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Crews handle variety of tough conditions on remote power line project

By Kevin Sharp, Don Henry and John Wilson

Drilling 67-foot shafts and installing large-diameter caissons into solid rock can be a challenge for even the most skilled drilling contractor. Doing so in an extremely remote location nearly 400 kilometres (250 miles) from the nearest town as winter weather quickly encroaches adds a new level of complexity. Steep cliffs, demanding soil conditions and limited equipment repair services meant the project team was forced to adjust the project’s trajectory on the fly. Without cell coverage, crews communicated with each other and the concrete batch plant via two-way radios.

Crews from Sharp’s Construction Services (Leduc, Alta.) and Henry Drilling (Langley, B.C.) joined forces to drill the foundations for transmission towers for the new 344-kilometre (213 miles) Northwest Transmission Line project in B.C. The work was performed under the general contractors’ joint venture of Valard Construction (Edmonton, Alta.) and Burns & McDonnell (Kansas City, Kan.) for the owner BC Hydro.

The monopole and self-supporting towers, each averaging 27 metres (88 feet) in height, are along a route that traverses steep hillsides, crosses rocky plains, dips into marshlands and meanders through a creek. The project is the longest transmission line in BC Hydro’s new capital management plan.

Well-equipped
The drilled shafts ranged from 167 to 304 centimetres (66 inches to 120 inches) in diameter and were spaced approximately 300 metres (984 feet) apart across a total distance of 40 kilometres (24 miles). A slew of equipment – from lattice-boom crawler cranes, hydraulic crawler cranes, drills, excavators, vibratory hammers and loaders – was deployed to tackle the job. Sharp’s and Henry Drilling crews used three Soilmec drilling rigs – the SR 80, the SR 65 and the SR 50 – to drill through a variety of soil conditions. Soilmec designs and manufactures a broad range of equipment to provide high performance for even the most technologically complex and challenging projects.

The SR 80 was deployed predominantly on large-diameter shafts between 250 and 300 centimetres (98 and 118 inches) through rock and hard soil, while the more nimble SR 65 focused on smaller diameter holes and rock holes that were perched on ledges and blasted shelves. The SR 50 was used to drill smaller shafts – most between 120 and 182 centimetres (48 and 72 inches) in diameter – through softer soil. The SR 50 enabled the team to deliver an average of one shaft per day, while the larger SR 65 and SR 80 were battling difficult drilling conditions through solid rock for upwards of a week per location.

Rock stars
With minimal geotechnical information available, crews had to assess drilling conditions on the fly. Over the vast 400-kilometre distance, the team naturally encountered soil conditions ranging from peat and soft silt overburden to gravels and heavy boulders overlying competent rock and fractured rock up to 15,000 psi in strength. Gravels were very loose in places, and to provide permanent casing, contractors had to seat into the highly sloped underlying rock to place rock sockets into the material. On the larger, 300-centimetre (118-inch) shafts, crews often had to pre-drill with 76-centimetre (30-inch) cored holes around the perimeter, then follow up with 300-centimetre (118-inch) core barrels. The team used every tool at its disposal, including vibratory hammers and cranes, to seat the casings. In some situations, the casing was modified and rotated into place with the drills. Luckily, the team had the three Soilmec drills onsite – each with varying capabilities – that allowed the crews to address any condition they encountered.

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Piling Canada is the premier national voice for the Canadian deep foundation construction industry. Each issue is dedicated to providing readers with current and informative editorial, including project updates, company profiles, technological advancements, safety news, environmental information, HR advice, pertinent legal issues and more.