The completed West Charlestown Bypass project comprises a 6.5 km length of dual carriageway urban freeway south west of the Newcastle CBD, in New South Wales, Australia. This section of State Highway 23 extends from the Pacific Highway at Windale in the south, to Charlestown Road at Kotara in the north. A 1 km length of the Bypass, at the northern extent of the works, traverses old underground mine workings with overburden depths as shallow as 5 m ranging up to 30 m. The mine workings are within the Australasian Coal Seam and they were undertaken by two collieries, initially in the 1800’s by the Australasian Coal Company and later in the early 1900’s by the Newcastle Myall Colliery. The presence of these old mine workings and associated subsidence effects posed a difficult problem for the design and construction of the Bypass.
The Bypass design was required to be safe and serviceable, considering the potential impact of ongoing mine subsidence associated with these workings. A number of treatment options were considered, with the adopted design including a combination of excavate, collapse and backfill of shallow mine workings, and grouting of voids where the workings were located at greater depths. This paper presents a case study of the pre-construction investigations, including discussion of the methodologies adopted and their success, the design requirements and options considered, and the completed mine void treatment works.