August 10, 2011

Segmental casings used more often

ANCHOR SHORINGS & CAISSONS LTD.

At the Union Station upgrade for Carillion Canada, Anchor Shoring, in a joint venture with Hayward Baker Inc., is installing nearly 900 micropiles using low-headroom equipment.

JESSICA KRIPPENDORF

correspondent

New technology is always on the horizon in shoring, a specialized industry with growing opportunities across Canada.

One of the keys to success in what has become a highly specialized industry is having the right equipment for the job at hand, said Dawn Tattle, president of Anchor Shoring & Caissons Ltd.

The company was the first in Ontario to import Bauer rigs from Europe.

The drill rigs twist 10 to 20-foot lengths of double wall casings into the ground and each segment is bolted onto the top of the next.

Segmental casings are becoming a more commonly used technique than vibrating liners into the ground, she said.

Typically, if the project is next to other buildings or below the water table, the company uses a caisson wall — a series of interlocking holes. If the ground is stable, the company uses a piling and lagging system, where piles are drilled and horizontal wood lagging is installed during excavation.

The demand for shoring on condominium and historical sites is on the rise in the Toronto area.

When maintaining historical buildings, there is more need for equipment that fits into a small area.

The company does a significant amount of work for expanding rail lines, which, along with many infrastructure projects, require noise and disruption to be minimized.

Excavations are also getting deeper and sites are getting smaller as people need more parking and land is at a premium, said Tattle.

“One of the complications is quality control and making sure the extra depth is accommodated for in the design,” she said. “The equipment has to be heavier and bigger.”

Anchor Shoring is currently installing 52,000 square feet of caisson wall around the perimeter of the X2 condominium site, along with about 222 tiebacks with a drilled length of 14,000 feet.

Tucker Highrise Construction, Inc. is the builder on the project and the shoring was designed by Isherwood Associates.

Drill depths for more than 300 interlocked vertical 1,000 mm diameter holes approach 90 feet, and drilling conditions are extremely challenging due to the depth of holes, the wet sand and gravel conditions and the proximity to nearby buildings and utilities.

About 7,000 m3 of caisson mix was required for the caisson wall.

Two Bauer BG-40 drill rigs were used, each providing 390 KNm of torque and the ability to drill holes up to three metres in diameter to a maximum drill depth of 80.5 metres.

“This equipment is able to install steel casings in tough ground conditions without the need to use a vibratory hammer,” said Tattle. “The casing can be advanced ahead of the drilling process.”

This is the method of choice in conditions, where it is necessary to drill adjacent to existing structures in areas of loose soil, or where vibratory methods can’t be used.

Sam Daaboul, project manager for Anchor Shoring, said the restricted access and traffic of the downtown location, the depth of the excavation and difficult soil conditions make it an exciting project.

The 22-metre excavation is next to existing streets, services and buildings and extends below the water table making drilling of the vertical holes and tiebacks challenging.

“The west end of the site will be braced by a waler strut system with the remaining sides braced by regroutable soil anchors,” he said. The walers and struts require 400,000 pounds of steel bracing.

The company is using much smaller equipment at the 90,000-square-foot Union Station upgrade for Carillion Canada.

In a joint venture with Hayward Baker Inc., Anchor Shoring is installing close to 900 9-1/2-inch diameter micropiles using special low-headroom equipment that can work in heights as small as 12 feet. Electric drill rigs maintain air quality in the congested areas with low ventilation.

The micropiles are drilled through fill overburden and into the rock. High strength steel bars are placed into the holes and grouted in place.

Due to the low headroom, the bars are placed in lengths as short as 10 feet and are connected in place to make the full length sections required using couplers.

“The total drill length for the job will be about 31,000 feet,” said Tattle. “Grout is mixed on site and we anticipate using over 16,000 bags of cement for the project.”

The micropiles are installed in four pile groupings around the existing columns.

Shoring posts are placed on each micropile and jacked, then the existing columns are removed.

A new footing and column are then installed.

Load testing was done in December 2010 with production drilling starting immediately after.

“We are also installing double corrosion protected (DCP) micropiles to support future walls of the structure,” said Tattle.

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