Bearing Capcity for Soft Clay over Stiff Clay 1

[indyhans]

I am looking for references or equations to compute bearing capacity for layered soils.

I have already looked up the following and looking for some more......

1. Myerhoff & Brown, 1960 Geotechnique
2. Myerhoff & Hanna, 1973 Geotechnique & Das's Book
3. Vessic 1973 (Winterkorn and Fang)
4. Army Corps Shallow Foundation Manual
5. Navfac Shallow Foundation Manual

Are there any other sources..........

 

[BigH]

Button (1953)(3rd ICSMFE, Zurich, Vol 1) was the classic paper followed by Reddy and Srinivasan (1967)(ASCE JSMFD Vol 93, SM2). You'll find this discussed in Fang's Foundation Engineering Handbook Fig 4.14. Can also see Meyerhoff and Brown (1967) discussion given in ASCE JSMFD, Vol 93, SM5). You can get some information on this from Bowles' 5th Ed. One thing to remember with soft clays overlying stiffer clays is that you might get a "sliding/squeezing out" problem occuring before traditional shear; or punching shear.

 

[jdonville]

indyhans,

There is a procedure in AASHTO Standard Specifications for Highway Bridges 2002/17th Edition. I don't have the LFRD edition, but I can only imagine that they would have it in there as well. I don't know where they got it from.

Hope this helps,

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

 

[indyhans]

Thanks BigH and JD...

I am calculating bearing capacity for 22 feet wide retaining wall footing embeded 3 feet over a soil layer that has a strength of about 500 psf (4 feet layer) over 1500 psf. Each one of the methods result in different answers. Meyerhoff and Hanna (Das' book) results in twice the value of (Vesic 1973; Winterkorn and Fang) and AASHTO LRFD's manual (ref; EPRI 1983- Electrical Power Research Institute).

So I called up Dr. Das, He believes EPRI 1983 which is in AASHTO is an improved on Vesic 1973; which is based on semi-emperical theoretical calculations. He believes Meyerhoff and Hanna because they came up with those equations by conducting lab tests in using a box.

I am confused on which equation is appropriate. My google research tells me there are onging studies conducted in several countries including India, Australia, etc. till 2005.

Is there a study out there that does a parametric study /review of each method and its applicability?


Sincerely
Indyhans

 

[BigH]

If you only have a 4 ft layer of soft soil under this 22ft wide retaining wall footing - and for a footing that wide, it must be pretty high - I think that you owe it to the client (and your own desire to sleep sound at night) to excavate out the 4 ft of soft soil to the stiff clay - then place your footing there. I wouldn't found a retaining wall footing in a situation like this on the soft clay. There is no justification in my mind, given the data to date, that you should leave in the soft clay. This is a situation where there would probably be a sliding condition governing over a simple bearing capacity (even two layer system) formulation. As a side note, it would have been helpful if you had indicated the geometry/stratigraphy at the beginning - while I would have still given you the references, I would have also given you the suggestions as per this note.

 

[VAD]

Indyhans:

I tend to agree with comments made by BIG H re the soft clay layer. Here are some of my comments.

It appears that you are building a wall that is vey high 50 ft at least based on the base width chosen or you are trying to invoke the lower stronger layers in your bearing capacity evaluation, or on the other hand you are concerned about settlement and have used a wider base. This has the effect of invoking characteristics of layers with depth, at least 1.5 times the footing width based on classical theory.

It may be that you are building an MSE wall, even so the base width suggests a tall wall.

Based on some quick calculations it would appear that if you do not want to remove the soft layer then your wall height for an FOS of 1.5 in BC would would have to be no greater than between 12 and 15 ft. and perhaps as low as 10 ft re BiGH's comments of the weak layer governing.

It is always of interest to see the variations one obtains using different equations in the literature and the uncertainty of which one to choose. That is the beauty about geotechnical engineering. Finally, you have to make the judgement call which ranges somewhere between optimistic and pesimistic values and invokes other concerns as well. Settlement issue could be a concern for both cantilever and MSE wall types. This has to be considered as well.

Removal of the weak layer is a solution if wall is to be much higher or other types of foundation improvement-short piles, stone columns, stone cement columns etc. The choice will depend on site factors/considerations.

 

[jakin]

I doubt that this would be an MSE wall support ... to be underlain by soft clay. My reaction is same as BigH, .... byepass the soft clay layer, what with frost depth, etc. Also, even with a 7ft foundation depth, your B/D is still reasonably low. Just wondering, has it occured to you maybe you need to consider a different type of foundation?

 

[fndn]

Have you considered two rows of drilled shafts and a cap supporting the retaining wall?

 

[BigH]

Actually jakin, we are working presently on a site where we have 6 to 7 m of soft clay (Su <25kPa) where we are putting MSE (RE Wall) on. Walls up to 12m high. We've done this by using wick drains and stage loading; have undergone up over 800mm total to date - walls remained straight. But, you might be right - even applied bearing of 150 to 175kPa (3 to 3.5ksf) might not be enough depending on the actual height of wall. For fndn's suggestion, we did this too - but it can be an overkill. Given the choice, I would put in one row of drilled shafts, a good grade beam and then put up solid and relatively thin wall with deadmen achors (like in wharfs) for taking the horizontal overturning forces. Lots less piling and more efficient than trying to take the bending moments at the cap/pile interface. Still, I would take out the soft clay under the retained soil so it doesn't squeeze out through the piles. Just some quick thoughts.