Paula Wallis, TunnelTalk

Repairs are complete following a 25m³ rockfall in the tunnel but planned maintenance keeps the mega TBM idle on Niagara's water diversion tunnel some two months after the partial collapse. Meanwhile, in an interview with TunnelTalk yesterday (November19), the contractor says the root cause of the collapse stems from early geotechnical investigations.

Section of high overbreak in crown of tunnel

"The crown fall happened in a area surrounding a 20-year old bore hole," said Ernst Gschnitzer, Project Manager for Strabag. "The hole had a long time to deteriorate and eventually the ground gave way."

Gschnitzer said it took some time to make the repairs in part because the contractor had to build up a ramp to reach the crown and the overbreak. "We used standard means and methods employed throughout excavation to repair the crown, including 6m (20ft) rockbolts and 6m (20ft) IBO bolts plus wire reinforced shotcrete," said Gschnitzer. "We will also increase rock supports in areas of known previously drilled bore holes and we have also grouted all these old bore holes." Gschnitzer said the bore hole in question hadn't grouted at the time it was encountered, but has been subsequently.

The partical collapse came just as the TBM was moving into more competent ground and achieving higher progress rates. About 25m³ or 100 tonne of temporary lining and Queenston Shale fell from the crown on September 11. According to the owner, Ontario Power Generation, no one was injured in the collapse and all of the workers left the tunnel safely, following established emergency procedures.

The fall occurred about 2km (1.2 miles) behind the TMB, and some 3,600m (2.2 miles) into the 10km (6.2 mile) long 14.4m (47ft) diameter tunnel, in an area that experienced some of the most severe overbreak to date.

Longitudinal section of the geological conditions along the alignment

It happened on the new alignment in 80m (262ft) of overburden and beyond the immensely challenging section under the St David's Gorge that cancelled out the original completion schedule and cost estimate of the contract. The revised schedule agreed to as part of the realignment. The original project cost was estimated at $985 million with a scheduled completion of June 2010. The revised project cost estimate is $1.6 billion and the revised schedule completion date is December 2013. The contract includes revised incentives and penalties tied to the new target cost and schedule.

A six-week maintenance outage planned for the end of September on the Robbins TBM was moved forward and combined with the crown repairs. "After 5.4km (3.4 miles), partially in very abrasive rock, the bucket lift scrapers were worn out, so we have some major refurbishment on the cutterhead and we are also making repairs to the conveyor system, trucks and trailers and other equipment," said Gschnizer. Meanwhile installation of the concrete lining is progressing ahead of the revised schedule.

Alignment of the 10km long 14.4m diameter water tunnel

Before the stoppage, ground conditions had improved markedly following a change in the vertical alignment that was initiated late last year. Progress rates jumped to a high of 468m (1,535ft) for the month of July. "That was excellent, and confirmed that changing the alignment was the right decision," said Gschnizer. "Since then rates have deteriorated a little to about 8m-10m/day (26ft-32ft), but not to the extent of what we experienced in the Queenston Shale, and we expect decent progress rates once we resume excavations in about two weeks."

Repairs to the crown were completed at the end of October and Gschnizer says the TBM repairs will be completed shortly, with excavation resuming in early December.

References

Accounting for slow progess at Niagara - TunnelTalk, July 2008

Ontario Power Generation