Bearing Capacity in Granural Soil

http://www.eng-tips.com/viewthread.cfm?qid=286567&page=1

New PostSMetcalfe (Geotechnical)
25 Nov 10 3:39
Hello, I am having a bit of trouble with a basic bearing capacity calculation;
Looking at a 14m dia. foundation founded at 1mbgl on loose granular deposits (N-value of 6, phi of 29degrees).
Based on the Brinch Hansen calculation, ignoring shape and depth factors and using a factor of safety of 3 I am obtaining a bearing capacity of around 1000kN/m2. This seems very excessive and when I compare this to other literature sources such as Tomlinson (7th edition) Fig 2.4, Pg 57, I should be looking at a value of around 50-150kN/m2.
Parameters I am using are a unit weight of 19kN/m3, bearing capacity factors; Nq - 16 and Ny - 18.

Can anyone correct me on where I am going wrong?
My shortened calc (Brinch Hansen) ignoring shape / depth factors is;

qn = (Po x Nq + 0.5 x yB x Ny) / 3
qn = ( ((19 x 1) x 18) x + 0.5 x ((19 x 12) x 18)) / 3
qn - bearing capacity
Po - Overburden pressure (Depth x unit weight)
Nq - factor (16)
Ny - factor (18)
yB - (Unit weight of soil x Breadth of structure)

New PostBigH (Geotechnical)
25 Nov 10 7:23
You are basically correct - you have a very large footing which gives a huge "gamma" term. Just think what it would show if the footing was a more normal width - say 5 m. Day's Fdn Engr HB (published by McGraw Hill and sponsored by ASCE shows a Ngamma value of about 15. This would push your value down a bit - so, in any event, you are showing an allowable bearing capacity of about 800 kPa (fudging a bit without calculating for the lower Ngamma (how did you get the greek symbols?). Anyway, as far as you go, you are correct.

However, this shows the point that shear seldom governs the permissible (allowable) bearing pressure. Settlement does. If one uses 25 mm of settlement for your size footing you would estimate an allowable bearing pressure of about 50 kPa (Terzaghi and Peck) - or perhaps as much as 75 kPa if one uses Bowles suggestion of adding 50%. For 50 mm of settlement (typical for mat foundation) one would say that the allowable bearing pressure (settlement based) would be in the order of 100 to 150 kPa - a far cry from the bearing pressure based on shear. One can draw curves of various allowable settlements vs footing width. See the attached chart. This is for a situation where the "N" value (corresponding Ngamma) is in the order of: 0 to 6m: 25 (35); 6 to 18m: 42 (95); 18 to 23m: 18 (23) and >23m: 49 (135). But it shows how to do it.

It has been my opinion that far too much effort is made on bearing capacity - and you see this even in the LRFD approach - you spend a lot of time analyzing the bearing capacity under many load variations - and in the end, it is the serviceability that will govern the permissible (allowable) bearing pressure.

For the chart that has been attached and its background, you can go to the following link and click on NCS Policy Memo 1 (they will then ask name etc so you can access their site (free).
http://www.ncsconsultants.com/projects/lrfd_policy_memoranda.php

One should also read Fellenius' papers
http://ww.fellenius.net/papers/219%20Delusion%20of%20Bearing%20Capacity.pdf
http://widener.parameterid.com/docs/courses/senior/F08_SP09/401files/FootingsinSand.pdf

* http://files.engineering.com/getfile.aspx?folder=f222ac63-e8ec-414a-bb03-51

New PostSMetcalfe (Geotechnical)
25 Nov 10 8:05
Thanks BigH that is really helpful. I used the tables within Tomlinson to obtain the greek values.

So which method would you recommend?

I don't really understand your second paragraph regarding settlement governing bearing capacity can you give me some more information on this or point me to a paper?

New PostRon (Structural)
25 Nov 10 9:56
Check your overburden pressure and unit weight...doesn't seem to fit with N=6 and soil description.


New PostBigH (Geotechnical)
25 Nov 10 16:30
I agree Ron that for N=6, a unit weight of 19 might be a bit high - but even if you drop it to 17 or even 16, it is not going to change the "answer" too much - for the big point is the very large (14m) foundation size. It is still the settlement that will govern the load than will be permitted on the foundation; it shows that large mats can carry, in shear, very large loads . . .

@SMetcalf - read the Fellenius paper. Why does settlement govern? Because if you exceed certain settlements (both total and differential) the structure you have might undergo distress (cracking, tilting, etc) that would make the structure unserviceable. Just think - if you have a limit on settlement of 40 mm and it takes only 75 kPa to achieve that - why would an an allowable bearing capacity of 300 kPa be relevant.