Preliminary bearing capacity 1

[nagatalluri]

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

 

[Ron]

Most of the classic geotechnical texts and some of the contemporary ones will lead you to this. Try Terzaghi and Peck, Sowers, and others. I believe Das has something as well but I'm not near my library and can't check until Monday.

 

[BigH]

The short answer is yes, with judgment.

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.

 

[Ron]

BigH...exactly.....thus my comment

Most of the classic geotechnical texts and some of the contemporary ones will lead you to this.

Your answer was much more complete and informational!

 

[oldestguy]

My two cents: Remember that N value is darned rough. I view the so called "corrections" as somewhat useful, but again rough. The charts are rough also. Most I know of are before any "correction". Result: your answer is doubly rough.

 

[BigH]

As oldestguy states - rough. I use most all correlations with grain of salt - engineering judgment is a necessity. And he is right - we never corrected our N values as reported . . . we would, in design, correct for seismic purposes.

 

[Retrograde]

10 x N as a rough guide (answer is in kPa).

Also refer charts here:

http://people.eng.unimelb.edu.au/stsy/geomechanics_text/Ch13_foundatn.pdf

 

[Okiryu]

BigH, what are the correlations that you normally use? Do you use correlations for granular and fine-grained soils? I normally try to take some undisturbed samples for fine-grained soils, but just curious to see what SPT correlations for fine-grained soils are used by others. Thanks.

 

[BigH]

For allowable bearing pressure for sands and granular soils, I'll typically use the charts as seen in the references I have noted . . . if it seems a bit "high", I'll adjust - I seldom would ever determine the ultimate bearing capacity for coarse grained soils with all the fancy equations and i factors as it is really settlement that governs - unless of course you have complex loading conditions.

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.

 

[Okiryu]

BigH, for well graded granular soils here in Japan we use this correlation for phi:

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.

 

[oldestguy]

My Cornell soils engineering professor and later employer was B.K. Hough. He wrote a text "Basic Soil Engineering" and later a ASCE paper that I am quoting from. The soils text has two charts dealing with "bearing Capacity" related to N value. I'll attach if possible. However, in an August 1959 ASCE paper in SM-4 #2135 he gave a chart of N value versus his "Bearing Capacity Index, C. C=(1+e))/Cc Two engineers from California DOT found the method in sand to be within 10% for 70 percent of their cases (over 40) and when water table was shallow the error was 30 - 40% low. In clays the error varied from none to 50% low (15 cases). See attachment. Remember no correction on N.

 

[oldestguy]

Here are the charts from "Basic Soil Engineering" by Hough.

 

[oldestguy]

With the C values and a soil profile you can set up a spreadsheet to calculate settlement as deep as you may wish, but Hough suggested stopping when the imposed pressure was 10 percent of existing. pressure was 10 percent of existing. I use a separate calculation for each foot of depth and spread the load simply by using an width of footing width plus depth below footing. That is rough but consistent with the rough N values.

 

[Okiryu]

OG, good charts. Thank you very much to share this info.

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.

 

[oldestguy]

Hey Oki: I have not tried any metric units for these types of things, but I would expect as long as you use the correct units in the input you end up with the correct unit in the answer, well yes. I will try to show the formula and how it is used for figuring settlements with the delta as this symbol ^. H being vertical dimensions, p being pressures, With actual lab test results on clays you won't be guessing. But for sands, even though the formula was set up for clays, it's amazing how it works.

^H=H(log(1+^p/pi))/C

 

[Okiryu]

Hi OG, thanks for the reply. It makes sense and as expected, I have to plug "C" into the 1-D consolidation settlement equation...thanks again !!

 

[oldestguy]

One more item. The California DOT checks were at highway bridge approaches, measuring the settlement of the embankments, but using the test borings at the bridge nearby. This probably is a better check than trying to figure stress distribution with depth from a footing, due to the wide width of the fill. I see I missed a ) in the equation. Then edited it now.

 

[BigH]

Oldestguy is tops!

 

[nagatalluri]

Guys:

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

 

[Retrograde]

The 10N correlation is only meant to be used for sands. It should still be OK for soils that are predominately sand.