Bearing Capacity of Reinforced Soil over Clays 1

[geot88]

Hi all,
I need to calculate the bearing capacity of a spread footing.
B= 2 ft.
L= 19 ft.
Df= 1 ft.
The underlying soil is mainly stiff clay.

The bearing capacity, using Terzaghi's theory, is kinda low. I can recommend using a geosynthetic as reinforcement, after replacing 0.7 ft. with granular soil.
What theory should I use for this calculation?
Das (1988), Patra et al (2005) or Omar (1993). The first one uses Geotextile, while the second and third use Geogrid.

I want to make sure I do the right thing. Do you have any recommendations to make: Geotextiles or geogrids? Or any other methods?

Thanks

 

[BigH]

What is your loading per linear ft? How much settlement to you expect to occur? Is the allowable bearing pressure (settlement driven) lower than your computed allowable bearing capacity (shear driven)? What is your undrained shear strength? I don't think I've ever seen or heard of geo-reinforcement in a stiff clay; although I am sure that someone has tried it.

 

[geot88]

Thanks BigH for your reply. I'm new in the site and I really like the support I've seen. [I'm sorry for my English]
The only info I have is: contact pressure (qb) is 109 000 psf.
The computed allowable bearing capacity (qu) was 67 000 psf.
I expect 1 inch of settlement.
- Let me correct the lenght: L= 5 ft.

 

[oldestguy]

This problem sounds fishy. Footing pressure seems very high and the undercut of 0.7 feet is impractical. Consider widening footings and any undercut should be on the order of several feet to be useful. Reinforcing thin layer also seems impractical. Is this some sort of class homework? If so we don't do homework here.

 

[geot88]

OK, oldestguy. Thanks for taking some time for writing, even though you were rude and I didn't receive any help whatsoever.

For all the others who are willing to help:
I've been called just to calculate the bearing capacity of some 'pipe supports' on a Poliduct here in South America. These pipe supports are steel I-beams over a concrete foundation that will act as a spread footing.
All they gave me was the support dimensions -which are several different combinations- width(0.55, 0.80, 1.05, 1.45 and 2.25 meters) and length (1.45, 2.00, 4.55, 5.95 m.). Df was also different: (0.30, 0.80, 0.9, 1.20 m.)
The requirement was calculate the allowable bearing capacity of each combination and check if they were higher than footing pressure. These people don't know too much about engineering. Their job is only construction. When I asked for the load or contact pressure, they said it was 20 T/m2, which is crazzyyyy. I told them to really check that value and they said that it will only support small pipes with oil inside. Then they gave me different footing pressures for each combination of pipe supports, from 2.4 to 6.4 T/m2. However, I still think it's too high for the kind of structure.
One of them did not pass on Bearing capacity, so I suggested increasing the dimensions of the footing, so the footing pressure will decrease. The requested to keep the dimensions and to improve the soil conditions. I've always preferred to modify the footing, however, I know I can increase bearing capacity reinforcing the soil with geosynthetics (Das, Omar, Patra, Meyerhof, Hanna, etc). I just wanted to make sure, or double check, with you guys.
Soils in the area are colluvial deposits, superficial deposits from the Quaternary geologic age. 80% of soil found is very dense gravel and sand, and 20% is mainly stiff or very stiff clay.

 

[BigH]

Fjlsu - you will get nowhere on this site if you exhibit the attitude of your first sentence to oldestguy. Let me tell you a little bit - oldestguy is our oldest guru - he began in geotechnical engineering before your parents were probably born learning from some of the greatest founders of the field that you call yourself a member of. He was involved at the very first with the nuclear density gauge - when it was little more than holding the isotope down the hole by a thread . . . he has forgotten more than you or I will ever know and I give him my utmost respect. And . . . we do get students trying to get free homework advice and this is like Warren Buffet collecting welfare - frowned on immensely.

Now to your problem, it sounds like they want you to develop a chart for any and all combinations that they might encounter - I think that a footing of some 0.55 m wide is too narrow from a practical point of view (what excavator/backhoe will easily dig a 0.55 m wide footing - I prefer in the order of 0.7 m minimum for a strip footing - just what I am comfortable with (and don't forget someone needs to get down and clean out the debris left by the excavator. 20tonnes/m2 is about 200 kPa. For a stiff clay (if stiff, undrained shear is in the order of 50 to 100 kPa). If 50 kPa, the allowable (SF=3) bearing capacity is about 50*6/3 = 100 kPa. At maximum allowable BC is about 200 kPa. 200/100 to 200/200 = footing width of 1 to 2 m, settlement notwithstanding.

Now with the second set of pressures you were given, say 5 t/m2 or 50 kPa, it makes sense that a footing about 0.5 m side would be okay (50/100 - 0.5). Again, I would use a minimum width of 0.7 m so you have some additional safety factor for only a slight marginal cost increase and an easier space for debris cleanup..

It would make much more sense as Oldestguy stated to increase your footing width rather than dig out the soil if you think it is needed to increase the bearing, add reinforcement, compact fill, etc. But, as I noted above, I personally do not see any reason that a stiff clayey soil cannot handle an applied bearing pressure of 50 kPa.

I would suggest that you just dig out to the depth, clean up the base and place the concrete - no need for formwork (see Tomlinson on using excavations for side forms). Makes no sense to do anything else.

Stiff clays are good foundations for relatively small footings and I see no special need to increase the footing width etc. but this is a call you will have to make. We can only suggest possible ways to attack the problem; we are not doing the design for you.

 

[geot88]

First of all, I'd like to apologize to oldestguy. I've been so stressed out with this, since I have to send this report by today and probably badly understood your message. Thanks for your help.

Second, Thanks BigH. You are right, this clay is stiff enough. I was afraid of placing it directly onto the clay layer. Thanks for width observation too. I know what I can do know.

Now that I got the point, how do I close this thread?

 

[jayrod12]

If you want the thread removed you can red flag it. otherwise it will close on it's own after a certain length of time.

 

[oldestguy]

Here the old guy is once more. At age 86, I sometimes am a little short, so bear with me please. As some times happens, the questions are not always fully explained, so multiple postings result in getting the straight scoop for then providing a useful answer.

In providing a client a design chart or similar information, one has to be careful to be sure they fully know its limitations. The conservative approach then may be to include a significant factor of safety, in case of such misuse. Good luck and come back often.

 

[oldestguy]

Back again. Under he circumstances you find herself in with this project, I can only suggest one thing. Remember Murphy's law (you can find a lot of info on Captain Murphy by doing a
Google search). Anyhow, once a plan or design is out there and you have no control of how it is used or mis used, Thus, the simpler you can make the recommendation, the less likely will they goof it up. Well meaning or ignorant workmen are amazing as to what they may do or not do and get away with it. In construction, I think this law applies to almost every job.

 

[mudman54]

in spite of the goofed up units, its a pretty good question for the forum.
it does not seem that we have resolved how the presence of the geotextile influences the bearing capacity.
i would be tempted to somehow increase the cohesion of the clay, but would like to hear from others what they have done.