model in FLAC3D, integrating SANISAND to capture the liquefaction response of saturated sand. The model explicitly represents the articulated connection between pile tops and caps and simulates both the frictional and end-bearing behavior of the piles. The study’s core strength lies in its cross-validation: numerical results are benchmarked against shaking-table experiments that mimic the soil–pile–structure system under earthquake loading.
Key aspects of the approach include:
- Use of SANISAND to replicate liquefaction-induced pore-water pressure buildup and shear-strength reduction in saturated sand.
- Accurate representation of articulated pile-top connections, which alter the distribution of bending moments along the pile length and influence overall failure modes.
- Comparison metrics spanning pore-water pressure responses, soil and pile accelerations, dynamic shear-stress to shear-strain hysteresis, and pile shaft bending moments.
Findings: Dynamic Response and Failure Mechanisms
1) Pore-water pressure and liquefaction dynamics: Under seismic excitation, the sand’s pore-water pressure ratio rises rapidly, signaling liquefaction onset. This reduces soil shear strength and modifies the soil-pile-structure interaction regime. While the pore pressure response is most pronounced near the surface, liquefaction effects propagate through the upper soil layers, influencing the behavior of shallow and mid-depth piles.
2) Acceleration patterns: The study notes that soil and pile accelerations do not exhibit significant amplification within the liquefied layer, but an amplification trend emerges in the upper portions of the soil profile. This has direct implications for interface stresses and the potential mobilization of pile-top connections.
3) Shear-stress–shear-strain hysteresis: The dynamic hysteresis curves flatten with depth, indicating a progressive reduction in soil shear strength during shaking. This degradation contributes to a higher likelihood of excessive deformations in articulated configurations, even when peak accelerations are moderate.
4) Pile bending moments and shear forces: The simulations reveal that the maximum bending moments concentrate in the middle to lower portions of the pile shaft, while the corresponding shear forces at these sections remain comparatively small. This pattern highlights a critical vulnerability zone for end-bearing friction piles in liquefiable sites.
5) Failure mode with articulation: When the pile tops are connected to the caps via articulation, the pile shaft tends to experience pronounced bending in the middle portion. This emphasizes the risk of bending-dominated failures in evaluated designs and underscores the need for careful detailing of articulated connections in seismic regions with liquefiable soils.
Engineering Implications: Designing for Articulated Pile-Structure Systems
The study provides actionable insights for engineers working in liquefaction-prone regions. First, the articulation between pile tops and caps can significantly influence bending demands; designs should consider mid-pile bending as a critical failure indicator. Second, liquefaction-induced strength loss in the surrounding soil can alter the distribution of internal forces within the pile group, necessitating robust vertical and horizontal load-path analyses. Third, the validated FLAC3D model offers a reliable tool for parametric studies, enabling engineers to explore how variations in pile spacing, pile diameter, and cap rigidity affect seismic resilience under liquefied-site conditions.
Concluding Remarks
By integrating shaking-table experiments with high-fidelity numerical simulations, this work advances our understanding of how articulated pile-top connections respond to earthquake-induced liquefaction. The key takeaway is clear: in liquefiable sites, the middle portions of piles can bear unexpected bending moments, and articulation details must be designed with these potential demand patterns in mind. This holistic approach helps bridge the gap between laboratory observations and field-scale seismic performance predictions, guiding safer, more resilient infrastructure design.
