sinking piles

[rittz]

A contractor built a garage attached to the house. He poured 12" piles 8 ft deep under an 8" x 24" grade beam for a foundation. The grade beam was lightly dowelled to the existing foundation. There was a 2 ft deep trench around the perimeter (excavation not yet backfilled) the trench filled up with water during a heavy rain, softened the clay soil around the piles (naturally lost the skin friction) and the piles subsided 2 or 3 inches. Obviously 8 ft friction piles are inadequate for starters. The garage wall and the roof pulled away 1 or 2 inches from the house. I have thought of a couple ways to underpin the grade beam. Anyone gone through anything similar? A void form is required under the grade beam to avoid grade beam heaving in highly plastic clay.

 

[SteveGregory]

Could you use Chance helical screw piles to underpin the grade beam?

 

[msquared48]

What does your geotech suggest? Hopefully you have one.

This sounds like a problem for him to solve considering the problem with the embediment depth of the pile and the implied seasonal loss of skin friction.

I would certainly look at 2 to 3 inch diameter steel pin pile in that situation though.

My other question is what about the slab? Is it poured? Is it structural? If not on both cases, is it likely to fail if a similar storm happens again?

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[BAretired]

The problem with helical screw piles is that you cannot install them under the grade beam. They must be offset, so you need a bracket on the side of the grade beam.

For offset piles, you might consider drilled, cast-in-place piles with a haunch under the grade beam.

There is one contractor in my area who jacks steel piles into the ground under the foundation. He uses short lengths of pipe which he welds together in situ. This is usually quite expensive compared to other types of pile.

The answer to your question depends partially on what type of expertise the foundation contractors have in your location.

You mention a void form under the grade beam to avoid heaving in highly plastic clay. We have plastic clay too, but we also have the potential for frost heave and frost jacking of shallow piles. The existing piles at eight feet in length are too shallow and will be subject to frost jacking if the building is in an area with significant frost penetration.

BA

 

[SteveGregory]

Chance and Ramjack are two companies that supply helical screw piles. They both supply a bracket that mounts on the side of an existing footing or grade beam.

 

[vandede427]

helicals have L brackets the seat on the bottom exterior corner of the existing foundation, not to the side. helicals are also very good because the support the structure vertically and also resist uplift from expansive clay.

I frequently use helical pile to remedy a foundation issue. It's my preferred choice and they're are relatively cheap and easy.