Pile Capacity Quick Help!!!

[jgeng]

I am not a geotech...I need a ball park figure for the capacity of an 8" tip timber pile driven below water into fine sand 6ft. The first 2 ft was reported to be dense fine sand with N=40. After that it is medium dense fine sand with silt with N=20.

What would be a resonable bearing capacity using end bearing alone and then using the skin friction too?

I was given very high numbers that I don't think are right. Are there any rules of thumb for end bearing and skin friction?

 

[csd72]

allowable end bearing pressures of a pile are generally much higher than that of a footing due to the constraint of the surrounding soil.

 

[BigH]

I haven't done the calcs, but I would believe you'd be good for at least 25 tons (US). I drove timber piles into a 10 ft sand lens - about half way in and we used 25 tons on the load; no problems. As the material below was a very soft clay we didn't put in pile groups - but placed the piles along grade beams at wide spacings so we had no overlap - had done a pile load test on this to confirm. If you do the calcs use the Nordlund approach for the skin friction - due to the taper of the timber piles you will get some "bearing" along the shaft as well.

 

[jgeng]

These piles were likely jetted in, what effect would that have on the capacity?

 

[jgailla]

Jetting reduces the skin friction significantly. I have in the past ignored the skin friction component entirely and used the end bearing only.
You may not be able to design for more than 8 or 10 tons out of a pile that is jetted.

 

[oldestguy]

Your first posts says "driven", then jetted. You should have some way of getting some load into them before use if possible. Even a back-hoe with full bucket has some pretty good hammering effect.

A common way to install docks is by jetting. Can you check out any similar sites where jetting was used and see how the job looks? That jetting will destroy a lot of that density that is now present. It even will mess up the end bearing. 10 tons is plenty then. What's going on top? Can that settle a little?