Challenges Associated with Construction of Highways on Steep Side Slopes Covered with Colluvium from Hunter Expressway Project Australia

A section of the recently completed Hunter Expressway in the Hunter Valley, Australia, traverses steep side sloping topography covered by colluvium overlying weathered coal measures rocks. The rock mass contained seams of very low-strength and highly moisture-sensitive tuffaceous claystone interbedded in sandstone, siltstone and coal. The blocky and transmissive sandstone beds overlay the weaker, aquiclude siltstones and claystones creating a geological terrain vulnerable to rockfalls, surficial colluvium instability and deeper translational failures on the slope next to the alignment. The design of the motorway vertical alignment required a 10 m high reinforced soil wall (RSW) to retain the fill on the downslope side. Foundation works for the RSW were an early construction activity in the area. The construction of the RSW involved a large foundation excavation into the colluvial capped slope which had the potential to destabilize global stability of a marginally stable slope whilst boulder fall hazards presented both a construction and operational risk to be managed. Therefore, the construction of the RSW foundation was assessed as a high-risk activity. The team had to deal with challenges of additional hazards associated with inclement weather during excavation of the foundation. This paper discusses how the risks and challenges were successfully managed during construction through a broad range of mitigation measures including implementation of a responsive design development strategy, stakeholder engagement, on-site close geotechnical engineering oversight and staging of works which led to good design outcomes and allowed the works to be completed without any occurrence of slope instability or safety incident.