Screw Piles in Okotoks, AB

Below the surface in Okotoks

Okotoks is split down the middle by the Sheep River valley, and that divide gives the town two quite separate foundation questions. On the terraces flanking the river — the original downtown, the flats and the lots that back onto the valley — you are building on alluvium: layered silt, sand and gravel that changes over a few metres, sometimes with perched water sitting above the regional water table. Climb onto the uplands, where most of the recent subdivisions are, and it is glacial till over clay again.

Terrace gravel is a mixed blessing. Dense, clean gravel bears load very well, and torque rises fast as a pile goes into it. That same gravel holds cobbles, though, and a cobble can stop a helix above design depth without any clue from the surface. For that reason no screw pile in Okotoks is specified by depth — it is specified by torque, and a pile that stops early is moved, upsized or written up, never simply accepted wherever it quit.

West of town on Highway 7, the Big Rock erratic shows what the glaciers carried through. The ice that left a quartzite boulder that size on the open prairie also laid down the till, with smaller boulders scattered through it. Walking the property will not reveal which lots are hiding them.

The projects we see locally

The projects in Okotoks split the same way the valley does. Inside town they are decks off walkout basements on the sloping lots near the river, detached garages, and additions to 1970s and 1980s homes in the older core, where the original footings have often shifted already. Piles handle sloped ground well, because each one is cut to suit the grade instead of stepping a concrete footing down the hill.

Past the town limits in Foothills County, the work becomes acreage construction — post-frame shops, storage for equipment, shelters and arenas — along with underpinning older homes on the river flats where a strip footing in alluvium has sunk unevenly. Helical underpinning carries that load down past the soil that moved and into competent strata.