The proposed alignment for the Ipswich Motorway Upgrade (IMU) from Dinmore to Goodna west of Brisbane, Queensland, Australia, traverses over three abandoned underground coal mines from different eras of mining (from the 1860's to 1987).The underground coal mines vary in depth/cover, age, and hence the mining methods used from Welsh bord and pillar mining using chiefly hand tools, through to modern mechanical mining methods. The three mines pose different risks to motorists from potential mine collapse/subsidence during the motorway's 100 year design life due to varying overburden cover, geometry, subsoil/rock conditions and proximity to the motorway. This paper presents a case study of a risk assessment-based approach used to determine the requirements of remediation (type and extents) to ensure safe driving for motorists in the event of potential mine subsidence. The risk assessment was undertaken at the various design stages based on an evolving knowledge of mine geometry and extents as the investigations proceeded in-parallel with the design. The decision-making process at each design stage was managed by assessing the risks associated with mine subsidence based on the information then available.
The Ipswich Motorway Upgrade – Dinmore to Goodna (running in an East-West direction, refer Figure 1) involves the upgrade of 8km of constrained urban motorway from four lanes to a minimum of six lanes and also includes two motorway-to-motorway interchanges. This is one of the largest roads projects undertaken in Queensland, Australia, with estimated costs of AUD1.95billion. The motorway upgrade is part of the national and regional infrastructure investment strategy to provide the transport corridor for the growing urban population west of Brisbane. The upgrade is within a largely confined corridor, thus requiring careful planning for implementation whilst maintaining connectivity for up to 80,000 vehicles per day through the corridor. Numerous demolition (of some of the existing infrastructure) and construction stages are required with temporary re-alignment of live traffic lanes throughout the construction period. The proposed alignment traverses immediately over the relatively shallow (20 to 35m cover) Goodna mine (mined from the 1860's to the 1920's), skirts over a section of the New Redbank mine (1920's) and traverses in close proximity over the Westfalen No. 3 mine (1970's and 80's), both with 70 to 120 m of cover. The presence of these mines has complicated the design and construction sequence. Because of safety in design and construction considerations, anymine remedial works would need to be constructed prior to the civil and structural works, requiring remedial works to be on the critical path of the construction program. The Queensland Department of Transport and Main Roads (QDTMR) established the Origin Alliance (OA) comprising the QDTMR (client), Abigroup, Fulton Hogan and Seymour Whyte (contractors), and SMEC and PB (design consultants) to deliver (design and build) the proposed motorway upgrade through an alliance framework. Origin Alliance commenced work in June 2008 with an expected project completion date of 2012. Prior to OA's engagement, only preliminary investigations and risk assessment was carried out for the abandoned underground mines.