Toronto-based Deep Foundations Contractors has received prestigious awards since its founding in 1971, from the local Toronto Construction Association to the Canadian Construction Association and the International Association of Foundation Drilling. The company can now add another achievement to its long list of accolades.
Deep recently drilled one of the deepest large-diameter caissons ever installed in Ontario at Leaside Station (formerly Bayview Station) for Crosslinx (Aecon, Dragados, EllisDon and SNC). This achievement was just one element in Deep’s $20-million shoring and decking scope at this station stop on the new Eglinton LRT.
The Eglinton LRT, a light rail transit line being constructed from Kennedy Station to Mount Dennis (Weston Road) in Toronto, is the largest transit infrastructure project currently in progress in North America. As the foundation contractor for the majority of the underground stations, the Deep team continually faces geotechnical and logistical challenges on the project that require some of the most sophisticated techniques ever employed in Canada.
According to Deep project manager Matt Byford, “The Crosstown Light Rail Project is part of Toronto’s ‘Big Move,’ stretching 19 km and including 25 stations – 15 of which are underground. We are responsible for the design and installation of shoring and decking at eight of those underground stations, including Leaside. This is the largest portion undertaken by any shoring subcontractor.”
Two years and counting
The Deep team began work on Leaside Station in December 2016. The scope of work at Leaside includes a 9,200-square-metre secant shoring system; 2,000 square metres of traffic decking spanning Eglinton (approximately 20 metres); seven 1,320-mm-diameter, 50-metre king piles, eight 1,000-mm-diameter, 50-metre crane base caissons to support two tower cranes and temporary utility support under the deck. To date, four of the seven king piles are complete, in addition to 65 per cent of the station’s total drilling and approximately 35 per cent of the decking.
“This is all to facilitate the station’s construction,” said Byford. “The main station box will be excavated to a depth of approximately 22 metres. The entrance ways are to be excavated approximately 15 metres. The tunnels have already been bored in this area by TBM, which means much of our drilling is in close proximity to the completed tunnels.”
Equipment access between the main station box and entrance shafts was critical. It was this need that drove Deep’s design and implementation plan towards king piles.
“In order to carry the heavy traffic load of two major roadways, the project’s ‘cut and cover’ design required tight spacing of the steel piles for the hybrid shoring/decking support system,” he said. “A move to fewer structural members (king piles) meant a greater load on each element, but the increased spacing opens the station box to larger equipment, allowing for better access and faster construction.”
The king piles will serve two purposes: they will be incorporated in the excavation support system and they will support traffic from Eglinton and Bayview avenues over the station excavation.
Challenges
As part of this design/build, Deep worked closely with the station’s shoring and decking engineer, Terraprobe, to identify key challenges, which included:
King pile locations meant drilling would be in close proximity – less than 300 mm – of the previously bored LRT tunnel, 15 metres below the surface.
Geotechnical conditions included complex overburden with intermittent layers of dense tills and loose saturated sands offering varied capacity and installation characteristics between king pile locations and founding depths.
All work was to take place adjacent to and within the congested intersection of two large urban roadways.
The result was three potential solutions: 50-metre deep king piles that could reliably transfer vertical load using skin friction, 30-metre deep belled king piles that had the potential to work in end bearing – where localized geology, including increased clay content would allow – or supplementing a pile of intermediate depth with micro-piles.
In each scenario the caissons would be drilled and cased through complex overburden, with intermittent layers of dense tills and loose saturated sands. The sectional casing itself was to pass within 300 mm of the existing LRT tunnel at a depth of approximately 15 metres. As well, 1,320 mm sectional casing was used to accommodate spacing, pile sizing and allow drilling through soil layers that were not self-supporting.
Using a Bauer BG-40 drill rig with a casing oscillator, the 1,320 mm king piles were installed to depths up to of 50 metres.
In addition to the elements sizing needs, all options used a modified drilling procedure to address the proximity to the tunnel. Included in this procedure was fitting the drill with a diagnostic system to provide extremely accurate digital readings of casing verticality.
“This ensured location and plumbness of the pile drilling operation could be verified in real-time,” said Byford. “A communication plan was established so that tunnel activity could be halted at critical points during the drilling. The crew used monitoring data to verify the plumbness of the drilled shaft a few metres before passing the springline of the tunnel.”
Depending on the results observed, three actions could be taken: drilling could proceed, additional readings could be taken or the hole could be backfilled and redrilled at a later date. In practice, Deep’s high-quality performance meant that no holes were required to be abandoned for this reason.
“The congested, urban site also presented additional challenges,” he said. “The team’s project planning and safe work processes allowed safe execution of the work.”
This included potential contingencies should the king piles need to be installed over multiple days, as well as detailed coordination with all workers on site to allow the smooth installation of steel pipes.
When all criteria had been accounted for, the choice was 915 mm HSS pipe piles, selected for their resistance to buckling in all directions, combined with a flexible procedure that permitted drilling to any of the three depth possibilities. This was made possible by a design decision to install the HSS section perched within the concrete pile, with nelson studs transferring the load into the concrete. The piles installed were in excess of 12,000 kgs and 27 metres in length.
At present, all three options have been used. Where the in-situ soil evaluation required it, Deep relied on its high-capacity hydraulic drill fleet to advance casing to significant depths, not accessible to most. Elsewhere conditions permitted use of shorter, end-bearing installations.
“These were faster to install making them a higher value product,” said Byford. “Our experience, expertise and resources allowed for on-the-fly evaluations of ground conditions and enabled quick decisions. Deep Foundations was able to successfully deliver an optimized product for our client on a challenging site with variable geotechnical conditions.” 