Tunnel Boring and Underground Construction Equipment Advanced Machines play a critical role in modern urban transit infrastructure, but before these machines can begin their subterranean journey, deep foundation contractors must prepare the ground. On the Eglinton Crosstown Light Rail Transit Project in Toronto, Canada’s largest transit expansion, Deep Foundations Contractors undertook the complex task of constructing launch and extraction shafts that would allow two massive tunnel boring machines to operate uninterrupted through one of North America’s busiest urban environments. This article examines the engineering methods, drilling technologies, and construction techniques employed to keep the TBMs crawling beneath the streets of Midtown Toronto.
Project Overview and Scope of Work
The Eglinton Crosstown Light Rail Transit Project
The Crosstown is a CA $8.4-billion (US $6.6-billion) infrastructure project featuring a 19-kilometer (11.8-mile) corridor, of which 10 kilometers (6.2 miles) run underground. Upon completion, Metrolinx projects that 5,500 passengers per hour will benefit from what stands as the largest transit expansion in Toronto’s history. The Phase 2 contract, awarded to the Aecon and ACS Dragados Joint Venture in November 2013, encompassed 3.25 kilometers (2.1 miles) of 6.5-meter (21-foot) diameter twin tunnels, precast concrete tunnel lining segments, launch and retrieval shafts, headwalls for future stations, emergency exit buildings, cross passages, utility relocations, traffic staging, and extensive ground and building settlement monitoring.
Deep Foundations Contractors Role
Deep Foundations Contractors, one of Canada’s largest foundation contractors, specializes in shoring for building and infrastructure projects. The Joint Venture awarded Deep a roughly CA $21-million (US $16.7-million) contract encompassing three critical scopes:
- Excavation and shoring of launch and extraction shafts for two Tunnel Boring Machines
- Construction of two emergency exits for the TBMs
- Building of head walls for two future stations
The contract ran from April 2014 through March 2017, and its successful execution was essential to keep the two TBMs, named Don and Humber after two of Toronto’s rivers, progressing on their underground journey without interruption. The costs associated with stopping TBM progress would have been prohibitively high, making failure not an option.
Equipment Strategy and Drilling Rig Selection
Canada Largest Fleet of Bauer Drilling Rigs
The competitive advantage Deep held was Canada’s largest fleet of Bauer Drilling Rigs, all acquired through Equipment Corporation of America (ECA), a century-old leading supplier of foundation equipment in the Eastern United States and Eastern Canada. The fleet ranged from the largest rigs to compact machines suited for confined spaces.
Key acquisitions during the project period included:
- BG 55 – the most powerful rotary drilling rig on the market at the time, delivering 553 kNm of torque (acquired September 2016)
- BG 11 H – one of the smallest rigs, ideal for low headroom applications (acquired August 2017)
- BG 46, two BG 39s, a BG 40, and a BG 30 (purchased between late 2015 and early 2016)
- Seven Klemm Drilling Rigs from ECA
Why Bauer Technology Was Selected
Deep began analyzing the drilling rig manufacturing market in the early 2000s and made a calculated decision to acquire Bauer drilling rigs starting in 2003. The decision was driven as much by the relationship with the manufacturer and distributor as by the equipment capabilities. ECA and Bauer representatives brought real-world drilling experience to the table. Prior to adopting Bauer technology, Deep relied on conventional crane-mounted drilling equipment combined with vibratory hammers to drive and extract casing in urban environments, a method that proved more disruptive to residents and motorists. Bauer technology allowed contractors to install and remove sectional casings using powerful hydraulic rotaries, significantly reducing noise and vibration impacts on the surrounding community.
Ground Conditions and Drilling Challenges at Extraction Shaft 3
Soil Profile and Geological Conditions
Midtown Toronto is notorious for its troublesome ground conditions, and Extraction Shaft 3 (ES-3), located at the intersection of Yonge and Eglinton Streets, presented a particularly challenging soil profile. The ground conditions encountered during drilling were as follows:
| Depth Below Surface | Soil Type | Engineering Considerations |
|---|---|---|
| 0 to 10 meters (0 to 33 ft) | Granular and silty sands | Moderate stability, manageable with standard drilling techniques |
| 10 to 16 meters (33 to 52.5 ft) | Water table with flowing silt | High fluidity required careful slurry management to prevent cave-ins |
| 16 to 26 meters (52.5 to 85.3 ft) | Saturated flowing silt | Constant polymer slurry circulation needed for hole stability |
| 26 to 32.5 meters (85.3 to 106.6 ft) | Dense glacial till | High torque requirements, specialized drilling buckets and tooling |
The contractor overcame this variety of soil conditions through careful drilling techniques and an assortment of drilling buckets and tooling. As Conor Foy, Deep’s supervisor on the Crosstown project, explained, the silty sand became very fluid when wet, requiring proper procedures and consistent drilling techniques to prevent cave-ins. Deep relied primarily on Bauer drilling buckets for the material, with augers used as backup and bailers employed to remove excess water.
The Shaft within a Shaft Engineering Solution
Deep made a strategic decision during the tender process to engage Ontario-based Isherwood Geostructural Engineers as the shoring engineer. This partnership proved invaluable. Working collaboratively, the team engineered a creative solution whereby the shaft could be deepened just enough to allow the Bauer BG 11 H to descend below street level and install a secant wall from within the excavation. After establishing the dimensions of the rig and surveying the lowest utility supports, the team determined that lowering the grade to 15.5 meters (50 feet) and pouring a temporary slab would allow the compact rig to complete the secant wall.
