The specification of unprocessed rockfill for highway applications – A case study from the Coffs Harbour Bypass

Efficient utilisation of site won materials is the key component for successful delivery of any earthworks scope. Materials and construction methods need to be fit-for-purpose, providing long term performance confidence for use in earthworks design. The Coffs Harbour Bypass project presents a unique materials management challenge with over 1M m³ of material in cuttings requiring blasting within high strength siliceous metamorphic geology (Argillite). To meet the earthworks Specification for embankment construction this quantity would need to be crushed and processed, placing significant strain on delivery program and cost.

A project-based method specification was developed to suit the site geology and unprocessed blast condition, providing an efficient mass haul while ensuring performance requirements can be met over the design life. Laboratory testing, compaction trials and routine engineering geology assessment of the source cuts show that an unprocessed rock fill material can be produced that exhibits a well graded particle size distribution with high strength and durability properties. Analysis of these attributes show that the contract performance criteria including stiffness and post construction settlement can be met.