Helical pile failure - boathouse in cold climate

[Prairie Boy]

Hello:

We have a boathouse built in NW ontario that was installed on 22 helical piles, most of them 4.5" pipe w/24" helicals. The piles are topped with a steel I-beam frame.

The structure is light wood-framed, 24'x28', standing seam metal roof and fibre cement siding, so very low loads. The water depth is about 5' on the lake side and 1 foot or less (depending on water levels) on the shore.

The structure has experienced major shifting since it was built in 2013 - with shoreline piles appearing to heave (up to 9"), and water side piles appearing to sink several inches.

Unfortunately the pile company took very poor records so we only have a very general, anecdotal idea of pile depths.

Their attempts to stabilize the structure includes adding 6 more piles on the lake side, including 3 piles down to "resistance", which we assume is Canadian Shield rock, at a depth of 70'!

Wondering if anyone here has an idea of why a foundation that should theoretically not be failing be experiencing so much movement. I suspect the shore piles are heaving, as some may have only been driven to 8-10' and locals have indicated that frost in our sandy silty soils, especially where snow cover is poor, can be as deep as 8-10'. However - the dropping on the lake side is a real head scratcher.

Thanks in advance!

 

[bones206]

It does sound like frost jacking. I see some companies providing jackets or sleeves to help isolate the pile shaft and prevent this type of failure.

With the apparent settlement on the lake side, perhaps every pile settled the same amount but the shoreside ones also heaved and negated the settlement. The lakeside settlement would be protected from frost jacking and remain with a net downward settlement.