July 22, 2008

Above is a tunnel boring machine similar to the one trapped in a tunnel dug for construction of the York-Durham Sewer System project.

A tunnel boring machine, similar to the one pictured above, remains trapped under Langstaff Road after 1,000 cubic metres of soil collapsed on it during construction of a portion of the York-Durham Sewer System project.

McNally and Aecon work to extract tunnel boring machine trapped under Langstaff Road

McNally Construction and Aecon Group, working as a joint venture, have begun preparations to remove the $5-million tunnel boring machine that has been trapped under Langstaff Road since May 2, 2008.

A five-by-30-metre secant pile wall shaft is being built to help remove the buried 100-tonne tunnel boring machine (TBM) and its assorted trailing equipment to allow completion of the project.

The 60-metre train created by the TBM and its trailing equipment of backup pumps and electrical equipment is trapped under approximately 1,000 cubic metres of soil.

“Work commenced on a recovery shaft in late June, and will be ongoing until spring 2009,” explains Laura McNally, of McNally Construction.

“In order to minimize impacts on the project schedule, another TBM will be launched (in August) from the shaft at the east end of the tunnel drive (Bathurst Street, south of Highway 407).

“The TBM will mine west and be removed at the recovery shaft, and the two sections of tunnel will be connected at the shaft.”

McNally/Aecon has tackled three sewer expansion projects in York Region. Main pieces in the group’s underground project arsenal were Earth Pressure Balance TBMs designed by Lovat.

The TBMs used by McNally/Aecon are 3.3 metres in diameter and erect a precast concrete segment liner system that is 1.21 metres in length with a 2.74 metre inner diameter and 3.12 metre external diameter.

Two months ago, six workers operating a TBM 22 metres under Langstaff Road were progressing through “the most challenging ground conditions on the projects,” a very fine sand and silt with water pressure of 1.5 bar (21.7 psi), explains McNally.

“A breach of the tail seals occurred approximately 150 metres west of the intermediate shaft,” says McNally. “This resulted in an inflow of material, which the crew was not able to control. The tunnel was safely evacuated and secured [and] no one was injured.”

Approximately 1,000 cubic metres of material entered the tunnel over a 48-hour period, causing a large sinkhole on the surface and damage to a local watermain. McNally/Aecon is still investigating how exactly the breach occurred.

Soil conditions for the second TBM bore are different, consisting of sand and till for the majority of the drive. However, mitigation measures are being reviewed for when fine silt and fine sand are encountered closer to the recovery shaft.

“Recovery of the tunnel is paramount to completion of the project. The TBM will be removed during this tunnel repair process,” says McNally.

Dealing with sensitive ground to bore through was an issue in completing the 4.1 kilometre tunnel project under 19th Avenue.

“This project involved some highly sensitive environmental features, including tunnelling through the Oak Ridges Moraine,” notes McNally. “Diligence in all operations, combined with good cooperation with the owner’s consultant (Earth Tech Canada), resulted in completion of the project with no environmental impacts.”

The ground conditions on the 19th Avenue project varied between till and sand with water pressure up to 1.8 bar (26.1 psi). Several watercourses, which are fish spawning grounds and form the upper tributaries of the Rouge River, were challenges to contend with. Also, the presence of Redside Dace, a species of minnow which is threatened provincially and endangered nationally, required close attention.

The five-kilometre Bathurst Trunk Sewer tunnel from Steeles Avenue to Autumn Hill Boulevard (north of Hwy 407) primarily consisted of clay, till and sand, with water pressure up to 1 bar (14.5 psi). Completion of the trunk sewer is expected sometime in November. Any silts encountered here and on the 19th Avenue project have been “coarser grained” than those on the Langstaff project, notes McNally.

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