On 22 September 2018 a leak developed during construction of the first tunnel cross passage, Cross Passage Dundas, in Tunnel One approximately 200m north of the Forrestfield Station site. As a result, water and silt entered Tunnel One (TBM Grace’s tunnel) leading to the formation of a sinkhole at the surface alongside Dundas Road.
The pressure of the groundwater, and the associated loading, caused permanent localised distortion to the tunnel shape and movement of the segments which make up the tunnel lining, ultimately damaging a 26m-long section of Tunnel One (16 rings). Rectification works were successful in repairing both the tunnel leak and sinkhole and temporary steel frames were installed to support the tunnel. These rectification works allowed tunnelling and cross passage construction works to continue whilst the Contractor investigated the incident and developed a proposal for the permanent repair of the damaged tunnel section.
Incident investigations have determined several contributing factors that are likely to have given rise to this incident. Given ongoing contractual discussions, and potential legal implications, it would be inappropriate to make statements concerning these as it could prejudice an outcome or weaken the Public Transport Authority’s position. These investigations have influenced work procedures project wide to ensure that a similar incident does not reoccur.
The Contractor, Salini Impregilo – NRW Joint Venture (SI-NRW), engaged the support of specialist engineering consultants to provide a comprehensive analysis of the incident and a proposal for rectification. The reinstatement works proposed include the installation of a secondary spheroidal graphite iron (SGI) lining within the existing pre-cast concrete segmental lining. PTA accepts SI-NRW’s proposed solution and accepts the proposed specification departures which do not significantly compromise the design and retain the 120 year asset design life.
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