Drained Timber Pile Ground Improvement for Liquefaction Mitigation: Fid-Bau Portal

Drained Timber Pile Ground Improvement for Liquefaction Mitigation

A. W. Stuedlein / T. Gianella

Excess pore-water pressure induced by rapid shearing often leads to liquefaction of granular deposits, resulting in excessive deformation (settlement, lateral spreading) and loss of stability of supported structures. Since several devastating earthquakes in the 1960s, practitioners and researchers have developed and evaluated numerous approaches for the mitigation of liquefaction and its deleterious effects on civil infrastructure. Innovations include vibro-compaction and vibro-replacement of granular deposits, compaction and permeation grouting, deep soil mixing and jet grouting, and installation of large-diameter, high-density polypropylene (HDPE) earthquake drains (EQDs). These mitigation techniques attempt to improve the ground such that the soil is densified, reinforced, or drained, lowering the potential for excessive ground deformation. Although the foregoing mitigation techniques enjoy strong theoretical and empirical evidence of their effectiveness, each of the methods exhibits the limitation that they use one mode of treatment (densification, reinforcement, or drainage). To overcome these limitations, the effectiveness of conventional and novel drained timber pile ground improvement for the mitigation of liquefaction was evaluated. The results of this study showed that drained and conventional piles could effectively densify liquefiable soils, with increases in relative density ranging from 60 to 95 percent immediately following installation of timber piles, depending on the pile spacing and use of pre-fabricated vertical drains (PVDs). Long-term measurements of corrected cone tip resistance showed increases of approximately 30 percent for piles spaced at four to five diameters, D, with and without PVDs, 125 percent for piles at 3D without PVDs, and about 145 percent for piles spaced at 3D with drains and 2D without drains. Closely-spaced drained piles produced larger improvements in cone tip resistance than conventional piles at the same spacing (i.e., 3D). Controlled blasting of the timber pile treated areas showed that the treated soils responded in a dilative manner, resulting in decreases in excess pore pressure relative to an unimproved zone, and resulting in significantly smaller vertical ground deformations. Although areas for improvement in the drained pile prototype were identified, there are no barriers to the immediate implementation of drained and/or conventional, driven timber displacement piles. Because there is no proprietary information associated with this innovation, state departments of transportation and their design consultants may begin to implement this technology immediately.