Deep foundations drills keep tunnel boring machines crawling
By Brian M. Fraley
To say that Deep Foundations Contractors (deep) was drilling in a confined space within an extraction shaft on the Eglinton Crosstown Light Rail Transit Project (Crosstown) would be an understatement.
It was the first of many challenges the firm would overcome while working above and below the buzzing streets of Midtown Toronto.
The centre of the action was a 32.5-metre-deep extraction shaft where two massive Tunnel Boring Machines (TBM) would be lifted from the ground at the end of a long subterranean journey.
While deep has experience working in Toronto, drilling in an urban environment isn’t simple.
The company needed to conquer challenges including inconsistent soil conditions, low clearance drilling, unmovable utilities, maintaining traffic and dark confined working spaces to provide underground access for two TBMs.
Canada’s largest transit expansion
The Crosstown is an $8.4 billion infrastructure project with a 19-km corridor, 10 km of which are underground. Phase 2 of the project included 3.25 km of 6.5 metre diameter twin tunnels, precast concrete tunnel lining segments, launch and retrieval shafts, headwalls associated with future stations, two emergency exit buildings, three cross passages, utility relocations, traffic staging, ground and building settlement monitoring and site restoration.
With previous experience on large transportation projects, deep was awarded a $21 million contract to excavate and shore launch the extraction shafts for two TBMs; build two emergency exits for the TBMs; and construct head walls for two future stations. The job began in April 2014 and concluded in March.
Deep’s work would allow the two TBMs, known as Don and Humber, to continue boring without interruption. The costs associated with stopping their progress would be incredibly prohibitive.
The Caterpillar TBMs — named after two Toronto rivers — began tunneling on Sept. 30, 2015. The pair would crawl westbound, digging 3.5 km from the launch shaft at Eglinton and Leslie streets before reaching their destination. The TBMs would eventually be hoisted out of Extraction Shaft 3 (ES-3), at Yonge and Eglinton streets, Midtown Toronto’s busiest intersection. Deep’s end goal was to prepare the shaft for their arrival.
Canada’s largest fleet of Bauer drilling rigs
The ace deep held in its pocket was Canada’s largest fleet of BAUER Drilling Rigs, all of which were acquired from Equipment Corporation of America (ECA) and includes the BG 55 – the most powerful rotary drilling rig on the market at the time with 553 kNm of torque.
The company also owns a BG 11, one of the smallest drilling rigs. As well, ECA sold deep seven Klemm Drilling Rigs.
According to ECA Canada’s branch manager Ray Kemppainen, deep geared up by purchasing five more rigs, including a BG 46, two BG 39s, a BG 40, and a BG 30.
Kemppainen recalls the 1970s when the firm was buying foundation equipment from his former employer, Special Construction Machines, known today as ECA Canada. Deep is now ECA’s largest Canadian client.
Deep previously used conventional crane-mounted drilling equipment, combined with vibratory hammers, to drive and extract casing in urban environments, which is more disruptive to residents and motorists. Bauer technology allowed contractors to install and remove sectional casings using the hydraulic rotaries of the Bauer drilling rigs.
ECA was instrumental in matching equipment to the complexities of the Crosstown, according to Conor Foy, deep’s supervisor on the Crosstown.
“There was a lot of back and forth,” he said. “We had to provide a lot of information and work with them to get the right equipment.”
Midtown Toronto is notorious for troublesome ground conditions. This reality sunk in as deep began drilling ES-3. The first 10 metres included granular and silty sands.
Then, deep hit the water table and encountered a flowing silt, which went down 16 metres. A layer of dense glacial till stretched from 26 to 32.5 metres below the surface. The contractor overcame the variety of soil conditions with careful drilling techniques and an assortment of drilling buckets and tooling.
“The problem was when the silty sand got wet,” Foy said. “It was very fluid so you had to follow proper procedures and techniques to make sure the drilling was consistent and you didn’t have any cave-ins.”
Deep relied primarily on Bauer drilling buckets to conquer the material. Augers were used as a back-up and bailers helped to remove water.
ES-3 was built in two sections: north and south. Each presented its own unique set of challenges.
The conflicting utilities in the northern half were relocatable, which allowed deep to build a secant pile wall by drilling from the surface with one of its five Bauer BG 40 drilling rigs.
Fibreglass pile sections were used on the east wall to prepare for breakthrough by the TBMs.
Deep kept traffic flowing on Eglinton Avenue by building a temporary bridge deck over the northern half once drilling was completed. It consisted of timber mats supported by steel beams and bracing, and was locked in place with steel angle.
Once utilities were surveyed, located and protected with steel sheeting, deep relocated its Bauer BG 40 to commence drilling in the southern half. It was important to compare pile and utility locations to avoid any conflicts.
Supporting utilities was deep’s top priority when excavating the southern half of ES-3. The crew built a lattice of steel support beams to support the utility ducts, conduits and mains running across the shaft before beginning work on the deck.
Deep installed an internal waler and lagging design to continue shoring below the utilities so the slab could be poured for the BG 11 H. The system, designed by Isherwood Geostructural Engineers, allowed lowered excavation to accommodate a compact air track drill to drive soil nails. The crew worked within a confined space to install the nails and spray shotcrete in a panel sequence to keep the ground stable as excavation continued.
Alternating panels allowed deep to vertically excavate up to 1.2 metres of material every week.
Most secant piles were accomplished from street-level, but three windows remained in the southern half where that was not an option. Closing them during the remainder of the excavation was the greatest challenge of the ES-3 shaft construction.
The client dewatered ES-3 to lower the water table by one metre so the working platform could be poured at the correct elevation. The equipment ran until the secant wall was finished to keep water at bay.
One of the most interesting spectacles for passersby was the sight of a BAUER BG 11 H dangling from the end of a crane line as it descended below street level. Drilling within the ES-3 was also one of the more complex engineering challenges.
“You barely had room to breathe, especially inside the shaft,” Foy said of the working conditions within ES-3, adding the mast of the BG 11 H was within inches of the lowest utility support. “You had to put in the preparation work and make sure everyone knew what they were doing and had clear procedures for what was happening.”
The BG 11 comes standard with the ability to remove the upper section for low headroom applications.
“The BG 11 H was the only rig that could fit under the decking and utilities and still be able to drill the required depth,” Foy said. “This machine was dropped in 15 metres below ground surface, but it still had to drill holes from inside the shaft down another 17.8 metres.”
The size of the piles forced deep to improvise. At 18 metres in length and 8,400 kg, they 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 each of the three six-metre sections to be bolted together before descending to their founding elevations. The hoist could lift three six-metre sections together at a weight of 8,400 kg.
The concrete was placed under polymer slurry using the tremie method. The crew poured each pile with a flexible hose connected to a sectional tremie pipe fed by a pump that was positioned above the shaft. The pipe was then lowered to the bottom of each hole. As the concrete was placed, the slurry in each hole was displaced. Deep scrutinized the volume and rate of concrete placement to guarantee the piles were uniform and high-quality.
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. The Southbound TBM and a 50-person crew were crawling quickly toward ES-3. Deep had to be out of the shaft before breakthrough.
On March 13, the lower drilling came to a successful conclusion.