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Foundations on C-phi soils

Foundations on C-phi soils

Foundations on C-phi soils


I have a case in which it is planned to build a bridge on an existing embankment. The embankment soils are clayey sands (SC). According to some lab test, this material has cohesion value = 500 psf and phi = 30. Fine content of this material is 35%.

After talking about typical bearing capacity computations with another engineer, he mentioned than other than using the typical bearing capacities (meyerhoff, terzagui, etc), he would also consider the bearing capacity for a clays, such as, 5.14xC / FS.

By comparing both, the BC formula for clays results in way much lower Bearing capacity values. Using this formula, recommendations would be deep foundations, otherwise, using the typical BC formula, recommendations would be shallow footings.

My question is, do you agree with such approach and If so, after how much fine contents in the sandy or gravelly soils, would you consider in order to use the BC formula for clays?

RE: Foundations on C-phi soils

I'm not sure I'd consider undrained analysis for a coarse-grained soil above the water table. I would likely use the drained friction angle and I would likely ignore drained cohesion. Too much risk of losing cohesion over time owing to wetting/drying, freezing/thawing and etcetera.

Once you figure out all of that, then the real work begins. How to limit the settlement. In what manner do you assign deformation criteria to the SC? Do you just guess modulus? Do you ignore the subject as if it'll be moot?

To a great extent, I rarely see bridges designed on spread footings bearing on soil (I work for a highway department). Our performance criterion is less than 1 inch of settlement during the 20 years after delivery to the public.


ípapß gordo ainÆt no madre flaca!

RE: Foundations on C-phi soils

thanks fattdad for your reponse,

In this case the water table is very deep, 25 feet below ground surface.

Just for curiosity Would you do the same approach if embankment fill is silty sand (SM) instead of clayey sand (SC)?. Silt content = 35%.

Yes, I know the nightmare for selecting the modulus would be the next step. I could be able to open a new thread for the E modulus selection, but just wanted to focus on how would different engineers approach this.


RE: Foundations on C-phi soils

Who owns the bridge? Any stream crossing? Will this area be built up in future with possible trenching for utilities? If any question, it is a piled foundation in my view. How about moisture changes in the future and shrinkage-expansion?

RE: Foundations on C-phi soils


It is owned by a mining company. Even though Water table is about 25 ft below the top of the embankment, the intent is to have water (stream) under the bridge. The top of that stream would be about 15-20 feet under the bridge.

In understand that most of the bridges nowadays are built on deep foundations due to scour or due capacity issues, but, would you consider undrained analysis for this SC material (35% fines) in that situation?, fattdad seemed to do so.

RE: Foundations on C-phi soils

(reread my post. I did not think undrained analyses important.)


ípapß gordo ainÆt no madre flaca!

RE: Foundations on C-phi soils

Typically I’d just use the drained conditions to run the analysis. Anything extra provided by cohesion just increases the FOS but as fattdad mentioned, it may not always be there. If push comes to shove and I need a little bit more, I would consider using a heavily factored c, probably around 100 psf in this material, assuming that the material is uniform throughout the zone of influence. But if this is for a bridge, I would just stick to using 30 degrees and 0 cohesion.

RE: Foundations on C-phi soils

What I find interesting is that the "other" engineer said to use 5.14*Su/FS - yet, isn't this foundation on an embankment. Shouldn't you consider, in either case, a footing near a slope?

RE: Foundations on C-phi soils

For the same reasons f-d mentioned, I would also use drained conditions based on real lab data from the project.

Also, are there any existing bridges/buildings close to your site? Are there on deep or shallow foundations? Are the foundations performing ok?

RE: Foundations on C-phi soils

Thanks for your responses,

Bigh, yes, the footing would be near a slope. After running few slope stability analyses, it was concluded to push the footing in, as it was becoming critical its original position near the edge of the slope. Either way, since the other engineer preferred to use the undrained analyses as the critical one, he was pushing to use deep foundations.

Okiryu, no, this is the first bridge around this mining site. We did some research if there were similar structures around the area.

thanks again.

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