Field Issue: Straight Shaft Pier Foundation Installation

[oengineer]

The contractor was in the process of drilling a straight shaft drilled pier when weather systems caused the drilling to cease. The drill depth achieved was roughly 29ft in which bearing strata was not encountered. The pier is supposed to have a total length of 40 ft. The pier hole was filled back in with dirt.

I am thinking that we could just have the contractor re-drill a slightly larger hole to ensure adequate and competent surrounding earth and also deepen it a bit to also ensure adequate capacity.

Any suggestions on additional resolutions on how to proceed with this pier penetration are welcomed.

 

[jayrod12]

Personally I don't even know if larger diameter would really be necessary, but it's probably not a bad idea. You mentioned bearing strata was not encountered. Is the pier/pile end bearing or friction?

 

[oengineer]

It is both. The bearing strata is 30 ft below the finish floor. The foundation design is based on an allowable end bearing value of 25,000 psf and 4,500 psf in side friction. Both end bearing and side friction come into play once the piers have meet a minimum penetration into the bearing strata.

 

[jayrod12]

So we're talking rock socketed Caissons. If you don't require the length drilled yet in side friction, then I don't think you necessarily need to go larger but again, not a bad idea.

I do not feel you need to go deeper than originally designed for. They need to reach the appropriate bearing strata, if they haven't yet they were going deeper anyway. I don't think you need to tell them to go deeper than that just because they backfilled the hole once.

 

[JAE]

Why did you start a new thread on this topic - you already had the same question posted here: thread507-450389



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[oengineer]

@JAE: This issue was a tab bit different since the bearing strata was not reached.