Skin Friction Concrete Pile (5' Deep)

[JohnnnyBoy]

I have two similar projects in regards to the depth of foundation and its skin friction resistance, one of them are 12'-0" 12" diameter piles for a storage building and the other is a 5'-0" 12" diameter concrete pile for a deck. For the storage building, I have a soils report and I have to exclude the first 5'-0" of soil leaving very little capacity for the storage building. Now excluding the first 5'-0" is a general practice that I've used many times for many different types of buildings but for a small storage building I have a hard time rationalizing needing to go almost 18'-0". Now on the other project where the load is minimal and a 5'-0" pile, using the same principle the pile has a capacity of 0 which is obviously wrong. At what point can we start rationalizing using part of the 5'-0" soil at the top. Thanks in advance for any insight.

 

[jgailla]

I would design a 5' deep pile as a column foundation bearing 5 feet below the ground surface, not as a pile.

 

[dik]

Here we typically neglect the top 10'... the savings in concrete and drilling for longer piles, unless you are using a hand auger, is little.

Dik

Dik

 

[WARose]

Almost every geotech report I've ever read told me to neglect the top 3-6 feet (at minimum) for skin resistance. You don't get a lot of capacity at those shallow depths anyway.

 

[JohnnnyBoy]

I should possibly have explained the situation a little bit better. For the storage building we are doing an As-Built design of the shop and at the existing 12'-0" the piles are inadequate therefore I was hoping to not make the owner repair the building by adding extra piles at this point. I agree with Dik the if this was a design for a building there would be little in cost savings.

In regards to jgailla how would I ensure that at the bottom of the hole, most likely hand augered, is flat so that I can use the full bearing area? If the end is augured it will most likely be cone like would it not?

 

[canwesteng]

The projected bearing area should be the same whether it's perfectly flat or not imo.

 

[dhengr]

JohnnnyBoy:
The important thing on any footing or end bearing pier is that it rest on undisturbed soil, so that it doesn’t have to compact and settle an inch or so before it really starts bearing. As others have said, I would not consider skin friction on these short piers, but it might be enough to hold the pier up a bit during early construction; in such a way that it would not settle down until more fully loaded, and you do not want that. The soil at the footing bearing elev. or the bottom of the augered hole should be hand dug to get to undisturbed soil, it should not be 6" of loose soil clods (backhoe teeth or auger teeth, etc.), which will compact when really loaded. We always had a gen. note on footings, etc. that they should bear on undisturbed soil and that all organic material should be removed.

 

[jgailla]

JohhnnnyBoy,
You are on the wrong track with this. A hand augered hole can't be more than 6 inches in diameter.
What kind of support are you expecting with a 6 inch diameter pile 5 feet deep?

 

[dik]

I've seen gas powered 2 man augers with 12" 'drills'.

Dik

 

[JAE]

A 5 ft. deep pier with no skin friction still has some end bearing so never is it zero capacity.
There is also some skin friction but we would negate that due to perhaps a high variability and uncertainty.

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[JohnnnyBoy]

Thanks all for the answers. The load is very minimal of a 4'x4' deck section less than 1500 lbs factored load. I will consider it as a bearing area of the pile and it will meet my needed capacity. dehengr yes I also have that general not for native undisturbed soil and no organic material and point of bearing.