Timber Piles for Ground Improvement in Fiji’s Liquefiable Soil

This paper presents the design of the ground improvement for the abutments of four bridges in Fiji, where the earthquake induced liquefaction and cyclic softening could cause slope instability. A geotechnical investigation with boreholes and Cone Penetration Testing (CPT) was undertaken to define the ground model at the eight abutments. Liquefiable sand was identified at four abutments and very soft clay up to 50m+ depth was logged under all of the considered bridge abutments. The available machinery and material supplies dictated using driven timber piles as a solution for ground improvement, as the crane on site for piling the foundations could also drive the timber piles, and treated timber piles could be easily imported from New Zealand. The slope stability analysis comprised four load cases for the eight abutments: static, earthquake Serviceability Limit State, earthquake Ultimate Limit State and Maximum Credible Earthquake (MCE) with the soil properties (drained and undrained) modelled according to the load case. The ground improvement design was based on the timber piles densifying potentially liquefiable sand and strengthening the soft clays. Swedish screw testing was used as a cost effective method to assess the post treatment densification within the sand layers as neither Standard Penetration Testing (SPT) nor CPT were readily available in Fiji. The required densification was achieved within the costs and timeframes required for the project.