Correlation between SPT and alowable bearing pressure

Hi, fkd.

You really need to know more than just the N values. Importantly, you must also know what types of soil you are dealing with.

One of my ancient texts, Peck Hansen and Thorburn, "Foundation Engineering", shows tables from which you can get bearing capacity factors Nq and Ngamma for SAND, based on N values.

For clays, PH&T say "the standard penetration test is not so reliable an indicator of the compressive strength of clys as it is of the relative density of sands, and the tests on tube samples are preferred".

For cohesionless soils, as austim says, mainly.

You have a freely available worksheet in the Mathcad's Collaboratory that cares of that.

See the graphical relationship in

Bridge Substructure and Foundation Design
Petros P. Xanthakos
Prentice Hall
p.534

I use a simple formula of allowable bearing pressure =10N where N is from S.P.T. Kieran Coyle
BEng(hons),CEng, M.I.Struct.E, M.I.E.I.

Hi Kieran,

In what unit is the ABP=10*N ?

The US Army Corps of Engineers has published many Engineer Manuals that can make you more than satisfied since they are based on current practical applications and have sustantiable references for further reading. For example, N values has been used mainly for sands to correlate allowable bearing pressure. The Peck Hanson Thornburn book has charts for those purposes, and Meyerhof has done extensive work on that too.

However, recently I read that for clays, Bowles have suggested to use cu = 0.12 Nspt (in ksf)which multiplied by two will be the unconfined compressive strength (qu), which in turn can be taken as the allowable bearing pressure having a safety factor of 3. Easy not?... but,

Remember that allowable bearing pressure is tied to allowable settlement and settlement estimates govern for clays while allowable bearing pressure using classical charts for sand are developed for an allowable settlement of 1 inch.

For in-depth free study of the issues and to get the USACE Engineer Manual of Bearing Capacity of Soils go to the following page:


You can get also many other manuals for free in their main web page of the Engineer Publications, you just need Adobe Acrobat Reader. If you have any problems accessing the info let me know. I know you and others will appreciate this link.

CIS Geotechnical CIS Geotechnical
jrodriguez@centralindustrialpr.com

A nice, conservative method for coming up with bearing values, based on N values, can be found in a couple charts published back in 1969 (2nd print) by B.K. Hough, "Basic Soils Engineering," pp 308 and 309. I used these charts for many years with success. As near as I can estimate, these values for bearing will result in about 1/4" settlement. And, these charts were developed with manual hammers. If your N values are from an auto-hammer, the results could be really conservative. As CISGeotechnical indicated, there are two modes of failure... bearing and settlement. As long as there is no bearing failure, the bearing pressure you select will be (somewhat) dependant on the settlement you can live with. Now that is probably more than you really wanted to know.....

I just want to say thank you very much for the valuable and precise information provided.
As RVT said it's really more than my needs, specially with that huge amount of info that can be found at USACE.


Best Regards

Fred

I am agree with Austim. For determine allowable bearing capacity you also need to know other soil properties. For you know if any correlation give trusable results, you need to know the hypotheses what is based the correlation. You need to know characteristics of the soil used to develop the correlation, too.