Preliminary bearing capacity
Preliminary bearing capacity
(OP)
Is there a way to estimate the preliminary bearing capacity from SPT blow count. I am looking at a soil profile where the bearing soil is Silty Sand with blow count 7. Below the silty sand layer there is an stiff sandstone/IGM material with blow counts in excessive of 50.
Are there any charts available, please let me know
Thanks
NT
Are there any charts available, please let me know
Thanks
NT





RE: Preliminary bearing capacity
RE: Preliminary bearing capacity
Check Bowles, too. Be careful in that you have asked for preliminary bearing CAPACITY. As has been discussed many times in the geotechnical forums for this site, CAPACITY is based on shear. You might have sufficient reserve of safety against shear but serviceability (i.e., settlements and differential settlements) might not be acceptable. Most of the charts that Ron has alluded to are set to deliver an allowable bearing PRESSURE (not CAPACITY) for a settlement of 25 mm (some Indian charts use 40 mm) as 25 mm has been the 'mainstay' of what constitutes generally acceptable settlement. Some structures will have lower values - say 10 mm; some more. As the footing gets larger, the allowable bearing pressure decreases (the depth of influence becomes greater).
You have not provided any data on the thickness of the silty sand - nor on the width of your footings, etc. If the sand is very thin over the sandstone, then you would likely have to consider something more than the generally accepted allowable bearing pressures.
As well, you can always obtain an approximate value of the deformation modulus, E, from the SPT values (again, see Bowles among other texts/manuals that have such correlations) and using this information and procedures as noted in Poulos and Davis' Elastic Solutions for Soil and Rock Mechanics, estimate settlements under your footing and footing loads. If you know what your service limit (settlement) is, you can estimate an allowable bearing pressure.
RE: Preliminary bearing capacity
Your answer was much more complete and informational!
RE: Preliminary bearing capacity
RE: Preliminary bearing capacity
RE: Preliminary bearing capacity
Also refer charts here: http://people.eng.unimelb.edu.au/stsy/geomechanics...
RE: Preliminary bearing capacity
RE: Preliminary bearing capacity
For fine grained soils, see attached. It is from a lecture at the Institute Engineers Malaysia given by Dr. Siaw Tien Ho (Bangkok). I've got others but this was sitting next to my desk. I typically like to make up my own correlations for areas in which I have worked - have one for the Calcutta flood plains. For a quick estimate of allowable bearing capacity (shear) with SF=3, I use 2xSu (2x undrained shear strength) - this puts you in the right ball park. Of course, you will have to check against settlement for the allowable bearing pressure. If you want to look more into this, Dr. Malcolm Bolton is pushing/has pushed the use of small strain theory - where you deal with allowable bearing pressures directly . . . If you wish, let me know and I'll try to find the various references on the subject.
RE: Preliminary bearing capacity
phi=[sqrt(20N)] + 15, then we use the classical Terzaghi's equation for bearing capacity, but for my next projects I will check the charts that you mentioned (I think that T&P book has that type of charts for 25mm settlements).
For cohesive soils, although I do not use it too much because I prefer to take undisturbed samples, we use cu=6.25N (kPa) which is close to the cu=(2/3)N from your attachment. I also follow your approach for SF=3, qa=2xSu (or just qa=qu, where qu is the unconfined compressive strength obtained from testing).
As you mentioned, then we check against settlement to determine the allowable bearing pressures.
Anyway, I like this forum because I think that it is very nice to know the approach that senior geotechs use. So, much thanks for your comments.
And yes, please, if you can post Dr. Bolton's references, it will be greatly appreciated.
RE: Preliminary bearing capacity
RE: Preliminary bearing capacity
RE: Preliminary bearing capacity
RE: Preliminary bearing capacity
One question: the Bearing Capacity index "C" can be used with any units? I am assuming that I have to use it with the 1-D Terzaghi's consolidation equation to get settlements. Also, I do not have the book so I am not sure how Professor Hough could get Cc values for sands and gravels. But it is very interesting chart. Maybe I have to look at this ASCE paper. Thanks again for the good info.
RE: Preliminary bearing capacity
^H=H(log(1+^p/pi))/C
RE: Preliminary bearing capacity
RE: Preliminary bearing capacity
RE: Preliminary bearing capacity
RE: Preliminary bearing capacity
Thanks for your input,.
Retrograde the correlation you gave 10N (Kpa), can we use this for all kinds of soils or only for sands or it can be used for mixed sands as well?
NT
RE: Preliminary bearing capacity
RE: Preliminary bearing capacity
Thanks for the settlement evaluation using the bearing capacity index.
Let's say the soil profile has both sands/silty sands/clays.
Sands/silty sands are going to undergo immediate settlement where as clays have time dependent consolidation settlement.
How do we determine/differentiate the settlement in this case?
For 2" allowable settlement criteria, how do we check and combine the immediate vs consolidation settlement?
Thanks
NT
RE: Preliminary bearing capacity