Geotechnical assessment of Pleistocene soils of the Mulgrave River Delta, North Queensland

The Mulgrave River Delta is a Quaternary age fluvial system in tropical North Queensland. The near- surface geology of the area comprises the Quaternary age (Pleistocene to Holocene age) Mulgrave River Delta soils. The soils accumulated to form deep infill of a distinct valley that is tens of metres deep. Geotechnical borehole, cone penetration test (CPTu) and geotechnical laboratory testing data were collected for use in geotechnical design strength assessment for displacement (driven) piles design for structures associated with the Edmonton to Gordonvale (E2G) Bruce Highway upgrade project, south of Cairns. This study focuses on the Pine Creek Yarrabah Road (PCYR) overpass bridge specifically. It is planned for the bridge abutments and piers to be supported on driven precast pre-stressed concrete (PSC) piles. As part of the site investigation twelve boreholes were drilled enabling standard penetration testing (SPT) and sampling. The SPT data identified a lower strength layer at depth. This was considered to be inconsistent with the initially anticipated geology, where increasingly stiff to hard cohesive soils had been expected. Supplementary geotechnical investigations were completed, including cone penetration tests (CPTu) and additional laboratory testing was scheduled on ‘undisturbed’ tube samples (U50). The aim was to assess implications (if any) of the identified lower strength layer on the design of PSC piles. Comparison of the field investigation and laboratory test data enabled greater confidence in the prediction of the soil strength profile and helped towards an optimised design for the PSC piles. To assist with the verification of geotechnical design strength, PDA testing (pile driving analyser – dynamic load testing) was completed during the driving of piles with CAPWAP analysis to help with confirming a prediction of shaft friction of the piles.