Paula Wallis, Reporter

After a titanic recovery operation, inspection of a buried EPBM in Toronto, Canada confirmed what engineers had suspected. The standard three-row wire brush seal on the machine was damaged. But was that alone enough to cause the catastrophic collapse of the sewer tunnel that entombed the TBM under more than 1,800 cubic meters of soil? It’s a question that plagues engineers more than 15 months after the collapse of the Langstaff Road Trunk Sewer for the Regional Municipality of York in Ontario, Canada.

“We still don’t know the actual cause,” said Steve Skelhorn, Project Manager for contractor McNally/Aecon JV. “We think it’s a combination of events. The damaged brushes were probably the catalyst that started the whole sequence of events, but the type of ground we were in is also a factor. The ground came in through the tail brushes, which started the whole sequence of events, causing the machine to sink, the tunnel to sink and ultimately to collapse.”

The questions remain as the owner orders four new LOVAT EPBMs for its next sewer project and the contractor completes the ill-fated Langstaff project that included roughly 3.6km of segmentally lined tunnel, three shafts and ultimately a recovery shaft and surface restoration work.

“We’re hoping to be finished this Friday (August 28, 2009) in terms of the permanent work,” said Skelhorn. “We’ve got a bit of clean up to do mainly fix a water main and restore the roadway, but basically the tunnel should be ready for use on Friday.”

Mitigation and Recovery
Following the May 2008 collapse the JV went immediately into recovery and mitigation mode, with the first order of business to resume tunneling to minimize delays in completing the $90 million contract.

By August, McNally had launched an available TBM from the other end of the drive to complete the 1.5km back to the recovery shaft. The available TBM was an identical LOVAT EPBM McNally/Aecon JV had purchased for its 4.1km long 19th Ave sewer contract several kilometers away, also for York Region. That TBM achieved final breakthrough on the 19th Ave job on the same day as the Langstaff tunnel collapse May 2.

The doomed EPBM was excavating east from Keele Street when roughly 1,800 cubic meters of liquefied mud came rushing into the tunnel. Six workers underground at the time escaped unharmed to the entry point 1.8km behind the machine. The compromised section of the 3m i.d. tunnel was sealed off with a bulkhead soon after the cave-in.

In December 2008, construction began on the 5m x 30m x 22m deep secant pile recovery shaft over the 100-ton waterlogged machine buried under more than 20m of soft, saturated, and now disturbed silt and sands.

On removing roughly 1,800 cubic meters of muck, enough to fill five Olympic sized swimming pools, the contractor realized part of the machine was lost forever.

“When we got down to the TBM and back up equipment, we found it had sunk below the tunnel alignment by more than 3 meters, which we did not expect, said Skelhorn. As we were restricted by the depth of our secant piles (22m) we took the front portion of the machine out along with the forward shell, the trailing shell and the tail can. We left the screw conveyor, the segment loaders, all the segments and bits and pieces in there and buried them under a mud slab. We couldn’t dig down to them so we really don’t know how far some of it went. It was not visible when we got to the excavation.

The remains of the $5 million dollar machine were recovered in March 2009 and are evidence in the forensic investigation of the collapse. The tail skin is of particular interest. According to the contractor, it was removed untouched, wrapped up and then uncovered and the brushes inspected in the presence of the client, engineers and the insurance company.

Tomislav Hrkac, Project Manager for the owner would not discuss the findings, but said the second TBM launched to finished the drive, was modified.

“I can tell you that machine was fitted with another row of brushes to help with any sort of challenging ground conditions,” said Hrkac.

Skelhorn confirmed the extra row of brushes and one other modification.

“We put more grease ports on the tail can and automated the system and added a fourth row of brushes. So if we did damage to a row we would have a spare one. It just gave us extra insurance.”

The brushes, located between the inside of the tail skin on the machine and the outside of the segments just built, provide a seal to prevent the ingress of ground water into the tail skin.

