Drilled Piers in Cohesive Soil - Brom's Method
Drilled Piers in Cohesive Soil - Brom's Method
(OP)
I'm constructing an Excel spreadsheet based on equations found in Federal Highway Administration's FHWA-NHI-10-016 for drilled shafts. I'm designing a drilled pier for the following loads: 15365 ft-lb Moment, 860 lb base shear, 19520 lb gravity (axial). The soil is very soft clay, so assuming 800 psf undrained shear strength. I'm using Brom's method for cohesive soil: theory and equations start on page 12-43. This set of equations doesn't consider an axial load, so I applied the alpha method for axial load in Chapter 13: incorporating both side resistance in clay, and base resistance. I'm getting what I find to be surprising results compared to a more conservative method I've tried that's similar to retaining wall design with triangular passive resistance.
I find this design to be adequate for the loads using their method: 2 ft dia., L=6 ft. (L being embedded depth). The other method says it needs to be 2.5 feet diameter and 10 feet deep. This being my first time doing drilled piers, I'd like to hear from someone with more experience. Thanks!
I find this design to be adequate for the loads using their method: 2 ft dia., L=6 ft. (L being embedded depth). The other method says it needs to be 2.5 feet diameter and 10 feet deep. This being my first time doing drilled piers, I'd like to hear from someone with more experience. Thanks!






RE: Drilled Piers in Cohesive Soil - Brom's Method
You have several problems.
First, an undrained shear strength of 800 psf isn't a very soft clay, more of a medium stiff clay by most descriptions. A very soft clay would have an undrained shear strength of less than 260 psf.
Second, even if your undrained shear strength is 800 psf, you need a drilled shaft that is about 4 feet in diameter just to handle the vertical load. I say about 4 feet because the allowable loading depends on the embedment depth.
Third, I would suggest using LPile for lateral analysis instead of other methods. A much better analysis method.
Best of luck.
Mike Lambert
RE: Drilled Piers in Cohesive Soil - Brom's Method
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The name is a long story -- just call me Lo.
RE: Drilled Piers in Cohesive Soil - Brom's Method
Thanks to both of you for your opinions.
RE: Drilled Piers in Cohesive Soil - Brom's Method
Now in reality, you would get contribution from both but it's tough to quantify.
RE: Drilled Piers in Cohesive Soil - Brom's Method
RE: Drilled Piers in Cohesive Soil - Brom's Method
As for combining shaft and end bearing resistance. You can do it, but as jayrod12 pointed out; they require different movements and that is really hard to quantify. Again, I only do that in very specific situations where I have a very good understanding of the soils, allowable movements, and the loads. Might be ok for transient loads, but would rarely use for static loads.
Mike Lambert
RE: Drilled Piers in Cohesive Soil - Brom's Method
For bridge foundations, a couple people in our design section went to the effort to calculate the combined capacity of end bearing and side friction using method from O'Neill and Reese - once. What we could gain wasn't worth the time to gather the required information and work through the calculations. We're typically designing for 12 to 20 shafts. I suppose if the design would affect 50 or so shafts, it might be worth being able to add the typical 5% of the end bearing capacity to the side friction.