Validation of Design for Liquefaction-Induced Downdrag on Piles

Validation of Design for Liquefaction-Induced Downdrag on Piles

Researcher: Sumeet K. Sinha (PhD, 2021)

Abstract:

Pile foundations are designed to transfer loads to deeper depths through skin friction and tip resistance while undergoing acceptable settlements. For sites where earthquake-induced (liquefaction) ground settlement is expected to occur, estimating the drag load and pile settlement becomes an important design consideration. The prediction is often based on simplifying assumptions that arise from the incomplete understanding of the phenomenon, resulting in over- or underestimation of drag loads and settlements. It mainly results from an incomplete understanding of the complex interplay and timing of the different mechanisms during/post liquefaction (Figure 1). Centrifuge model tests are performed to investigate these mechanisms and expand our current understanding and propose revisions of existing procedures to better and cost-effectively design piles.

Figure 2 shows the development of the drag load in a uniform liquefiable deposit. The centrifuge model consisted of two medium pile of diameter (D) 0.635 m embedded 0D and 5D into dense sand. With multiple shakings, the drag load on piles increased and attained saturation.  About 15-20 mm of soil settlement was found enough to mobilize the full negative skin friction. As far as the performance of piles is considered, the experimental results clearly showed that the presence of high excess pore-pressures near the tip can significantly reduce the tip resistance and result in excessive movement. Overall, most of the pile settlement occurred during shaking and < 5 mm occurred post-shaking. Currently, another centrifuge model test is being conducted to study the effect of interbedded deposits and interface gaps.

Figure_1
Figure 1: Illustration of liquefaction-induced downdrag on piles
 
Isochrones of excess pore pressure, axial load profile, and soil and pile settlement during and after an earthquake event
Figure 2: Isochrones of excess pore pressure, axial load profile, and soil and pile settlement during and after an earthquake event
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References:

  1. Sinha, S. K., Ziotopoulou, K., and Kutter, B. L. (2021). Downdrag on axially loaded piles in liquefiable soils. 20th International Conference on Soil Mechanics and Geotechnical Engineering (ICSMGE). Sydney, Australia. Abstract submitted.
  2. Sinha, S. K., Ziotopoulou, K., and Kutter, B. L. (2020). Centrifuge Model Tests of Liquefaction Induced Downdrag on Piles in Uniform Liquefiable Deposits. In Preparation.
  3. Sinha, S. K., Ziotopoulou, K., and Kutter, B. L. (2020). Centrifuge Testing of Liquefaction-Induced Downdrag on Axially Loaded Piles : Data Report for SKS02. Center for Geotechnical Modeling, Department of Civil and Environmental Engineering, University of California, Davis, CA, Report No. UCD/CGM-20/01, pp. 1-156.
  4. Sinha, S. K., Ziotopoulou, K., and Kutter, B. L. (2019). Numerical Modeling of Liquefaction-Induced Downdrag on Axially Loaded Piles. Proc. 7th International Conference on Earthquake Geotechnical Engineering (ICEGE), Rome, Italy.