The Lower Churchill Project is intended to develop the hydroelectric potential of the Churchill River. Phase I, referred to as Muskrat Falls, includes more than 800 miles of high voltage transmission line through the remote, rugged terrain of Newfoundland and Labrador, Canada.
Original foundation designs included grillage and driven piles. Variable bedrock depths and boggy ground conditions created foundation installation challenges at several structure locations. Soft surficial soils included organic peat bogs, and loose, saturated glacially derived sands and silts, which generally lack bearing support for tower foundations. Grillage foundations were unfeasible in these locations as they rely on a dense soil stratum below the footing bottom. Driven and auger piles were also problematic at numerous sites, as underlying bedrock materials consisted of strong to very strong granite, gneiss, limestone/dolomite, sandstone, and shales, prohibiting pile advancement.
QS Scope & Solution
The contractor engaged Quanta Subsurface (QS) to provide design-build foundation alternatives for sites predicted to experience installation challenges. QS designed micropile and drilled pipe pile options, as well as one-of designs for locations where subsurface conditions required a combination of foundation types. These included driven pile with rock anchors and driven pile with micropiles.
Micropile designs were developed for both self-supporting towers and guyed structures. They incorporated consideration of bog to depths of up to 25 feet, as well as variable bedrock quality. The approach provided a flexible solution that could be adapted onsite should conditions deviate from those noted in the pre-construction characterization borings, or where pre-construction borings could not be attained.
A drilled pipe pile is a single foundation element, combining properties of both drilled shafts and micropiles. Designs were developed for support of guyed structures where bedrock was anticipated to exist at relatively shallow depths, but where grillage support was deemed unfeasible.
Foundation designs were tailored to the installing contractor’s specific equipment, tooling and capabilities, allowing for cost and schedule efficiencies. In total, micropiles were installed at 161 structure locations and drilled pipe piles were installed at 131.