Foundation Design - Back to Fundamentals - Layered Clay Soils

[Prosser]

I am currently studying for an MSc and I am attempting to determine bearing capacity and settlement in a layered clay soil with weak clay overlying stiff clay.

I aim to design the foundation by fixing the length at 12m (assumption due to bridge deck = 12m) and determining width using Das equation. This is proving unsuccessful and I believe it is because Das equation is not appropriate for a weak clay overlying a stiff clay.

I also attempted using EC7 combination 1 but my values for B were not appropriate and I have been informed I cannot use this method for layered soils which is proving correct. I thought this would be appropriate because by balancing design resistance/acting forces = ODF the width could be calculated.

Does anyone have an appropriate method for calculating bearing capacity and settlement in soils with a weak clay overlying a stiff clay? Particularly a method which allows the engineer to determine a value for foundation width appropriate?

During my research I have only found method to solve situations with stiff clay over soft clay, sand over clay and stiff sand over weak sand.

Any help would be appreciated and apologies in advance if the details are unclear.

 

[oldestguy]

From what I know, we don't comment on or assist with homework.

 

[oldestguy]

For what it is worth, here in Wisconsin the DOT has a standing rule that all bridge foundations go on piling. Even at areas for overheads, no stream, one never knows what sort of trench will be dug alongside them in the future. Perhaps the professor needs some enlightenment.

 

[BigH]

Prosser - it is true that we do not help with homework on this site. But this is not a new topic and has been handled by Button (1953) among others. If you pick up a copy of Fang's Foundation Engineering Handbook, there is a nice section on how to handle this; and there are many other texts out there such as Tomlinson.

A problem I see today - is that few students and younger engineers have any knowledge of the "history" of geotechnical engineering - their idea of Bjerrum is his vane correction factor not his huge legacy to geotechnical engineering development. Similar to Bishop, Skempton, Leonards, Lambe, etc. If they can't get it on the web immediately, it never existed . . .

 

[jayrod12]

In terms of practicality does it not make more sense to either extend your foundation down to the stiff layer below. Or assume all of the soil is the weakest one. I would think that should simplify your calculations and provide more confidence that it will perform at least as well as designed.

 

[BigH]

Depends jayrod12 on the thickness of the strata, size of footing, etc., risk of the structure. You surely do not want to dig down 6 m or so for a footing . . . if that is necessary piles would be more practical.

 

[Prosser]

Thanks for the feedback guys,

I wasn't aware that studies were not discussed on here but to clarify, I am a Highways Engineer studying to progress my knowledge of topics that effect my day to day role, I am not a full time student looking for a shortcut.

In this particular case, the soft clay does not exceed, 3.5m and therefore I am aware that in reality, it may be good practice and sensible to use piles or situate the footing in the stiff clay below and I will comment as such in my report.

However, I will provide calculations to find the bearing capacity and settlement for this case and I appreciate your study material recommendations (BigH).

 

[jayrod12]

I agree that it depends on the thickness of the strata. But if the weak layer were say 5-6m thick would the stiff layer contribute significantly to the final bearing capacity or settlement calculations significantly? My bet is no, not appreciably at least. Hence my second recommendation.

Obviously every situation is different. It's been a while since I've done any geotech stuff but for some reason my brain is screaming at me that Peck did some stuff on weak overlaying stiff.

 

[BigH]

First - if the soft soil only goes down 3.5 m, you might find that it best if you remove it subject to the underlying stratum being able to support your structure - comparison of pile cap thickness, etc. to the 3.5 m. I sure wouldn't want to leave in a metre of so. If you have to go piles and well into the underlying stratum then leaving the softer soil in placed except for removal for the pile cap should be okay (depends on lateral loads and, as well, seismic).

For past studies - Button (1957 or so) is the first that I am aware. Fang discusses Reddy and Srinivasan (1967) and Chen (1975). Meyerhof and Hanna (1978) discusses sand overlying weak clay. Tomlinson points out the need to check for the soft clay to squeeze out rather than show typical shear failure (see Tomlinson, Foundation Design and Construction, 6th Edition).