Skin Friction in Drilled Piers
Skin Friction in Drilled Piers
(OP)
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?
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
is going to bedrock not feasible due purely to the cost?
RE: Skin Friction in Drilled Piers
RE: Skin Friction in Drilled Piers
RE: Skin Friction in Drilled Piers
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
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
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
RE: Skin Friction in Drilled Piers
RE: Skin Friction in Drilled Piers