Phased geotechnical investigations for a long HDD crossing under a drowned valley estuary

Abstract

A good understanding of ground conditions, through developing a detailed geotechnical model, is key to successful trenchless installation, particularly for long-distance crossings. Geotechnical investigations are undertaken for HDD projects to identify and manage risks such as steering issues, frac-out, blow-out and bore refusal. The Scotland Island Energy Reliability Project located north of Sydney, featured a 650m long HDD from the mainland to Scotland Island, crossing the drowned valley estuary of Pittwater. Preliminary review was useful to identify indicators of the subsurface conditions. In Pittwater, mapped igneous dykes and the many known geological unit boundaries created a complex subsurface setting for trenchless installation. This paper looks in detail at the benefits of phased geotechnical investigations, including desktop review of existing information and geological mapping to identify preliminary geotechnical constraints and geological features. In this project, geophysical investigations were used to increase the detail of information across the proposed HDD alignment, followed by targeted borehole drilling in discrete locations along the proposed alignment. The significance of managing ‘changing’ information needs during the investigation is highlighted. In addition to the above, the paper discusses the investigation’s challenges and lessons learnt including increasing the depth of investigation and land based drilling vs deep overwater drilling.

Techniques