effect of fill on existing pile capacity

[tw]

How does one calculate the effect of 3 feet of additional fill on piles that are already driven. The test pile tested out a 80 tons, but after that the project requirements dictated raising the building for flood reasons.

The piles are 50 feet long are driven thru clay & into sand, primarily skin friction. They are in groups of 2, 3 & 4.

I need info for preliminary look, will use geotech for final answer.

tw

 

[tw]

In my question above, pile length is actually 100 feet.

 

[dicksewerrat]

All you have done is surcharge the piles. subtract the weight of an inverted pyramid of soil for each pile group from the original capacity. The slope of the sides should be about equal to the angle of repose for the soil. Use the area of the pile group cap as a starting point. Truncate the pyramid at this area.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[MattRyan]

The downdrag will be relatively small, less than 1 kip. I'd ignore it. Assuming you are using the ENR formula for pile bearing capacity, you can rely on the fact that this pile has a big factor of safety.

 

[rochplayer]

Where are the clay layers? How thick are they? How compressible are they? How much settlement relative to the piles do you expect? 1/4 inch or more of soil settlement relative to pile is thought to be enough to mobilize negative skin friction on a pile. Your net ultimate pile capacity can be considered as the Ultimate positive shaft resistance plus the mobilized toe resistance, minus the negative shaft resistance.

How did you test the test pile? PDA? Static load test? Old, antiquated, unreliable, why-are-people-still-using-it ENR formula correlation?

For your preliminary look, assume the resistance from pile within and above the compressible layers is acting downward, and that the resistance below the compressible soil is activing upwards. Your net ultimate pile capacity is the difference between the two.