Shortly after construction in 2002, a concrete arch tunnel at the Mount Arthur coal mine below a reclaim stockpile experienced significant cracking. As a consequence, remediation works were designed and supervised by a large multidisciplinary engineering firm.
Four years later, the tunnel again partially collapsed, and a different large multidisciplinary engineering firm was engaged to design and supervise remedial works to the collapsed and surrounding section of tunnel. No recommendations were made in respect of the remaining sections of tunnel as no repairs to the remaining section of the tunnel were considered necessary at the time.
Less than 12 months later, further major cracking was observed in another section of the tunnel, with major concerns being expressed as to the structural integrity and stability of the concrete arches. As a consequence, extensive and restrictive temporary bracing was installed to prevent a further collapse of the reclaim tunnel.
SCE were then engaged to:
a) Undertake investigations to determine the 'root causes' of the cracking and damage to the tunnel.
b) Assist with the design of remedial works which would fix the tunnels permanently.
The forensic root cause investigation involved:
- Decommissioning of the affected section of the tunnel.
- Staged excavation and density testing of the backfill materials to determine the existing quality of the tunnel backfill.
- Inspection and photographic recording of the various service trenches and subsoil drainage systems surrounding the tunnel.
- Testing of the concrete forming the arch, including concrete coring at the crack locations.
The investigation revealed that:
- The various services trenches with gravel backfill were concentrating water flow into the area above the affected reclaim tunnel.
- Significant horizontal cracking had developed in the concrete arch structure.
- Insufficient draincoil subsoil pipes were used and they were not provided with any flushing points, leading to substantial blockage of the pipes.
- Inappropriate geotextiles had been used around a secondary subsoil drainage line, preventing water entering the drain.
In light of the deficiencies identified during the site investigations and 'root cause analysis', the design of the remedial works focused on both structural and drainage issues.
SCE were then engaged to design a subsoil drainage system for the reclaim stockpile and for the failed section of the tunnel, in conjunction with a structural engineer.
SCE prepared various design drawings which made provision for:
- The prevention / minimisation of the potential build-up of water around the repaired sections of the tunnel.
- The capturing of surface & subsurface water beyond the area of tunnel backfill.
- Appropriate maintenance of the drainage system over the design life of the coal export tunnel by the provision of flushing points within the reclaim tunnel.
It was also considered important to provide for a possible future extension of the drainage system at a later stage, so that appropriate drainage could then be provided for the entire length of the original and extended sections of the tunnel.