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Skin Friction in Drilled Piers

Skin Friction in Drilled Piers

Skin Friction in Drilled Piers

I am trying to design a drilled pier foundation (caisson) for a monopole.
The problem I'm having deals with the vertical load transfer.  The soil boring report identifies a large fill layer of poor soil from grade to approximately 60 ft. below grade.  For this layer they identify an allowable skin friction value of 200psf.  In this case the foundation weight per foot is larger than the allowable skin friction resistance per foot.  The boring report says that this fill layer shall not be used for end bearing (skin friction only), and that any foundation greater than 30' in depth shall be drilled to bedrock.  Since bedrock in this location is approximately 80' below grade, this isn't feasible.  Another engineer told me that the skin friction only needs to support the difference between the concrete weight and the soil weight.  In this case that would be 150pcf - 115 pcf.  I didn't think this was correct.  Can anyone offer a solution?  

RE: Skin Friction in Drilled Piers

what exactly is the foundation for and what kind of loads are you dealing with? also, how old and how "bad" is the 60' soil layer?

is going to bedrock not feasible due purely to the cost?

RE: Skin Friction in Drilled Piers

The foundation is for a 100' tall ultility monopole.  Basically it's just a 100' tall pole supporting utility lines.  Large overturning moments (3500 k-ft), and small vertical load reaction (30 kips) other than the weight of the foundation itself.  The 60' soil layer is all fill & is not to be used for end bearing.  Only the skin friction is to be used to support the vertical loads.  Drilling to rock which is 85' down seems very excessive & costly.    

RE: Skin Friction in Drilled Piers

My main question is whether or not you need to account for the total weight of the concrete (150 pcf) when you're using skin friction, or if you can take out the weight of the soil that you're removing to put the pier in.  So if the soil is 115 pcf, you would only need the skin friction to support 35 pcf of concrete.  I didn't think this was the case, but I'm being told by another engineer that this is correct.  

RE: Skin Friction in Drilled Piers

The questions that 'maucog' posted is important.  When we talk about fill, the quality and uniformity becomes a concern.  The components in a fill usually consist of unpleasant pockets/zones/thin layers of 'too hard' or 'too soft' materials.  Resting your piers over these, the settlement may not be predictable and the 10 kPa adhesion should not be account for (unless it is the minimum value).  However, if this is the case and you are reluctant to drill deeper, can you design this foundation to allow some settlements?

For calculation point of view, the ultimate resistance (unfactored) should be the resistace derved from soil strength subtract the pier weight.  The soil weight is not added back since, usually, a set of different values(factors) are used for displacement piers (vs. driven piles).

Hope this helps.

RE: Skin Friction in Drilled Piers

Thanks for the info.

Settlement isn't a big concern as long as we're only talking a couple of inches.

So you're saying that the entire weight of the pier needs to be accounted for in the skin friction calc.?  

RE: Skin Friction in Drilled Piers

1. Generally the weight of the cassion is not significant. The soil will suport in bearing, some amount over the weight of the soil reomed for the cassion. This is the net allowable bearing. The balance of the load, plus a margin of safety, must be carried by skin friction.
2. Due to variability of non engineered fills, it generally not desirable to terminate in them, as unpredictable settlements may occur.
3. If you do not want to go deeper you may need to go wider. However, I doubt bearing will control this foundation, It is probably controlled by bending. You need to check with the designer to be sure he is okay with this as simply going deeper will not solve the problem. we are installing some cassions now in an old fill that was not aticipated in design. We are in frequent communication through out the day with structurals and geotechs on the design team.
really need to run this through the designer as this is not a simple answer.

RE: Skin Friction in Drilled Piers

What happens if your monopole settles 2 inches, 12 inches or 3 feet?  If it doesn't matter to you, then design the pier for lateral load and don't worry about the vertical load.  If however, you can't or don't want to design for large (and unpredictable) vertical loads, then take the pier to rock.

RE: Skin Friction in Drilled Piers

mmm - you don't indicate the nature of the "poor" fill - is it soft clay?  organic? very loose sand?  To think out of the box - why not improve the ground in the vicinity of the pole. (e.g., like compaction piles to improve zones of liquefiable sands) The choice would be a matter of what type of fill you have.  By making a large ground improvement, you might get the "friction" you need and also more lateral support.  If in very loose sands, you could try vibrofloatation - then drive your monotube pile down the centre of the improved zone.  Costs are important, but if you can improve the upper zone sufficiently to "shallow" depths, you might get what you need for similar funds.

RE: Skin Friction in Drilled Piers

Skin friction is not the parameter to govern your design, rather the Horizontal subgrade modulus.  Vertical beams, vertical members with small axial loads and large moments, are typically designed with drilled piers for the efficiency in resisting the moments, the axial loads are usually incidental and are less than bearing from either skin friction or end bearing or both.

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