Estimation of Net Allowable Bearing Presusure from SPT "N" value

[nagatalluri]

Hi all:

Is there any way a preliminary estimate of Net Allowable Bearing Pressure obtained from SPT "N" value.

In our case the footing is resting at 8 ft depth. Do we average all the SPT values for whole bore hole or we consider SPT's in the footing depth range only ??

Because we have reasonable N values at shallow depths but at deeper depths we have low N's which is followed by a SHALE layer with good N's.

Thanks

 

[FixedEarth]

See attached to get a rough estimate. Calculate to see if induced settlement is acceptable.

http://www.soilstructure.com/

 

[oldestguy]

Interesting that B.K.Hough's "Basic Soil Engineering" first edition is referenced. In his second edition he cut the bearing values down somewhat. However, you can glean much more of his reasoning if you can get a copy of the ASCE Soil Mechanics and Foundations Division Journal August 1959 Paper 2135 SN4. There he relates blow count to factors related to compressibility and therefore related bearing capacity to compressibility, or in other words settlement. There are several discussors in paper 2391, among which is Ralph Peck. Peck stuck with his shear strength basics. However Hough's basic idea was evaluated by some and found to be quite accurate. In his paper he ties comprehensibility into blow count also.

 

[FixedEarth]

oldestguy- Do you know if these charts were based on glaciated soils? My former boss before he retired told me that Hough practiced in the east coast. The reason I ask is that it may be possible to have higher bearing capacity for overconsolidated soils, which may explain why these chart values seem high.

nagatalurri- You can get the paper from here:

http://cedb.asce.org/cgi/

http://WWWdisplay.cgi?11797

http://www.soilstructure.com/

 

[nagatalluri]

Thank you sir.

Naga

 

[oldestguy]

I'll check the link by fixed Earth. If needed I can copy the paper and see about best way to get it out. One thing I have done is make use of that paper for a spreadsheet program that inputs his soil parameters and also inputs for different footing sizes and loadings to compute settlements. There is a paper he published about this usage, but the spreadsheet method is better I think. Now some B.K. Hough Info. He was an MIT grad, under Cassagrande (I think) Might have had a Masters. Worked for the Corps of Engineers, generally during WWII. Then started the soils engineering program at Cornell about 1945. As that moved along, he kept a side business doing consulting and also started a test boring business that then branched off on its own. He had his own soils lab and made some use of the University facilities by developing a universal soil testing machine for the University, capable of Tri-axials to 3" dia, unconfined compression, direct shear (4" x 4") and consolidation. During all this time he of course was out on jobs and they were predominately on east coast, northern. However he did some jobs farther away. My familiarity was: as an undergrad student, later a grad student and somewhat later an employee. He left the University in about 1956 and was then full time consulting. Retired about 1983 and died about 1990.

As to how the chart was developed, it likely was based on many sources, among them building codes, his lab tests, as well as job experiences. In his text there are the results of many tests on soils generally from the New York State area. One thing I recall was his contempt for those with "diarrhea" of the pen that said nothing.

 

[oldestguy]

Correction. With age memory not so good. The change Hough made was in a different chart in the paper, not to the chart shown by Fixed Earth. What he did was reduce the "Bearing Capacity Index" "C" values from his paper to that same chart as published in the second edition of his text book. From my experience I don't think that was needed. That factor, while he relates it to N value, can also be computed on the basis of consolidation tests, yes even consolidation type tests on sands. That appears to be some of the basis for his "C" chart.

 

[nagatalluri]

Sir:

Thanks a lot for the information. I have one final question for you. Lets say I have a soil profile up to a depth of 100'. If I want to calculate total settlement using the constrained modulus, till what depth I need to calculate.

Is it at 10% of the stress increase or any empirical depth where there wont be any influence of footing pressure ??

Thanks
NT

 

[oldestguy]

NT In answer see the spread sheet sent your way this hour. You can expand this at will and once you see the results, you can then make up your own mind as to how important it is to run your calculations all the way to the bottom of the layer. I'd consider the precision or lack there of of the data you input from the N numbers in making this judgement. Hough sets that depth as where the added pressure from the footing is less than 10% of existing pressure. I can see no reason for tying to find a point of no increase from the added loading of the footing. That probably does not exist anyhow.

 

[FixedEarth]

Thanks oldestguy for that story. My former boss worked in Northern NY in the mid 60's to late 70's and then he moved to CA. It is good to know that several sources were used to make that chart. Incidentally, I only have Hough's first edition book but need to get his 2nd edition as well.

http://www.soilstructure.com/

 

[emmgjld]

My father supplied Hough's text to every Branch office he opened in Colorado & Wyoming in the 70's. Always the clearest, complete with good examples. I use those 2 charts for blow counts vs Bearing almost daily. added a few lines to account for local conditions.

 

[oldestguy]

NT For what it is worth, referring to Hough's paper, I dug into an old set of lab notes where I ran a series of "consolidation type" tests on sands and even glacial till. While this is not a neat summary, it shows that plotting some of my results on the Hough Fig 5-7 of his second edition that one can get "bearing capacity index" (C)...(1+e)/Cc values in his general range results, but sometimes showing less compressibility, thus larger value C, or, in other words less settlement that he might predict.