Shani Wallis, Editor - Toronto heading failure

Cause of an overwhelming inflow of waterbearing silt through the tailseal of the EPBM working on the Langstaff Road Trunk Sewer for the McNally/Aecon JV in Toronto remains under investigation and difficult to evaluate until the machine is retrieved by the planned recovery shaft.

The 3.3m o.d. Lovat machine was about 1.8km into its 3.6km long drive and about 150m from breakthrough into an intermediate shaft in early May when inflow of water, sand and silt from between the segment lining and the tail shield led to evacuation of the workers for safety.

Increasing flow delivered as much as 1,000m3 of material into the tunnel over a 48 hour period and created a deep sinkhole on the surface. The crater was filled with unshrinkable fill (low strength concrete). Continuing subsidence was stabilized with sand infill.

According to Laura McNally, Project Manager for the design-build contract, the TBM was not boring at the time. The machine was on a maintenance stop, the guillotine gate on the screw conveyor was closed, and there was no production activity going on. It was confirmed that replacement or repair of the standard three-row wire brush seal of the Lovat EPBM was also not underway at the time.

"It is difficult to know the root cause of the event until we retrieve the machine," said McNally, but it seems evident that highly saturated material, in combination with 1.5 bar ground-water pressure and very fine sands and silts particle sizes, was a factor. The incident is also under investigation for insurance purposes.

Annular fill behind the one-pass bolted and gasketed segmental lining is through grout holes in the segments using a non-accelerated sand/cement mortar and the tailseal is fed automatically with Condat’s heavy, viscous WR89 tailseal grease.

Ground conditions were known to be wet sands and silts under a high watertable. The 5m diameter x 22m deep intermediate shaft some 150m ahead "was a difficult shaft to sink but experienced no unexpected events", said McNally. The secant piled shaft was excavated in the wet with a tremmied concrete base and "has been finished for several months. We were planning an extended maintenance of the TBM, including inspection of the tailseal, once into the shaft."

Excavation of the intermediate shaft is not thought to have influenced the tunnel incident. Neither is existence of a highway runoff pond that lies immediately north of the tunnel line. "Water levels in the pond did not markedly change following the event," said McNally.

The tunnel at the point of the incident is parallel with and a few hundred meters north of Highway 407 and about 20m beneath Langstaff Road.

There are no buildings of any kind in the vicinity of the sinkhole and there were no injuries to any of the workers. A bulkhead was built about 300m behind the machine to contain the inflow and to June 3, the road above remains closed at the request of the tunnel owner. The tunnel was being driven on the gravity sewer's 0.1% down gradient.

Planning is currently underway to sink a 5m x 30m x 20m deep recovery shaft over the stranded machine and various ground treatment methods, including dewatering and ground freezing, are being considered to stabilize the soft, saturated, and now disturbed silt and sands.

Meanwhile, McNally is preparing to launch an available TBM from the other end of the drive and complete the 1.5km back to the intermediate shaft. "We are setting up to do that now," said McNally when TunnelTalk contacted the site this week (Tuesday, June 3). The available TBM is the identical Lovat EPBM that McNally bought for its JV contract with Aecon for the 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 incident occurred on May 2, and was lifted out of the reception shaft last week. The 19th Ave tunnel drive was very successful, mining through difficult sands and silts and under as much as 2 bar water pressure.

A third identical Lovat EPBM, bought at the same time for the Bathurst /Langstaff contract, is currently 4km into its total 5km in two drives under Bathurst Street as a perpendicular extension of the Langstaff Road collector. Record production rates have been achieved on the second drive, with a best shift of 17 rings and a best week of 129 rings. Both the Bathurst and Langstaff alignments pass underneath Highway 407 and the crossing are yet to be completed. “The Bathurst crossing is through till and sand while the Langstaff underpass is through dense till. Final breakthrough on the Bathurst tunnel drive is expected at the end of August.

The two contracts have a total value of some $Can150 million. The Bathurst and Langstaff Road contract is part funded by a group of private land developers and was awarded as a single design-build project in May 2006. Region of York awarded the bid-build 19th Ave contract to McNally/Aecon in July 2006.

Earth Tech/Hatch Mott MacDonald designed the 19th Ave contract for the owner and McNally/Aecon engaged local designer Genivar Consultants for the design-build Bathurst and Langstaff contract.

Halcrow designed the one-pass segmental lining for both contracts and Chris Smith of CRS Consultants managed set-up of the production process by subcontractor Boucher of Ottawa.

Having a TBM ready to launch at the opposite end of the stalled tunnel drive mitigates delay of contract completion. Total time lost to the incident and the eventual cost of the recovery is being assessed as part of current rescheduling and alternative recovery method studies.

Next line for expansion of the system
Meanwhile, another 15km long collector tunnel is in the planning stages by York Region to double capacity of the existing Southeast Collector ahead of approved urban development in York and neighboring Durham Region. A $Can500 million upgrade of the existing Duffin Creek Water Pollution Control Plant on the shores of Lake Ontario is currently in construction and the new Southeast Collector trunk sewer, estimated currently at some $425 million, is scheduled to begin construction in 2010-2012 once all regulatory approvals are in hand.