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Drilled Piers in Cohesive Soil - Brom's Method

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!

RE: Drilled Piers in Cohesive Soil - Brom's Method

penpe,

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

I don't know GeoPave, I've used Brom's method successfully for a number of projects in the past.

----
The name is a long story -- just call me Lo.

RE: Drilled Piers in Cohesive Soil - Brom's Method

(OP)
Nice to hear that Brom's method has achieved success for some. It seems the results are vastly different from what GeoPave, (and apparently LPile), would find: According to Brom's, even at 500 psf undrained shear strength, embedment depth of 11 feet and pier diameter of 2 feet load capacity is adequate for the axial load of 19520 lb., (base resistance and side resistance contributing almost equally).

Thanks to both of you for your opinions.

RE: Drilled Piers in Cohesive Soil - Brom's Method

Generally you can't account for both side friction and base support as the amount of movement required to activate each type of resistance is drastically different. In order to activate the end-bearing capacity, you essentially need to completely overcome the side friction (rendering the side friction useless). I've always been told, it's one or the other, not both.

Now in reality, you would get contribution from both but it's tough to quantify.

RE: Drilled Piers in Cohesive Soil - Brom's Method

(OP)
The Federal Highway Administration's Drilled Shafts Manual FHWA-NHI-10-016 references research on drilled shafts by O'Neill and Reese, and on driven piles by Vesic; where the resistance provided by side friction and base (tip) resistance are combined. However, I also just discovered that "it is unusual to locate the base of a drilled shaft in cohesive soil with Su (undrained shear strength) less than 2000 psf when compression loads are supported." That could mean that drilled piers are just not appropriate for the loads at the site.

RE: Drilled Piers in Cohesive Soil - Brom's Method

I've done a lot of foundations, never founded anything on very soft clay. Just don't do it.

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

I'll add my voice to jayrod's and GeoPave's: Combining side friction and end bearing is not usually worthwhile, even when it can be done. It requires very specific information on the geotechnical properties and careful construction practices that ensure the bottom of the hole is clean (no loose material), which in our experience rarely happens.

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.

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