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 exca-vated
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 sup-port
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 incor-porated
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 sta-tion’s
shoring and decking engineer, Terraprobe, to identify key
challenges, which included:
King pile locations meant drilling would be in close proxim-ity
– 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 con-gested
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 drill-ing
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 read-ings
of casing verticality.
“This ensured location and plumbness of the pile drilling
operation could be verified in real-time,” said Byford. “A com-munication
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 chal-lenges,”
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 buck-ling
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 condi-tions
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.”
GREAT CANADIAN PROJECTS
The congested, urban site presented additional challenges,
but Deep Foundations Contractors’ project planning and safe
work processes allowed for safe execution of the work
62 Q1 2018 www.pilingcanada.ca
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