can you combine pile for axial and spread footing for overturning? 1

[delagina]

I have a 3ft diameter, 20 foot vessel, weight is about 20 kips.
The soil report I have is old and has a pile capacity of 48 kips for 75' pile.
Basically 1 pile is enough for axial but the overturning confuses me.

I don't want to way over design this foundation. If I use piles, I will have to use 3 minimum piles for overturning per standard calculation.

This is in very soft soil. Soil bearing is 700psf per Geotech and it also has a settlement graph.
I can make this work with spread footing but settlement concerns me so I want to use piles.
But 3-75' piles looks like an over kill.

 

[Teguci]

I have designed a cantilevered retaining wall with a single line of piers. Rotation into the slope was restricted by passive pressure from the retained soil (the soil under the footing was assumed to settle away). In your case, I don't think you have the right situation to make a single or 2-pile system work. Any horizontal bearing should be assumed to settle away. Once it settles, you won't have anything to resist rotation around the 1 or 2 piles.

 

[structSU10]

you could involve a geotech to design a shorter pile maybe? May only be a day or so of effort if the data you already have is decent to work from. It may even be able to be a performance item (just state you need a pile capable of supporting X compression, X uplift and X shear) designed by the pile contractor.

 

[JStephen]

It seems to me that 3/4 of your cost is going to be moving a pile-driving rig into position, so once there, a couple of extra piles wouldn't amount to much?

 

[BigH]

As JStephen said, piling can be very expensive - due to mobilization of the rig. In concept - Is there anyway you can preload the soil - put on 5 to 10 ft of soil - say extending 15 ft each side of your intended location. Let it settle (monitor the settlement). When it "levels" off, remove the fill and build a spread footing. Your geotechnical engineer should be able to take the concept and refine it (i.e., how much settlement is needed to offset the expected settlement of your foundation, etc) so that you can build your spread foundation without worries of settlement. Any piping should to the vessel should be flexible - not rigid. Of course this takes time - if you have it. if you don'g have it, you can have a soils rig come in an install a series of vertical sand drains, then put on the surcharge - this will reduce the time for settlement.