Densification of Granular Soils Using Conventional and Drained Timber Displacement Piles

This paper presents the results of a full-scale field test program of driven timber displacement piles for the purpose of densification of loose, potentially liquefiable sands and silty sands. Both conventional and experimental timber piles fitted with prefabricated drains to help densify contractive soils during pile installation were assessed in this study. The piles were installed in groups with center-to-center spacing of two to five pile head diameters. Measurements of cone penetration tests (CPTs) revealed the variation of postinstallation increases in penetration resistance with decreases in pile spacing. Additionally, long-term measurements showed that pile spacing–dependent reductions in cone penetration resistance following installation can occur, which have been attributed to relaxation of horizontal stresses. Rough relationships between the average area replacement ratio and change in corrected cone tip resistance were developed. Pre- and postimprovement standard penetration tests conducted in each of the treated zones confirmed the long-term degree of densification noted using the CPT. Measured pre- and postinstallation shear wave velocities indicated that the soil fabric of the previously aged sands was effectively destroyed and that a reduction of small strain stiffness is possible in tandem with densification. The experimental findings in this paper may be broadly applied to displacement piles constructed of materials other than timber.