Select work resumed quickly after the incident, which happened more than 3km along the tunnel but well behind the tunnel boring machine, “Big Becky”. Both rock and the lining fell in the accident but no-one was injured.
The TBM has excavated more than half of the 10.2km long tunnel, which is a key part of a scheme to expand production at the Sir Adam Beck Complex. The utility plans to add 1600GWh of output per year.
The incident happened in a stretch of tunnel than previously suffered overbreak problems. The difficulties with the Queenston shale led to the remainder of the dig having to be realigned and a major contract renegotiation between OPG and contractor strabag.
Strabag has brought forward a six-week planned maintenance outage for excavation equipment, due for the end of the month, to minimise programme impacts. While repairs to the temporary lining proceed and work continues to install the permanent lining, OPG said no impact on project cost and schedule was expected.
At the half year, 30 June, almost 4570m of the tunnel, or 45%, had been excavated by the TBM. The client noted in its H1-’09 report that realignment of the tunnel had enabled the machine to improve progress, reducing rock overbreak to minimise excavation in the Queenston shale.
By then, tunnel lining was ahead of the revised schedule, which was negotiated along with the cost by OPG and the contractor, Strabag. Due to the poor ground and tunnelling difficulties, the project completion was re-negotiated from mid-2010 to the end of 2013.