Capacity of Jetted Timber Piles 1

[jgailla]

I was called onto a project with no geotechnical engineer or exploration. The project is a one-story wood frame restaurant built on a pond in up to 15 feet of water. The contractor jetted 40 foot timber piles and then got nervous and called for an exploration, which was completed last week, and wants an estimate of pile capacity for piles already placed. I can't find anything on the capacity of a jetted pile. Any way to proceed would be helpful in determining the capacity of piles which have already been placed. The only thing I can think of is hammer striking and PDA, but there is a soft clay layer about 2 to 5 feet below the pile tip and the hammer striking might keep going until the pile is under water. There might be no real capacity at all.

 

[rchanke]

If you already have 40 piles in,,, is it not possible to use adjacent piles as bearing reaction to apply a tensile load test on a pile as per ASTM D 3689-90? If this is logistically possible, it should yield good data.

 

[boo1]

Dont jetted piles react differntly?

 

[jgailla]

boo1,
Jetting a pile significantly reduces (I saw estimates reducing capacity by 50-85 percent) skin friction. A pile capacity has two elements; end bearing-the pile tip resting on the soils below, and skin friction-the soils "squeezing" the side of the pile. In cohesionless soil (sand) the skin friction increases as the relative density of the soil increases. Jetting essentially brings the density of the soil to zero and the soil gradually settles back into the void. How much does it settle back? I don't know, but I would assume it would be at a much reduced relative density. Jetting would be okay if the pile was primarily end bearing, but even then the jetting process must stop before the tip elevation and the pile must be driven the last 5 feet or so. The piles on the job I'm working on were jetted all the way in. In addition, just beneath the pile tip elevation there is a thin (5 foot) layer of very loose sand underlain by a 20 foot thick layer of soft clay. Hope this answered your question.

Any criticism from others regarding my explanation would be appreciated.

 

[GeoPaveTraffic]

I agree with rchanke. If you want to know the capacity, setup a load test and test a pile or two. I would not be surprised if the piles have very little capacity.

 

[Ron]

jgailla...as you mentioned, using PDA and re-striking should give you the most relevant data. Since these are in water, I would not do a conventional load test as the setup is quite difficult.

You're right that the capacity is significantly reduced....probably zilch! In any case with the profile you mentioned, you could be essentially point-loading the clay layer so settlements could be quite large without the skin friction resistance.

 

[BigH]

I'd be a tad nervous if you have soft clay about 2 to 5 ft below the pile tip - if the piles are acting as a group. First, do a borehole to find out what you really have - because you didn't even mention what he was jetting through! If it is clay there should be some take-up with adhesion - with sand, you may have just left yourself with a loose condition - but even loose has some capacity because it is sand (this is a likely not scenario!).

The one good thing, though, of timber piles is that they are somewhat tapered. You do get an affect of bearing due to the tapering (See Nordlund's method of determining pile capacity). If pile load test is NOT possible, then I'd restrike with a small drop hammer (say 2 tons or so) and get an estimate of the pile's set. Then (and I know, heaven forbid) use Gates formula to estimate the capacity (applying the corrections as per Flaate's paper).

 

[jgailla]

BigH,

I've been hoping I'd get your two cents. Above the clay is (was) firm sand. They want 6 tons, and my calcs indicate 6 tons is too high for these piles in these soil conditions even when driven, not jetted. Ron's probably right about the load test over water, the contractor won't buy it, especially because we're telling him he doesn't have theoretical capacity. Now we're trying to get the contractor and owner to agree to the cost of PDA, as my conservative calcs say to drive the piles into the clay a distance of about 20 more feet. With PDA they can have an answer about how much farther to drive immediately. I'm concerned about the PDA's use in clays, although clays should have more strength than a PDA would say. Right?