The modified EPBM heading west toward the cave-in finished the remaining 1.5m drive within seven months without incident.

“It went great,” said Hrkac. “There was on-going dewatering activity at the recovery shaft so the conditions were less challenging, but it went without incident and production rates were great.”

“It went extremely well,” said Skelhorn. “We didn’t break any records on it, primarily because we didn’t have a very good shaft to launch the machine. It wasn’t designed for it. We didn’t have a switch in the shaft so we had to remove the whole train bit-by-bit and then put it back down again when we emptied each train. We also didn’t want to arrive at the recovery shaft to early, so working two shifts we averaged about seven or eight meters a shift.”

The second TBM reached the recovery shaft in late February, 2009, and was parked for about two weeks as crews finished recovering the stranded TBM and installing the mud slab. With both TBMs recovered the tunnel repair commenced, said Hrkac.

“There was some jet grouting and some hand mining and other stabilization techniques in the slab area of the recovery shaft and ultimately the forming and pouring of the tunnel. The tunnel will be complete this week, with the repair of the water main and repaving of Langstaff Road to be finished by early October.”

New Contract
Despite the Langstaff Tunnel collapse, McNally plans to bid on the next major contract in York Region’s $800 million, 14-segment sewer network known locally as the "Big Pipe”.

The estimated $520 million dollar Southeast Collector Sewer (SeC) contract is scheduled for tender, January 10th, 2010 after a lengthy delay to acquire the necessary environmental approvals. In advance of the tender, York Region has purchased four LOVAT 3.6m EPBMs to excavate the 15km tunnel (Fig 1).

“This is the first time we have undertaken to purchase machines in advance of the tender call,” said Wayne Green, Project Manager for the owner. “In addition to the TBMs, we also have tenders out for the segmental liner system. We want to have the machines, material and equipment ready to go to work when we award the contract.”

The four machines will be launched strategically along the alignment to operate concurrently. The launch shafts are identified, but the contractor will determine the order of the launches.

With purchase of the machines the client has specified vacuum segment erectors and through the tail skin grouting, removing the option of grouting through the liner, which was the method employed on the collapsed Langstaff tunnel.

“It’s really contractor preference in many cases whether to tail grout or liner grout,” said Green. However we specified tail grouting for these new machines to maintain a good grout seal as the machine moves forward.”

Green says York Region plans to novate the four TBMs and the segmental liner to the successful contractor with the award of contract.

Ground conditions anticipated along the tunnel alignments consist of glacial tills both, plastic and non-plastic, sands, silt, gravel and clays. Cobbles and boulders of various sizes are also expected.

“For this particular project we chose a profile that allows us to stay mostly in the New Market till so we have some very good soil conditions to work with,’ said Green. “At the very start of the job and again at the highest elevation of the project we have some surficial sands. As we progress from high to low we go into a very tight till and move back into water bearing layers at the end of the drive.”

As the owner prepares to tender the Southeast Collector project it confirmed that McNally/Aecon JV, which has completed five major tunneling projects for the utility since 2000, would not be prohibited from bidding on this or future projects.

The contractor would not say how much the TBM recovery and repair of the Langstaff tunnel would cost, but did say it was unlikely to submit a monetary claim.

“It is more likely we will have a claim for additional time for a force majeure,” said Skelhorn. A force majeure is an event outside our control and we would be awarded time and time only. It mitigates liquidated damages which are being charge up front and have been since the original finished date of November 5, 2008.”

At $6000 a day for every day late, liquidated damages currently stand at close to $1.8 million.

“If we were put in the same position again the same tunnel with what we knew then. The same thing would have happened,” concluded Skelhorn. There’s nothing we could have been expected to know then, as far as I’m concerned, that would have prevented it. Obviously you can change the world with hindsight, so if we went back into that ground again we would do things differently, but we never expected the unexpected.”

References

Buried EPBM recovery in Toronto - TunnelTalk, Aug 2008