ES-3 was built in two sections, north and south, each presenting unique challenges. The conflicting utilities in the northern half were relocatable, which allowed the team to build a secant pile wall by drilling from the surface with one of its five Bauer BG 40 Drilling Rigs. Fiberglass pile sections were used on the east wall to prepare for breakthrough by the TBMs. Once drilling was completed, a temporary bridge deck of timber mats supported by steel beams and bracing kept traffic flowing on Eglinton Avenue, one of Toronto’s busiest thoroughfares.
Confined Space Drilling and Shaft Completion
Operating Below Unmovable Utilities
The southern half of ES-3 presented the project’s most complex engineering challenges because the utilities running across the shaft could not be relocated. Deep’s approach involved multiple coordinated steps:
- Surveying, locating, and protecting utilities with steel sheeting
- Building a lattice of steel support beams to support utility ducts, conduits, and mains running across the shaft
- Excavating beneath the utilities while maintaining structural support
- Installing an internal waler and lagging system, designed by Isherwood Geostructural Engineers, to continue shoring below the utilities
- Pouring a slab for the BG 11 H to operate from within the shaft
Once the supporting structure was complete, a compact air track drill drove soil nails. The crew worked within an intensely confined space to install the nails and spray shotcrete in a panel sequence, keeping the ground stable as excavation continued. Alternating panels allowed vertical excavation of up to 1.2 meters (4 feet) of material every week. External dewatering was required for the final two levels because they were below the water table.
Deploying the Bauer BG 11 H in the Shaft
One of the most striking operational moments came when the Bauer BG 11 H was lowered into the shaft by crane, dangling at the end of a cable as it slowly descended below street level. At 35,000 kilograms (35 tons), the rig could be safely lowered into the excavation. The height from the drilling slab to the underside of the utilities was only 9.8 meters (32 feet). As Foy recalled, there was barely room to breathe inside the shaft, with the mast of the BG 11 H coming within inches of the lowest utility support. Extensive preparation work and clear procedural documentation were essential.
The BG 11 H was selected because it was the only rig that could fit under the decking and utilities while still drilling to the required depth. Although it was dropped 15 meters (49.2 feet) below ground surface, it still had to drill holes from inside the shaft down another 17.8 meters (58.4 feet). The rig delivered 110 kNm (81,130 foot-pounds) of torque at depths up to 40 meters (131.2 feet). The compact rig excelled at drilling in the confined space below unmovable utilities due to its lower header configuration and the ability to remove the upper section for low headroom applications. ECA Canada also supplied a shorter Kelly bar specifically for this low headroom operation.
Polymer Slurry and Secant Pile Construction
Because the BG 11 H was too compact to use casing for drilling the remaining 17.8-meter-deep piles, Deep chose polymer drilling fluid to keep the holes open. A liner was inserted to keep the top of the excavation open, and the fluid was pumped for the remainder of the shaft. These steps, combined with dewatering, created enough head pressure to prevent the flowing silt from caving in.
The slurry was mixed and agitated in tanks at street level and regularly tested for pH and viscosity before the constant flow required during drilling was initiated. Perfection was mandatory, as any failure in the slurry system would jeopardize the secant wall integrity. Once the drill bit reached founding elevation, a drilling bucket scooped out remaining earth chunks while the slurry kept the hole open and clean.
The pile size forced further innovation. At 18 meters (59 feet) in length and 8,400 kilograms (18,400 pounds), the piles were too long to hoist in one piece and too heavy for the BG 11 H to lift. Deep designed a hoisting device above each pile location that could lower three 6-meter sections to be bolted together before descending to their founding elevations. Concrete was placed under polymer slurry using the tremie method, with a flexible hose connected to a sectional tremie pipe fed by a pump positioned above the shaft. Each pour was scrutinized for volume and rate to guarantee uniform, high-quality piles.
Completion and Breakthrough
Deep’s crews worked 24 hours a day, six days a week to complete the 29 secant piles under a rigorous schedule. By this time, the northbound TBM had already broken through, and the southbound TBM was crawling quickly toward ES-3 with a 50-person crew. Deep had to vacate the shaft before breakthrough. On March 13, 2017, the lower drilling reached a successful conclusion. Fiberglass crumbled and water spewed as the cutting head of the second TBM, Don, broke through the wall of ES-3.
The successful completion of this complex shaft demonstrated how deep foundation drilling expertise, specialized equipment such as Driven Pile Foundations Types Driving Equipment Capacity Testing, and innovative engineering solutions can overcome the extreme constraints of urban infrastructure projects. For a more detailed look at how these machines operate underground, see the Detailed Analysis of Tunnel Boring Machine Working of the tunnel construction process and the Tunnel Boring Machine Working of the Tunnel Construction giant.
The close working relationship between Deep and the general contractor Aecon and ACS Dragados Joint Venture, with support from ECA and a fleet of Bauer Drilling Rigs, brought the City of Toronto 3.5 kilometers (2.2 miles) closer to increased mobility through the largest transit expansion in the city’s history.