Logistical and technical challenges of the geotechnical investigation for the new bridge over the Clarence River at Harwood

The new bridge over the Clarence River at Harwood forms an integral part of the Pacific Highway upgrade in northern NSW and is currently under construction by the Acciona-Ferrovial Harwood Joint Venture (AFHJV). The proposed bridge extends 1.5km between abutments, spans the Clarence River with a water span of approx. 560m and is founded on 2m to 2.4m diameter driven hollow steel tubular piles to over 60m depth in some areas.

The underlying geology of the site consists of 30 to 40m of soft estuarine silts, clays and loose to medium dense sands underlain by saturated basal sands, gravel and cobbles up to 25m in thickness. Historical ground investigation information has thus far failed to provide confidence in the characterisation and engineering properties of this basal gravel and cobble layer due to limitations with conventional SPT, CPT and drilling techniques.

A non-conforming ground investigation (GI) scope was proposed to support the bridge detailed design phase undertaken in parallel with the field works. This paper presents a perspective on the challenges of developing a ground model within this complex geological sequence and is was addressed by a diverse, state-of-art GI campaign. It also presents the logistical challenges encountered during the campaign and how alternative approaches such as geophysical methods and sonic drilling can bolster engineering design as well as the efficiency of an investigation program.