Seismically-induced liquefaction represents a major cause of damage to civil infrastructure and can result in significant damage following major earthquakes. Liquefaction can occur when earthquake ground motions raise pore-water pressures in loose, sandy soil, resulting in a substantial decrease of soil strength. Owing to the potential for instability following liquefaction, geotechnical engineers often prescribe ground improvement programs to densify, drain, and/or reinforce liquefiable soils. Conventional driven timber pile ground improvement can provide a cost-effective liquefaction mitigation method, as it provides densification and reinforcement to an improved subgrade. The potential for drained timber piles to improve densification and potentially reduce in-earthquake pore pressures could allow all three improvement methodologies in one mitigation method. However, the soil densification possible with timber pile ground improvement is rarely incorporated into stability analyses because of the current lack of understanding of the amount of densification possible.
This study focuses on a field trial of driven conventional and drained timber piles to investigate the effect of pile spacing, time-since-installation, and drainage on the amount of soil densification. The test site consisted of clean to silty sands with a relative density ranging from 40 to 50 percent prior to installation. Following installation of the timber piles, which ranged in spacing from two, three, four, and five pile diameters, cone penetration tests were conducted to evaluate the degree of densification. These tests were performed at approximately one week, one month, three months, and eight months following installation to evaluate the effect of time and to understand the role of fines content on the degree of densification. In general, the relative density of the soils improved to approximately 60 to 100 percent depending on the pile spacing and the presence of drainage elements. A controlled blasting test plan was also conducted at an un-improved control zone and in the improved timber pile test area to evaluate the effectiveness of this ground improvement alternative at reducing the excess pore-water pressures and mitigating liquefaction. The treated zones were shown to mitigate liquefaction by reducing the peak residual r[subscript u] values 0 to 22 percent and lowering the soil settlements by approximately 75 percent compared to the un-improved zone.