Soil Bearing Capacity from N-values of SPT for Design Purposes 1

[artnkman]

Is there any reference material available that gives values of soil bearing capacity for various soil foundation types (e.g. compacted sand and gravel, etc) using  corresponding N-values obtained from Standard Penetration Tests?

 

[TTK]

I do not profess to be an expert in this area, and I am certainly not a geotechnical engineer, but I can say the following:<br><br>Any reference material that gives bearing capacities based solely on SPT values and such broad soil descriptions is not worth the paper it is printed on.&nbsp;&nbsp;<br><br>A bearing capacity should only be calculated when the following information is known, or can be reasonably estimated by a qualified person:<br><br>1. stratification of soil above and below the proposed foundation.<br>2. soil classification through lab testing or visual identification by a qualified individual.<br>3. depth of groundwater.<br>4. size and location (depth) of proposed foundation.<br><br>Since you have SPT values, I will assume that there was a soil exploration program, so you should have the stratification (hopefully to an adequate depth), and the location of the groundwater table (if applicable).&nbsp;&nbsp;The next step is to indentify the strength properties of the soil(s).&nbsp;&nbsp;<br><br>The best option is obviously to get the appropriate lab tests of the samples that were retrieved during the SPT sampling.&nbsp;&nbsp;If this is not possible, fairly accurate properties can be found in a number of texts (i.e. Geotechnical Engineering Investigation Manual, Hunt, Roy E. 1984) if the USCS classification is known.&nbsp;&nbsp;A qualified person who is familiar with the soils in your neck of the woods should be able to determine this classification fairly easily by visual identification (assuming we are not talking about a purely cohesive soil).<br><br>There are plenty of &quot;rule of thumb&quot; estimates out there for allowable bearing capacities, which are probably ideal for the design of the foundation for the storage shed in the back yard, but any permanent structure should be based on sound engineering and not a value that is picked out of a table.<br><br>Sorry for the lengthy commentary, but I hope this was helpful.&nbsp;&nbsp;If there are any geotech's out there, please let me know if I missed anything.<br><br>Regards,<br><br>TTK<br><br><br>

 

[CHANDRA]

artnkman<br>Yes you can refer to the book 'Foundation Analysis and Design' by J.E.Bowles by McGraw-Hill publications.Particularly you can ref to articles 4.10 and 3.7(I am refering to IV edition & v edition is out)

 

[ok]

As soil material under foundation or any where is not a homogeneous one itt is difficult to define exactly the properties of soil wihout making soil sampling and experimental studies for each idividual case,&nbsp;&nbsp;geotechnic studies shuld be carried out on the strata before defining the type of foundation that will be suitable, in some soils sometimes SPT is giving an idea about the stiffness of the soil and the scientists creat a relation between nos. of blows and bearing capacit y&nbsp;&nbsp;but this value of B/C should not be taken into consideraton in design without taking the other factors, that is not every thing about soil .

 

[gc]

You will find several books which provide bearing capacity correlated to SPT values.&nbsp;&nbsp;These should be taken only as references or 'base values'.&nbsp;&nbsp;If a geotechnical engineer was hired to do a soils investigation he should be able to provide you with this estimate.&nbsp;&nbsp;If there was no geotechnical engg.&nbsp;&nbsp;you could hire one for a nominal price and have him provide you with an estimate based on existing data.&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<br>Just out of interest, if you provide me with the relevant data I could see what I could do. ( I have some geotech. experience)&nbsp;&nbsp;&nbsp;

 

[RUSSRASCAL]

YOU NEED TO CONTACT A GEOTECH THAT DID THE TESTING OF THE SOIL. THE N VALUES ARE GOOD AS A RULE OF THUMB, HOWEVER SOME SOILS ARE SENSATIVE TO MOISTURE CHANGES AND OTHER SOILS ARE SENSITIVE TO RE MOLDING.&nbsp;&nbsp;CAUTION IS THE WATCH WORD

 

[Polecat]

If it's vertical bearing pressure you're looking for, then I would agree with those respondants who approach this conversion with caution, especially when long term settlement is a major consideration.<br><br>However, if it's lateral passive pressure you need (such as lateral loads on direct buried poles), then you might try these conversion estimates:<br><br>For sand, phi angle = 27.5 + N/4. From phi,of course, you can get the Rankine value.<br><br>For clay, C = N/7.5 in ksf.<br>In both cases, always consider the soil in a submerged condition for applying minimum density (well, maybe not in Arizona or west Texas!). <br>And last but not least, run it by a qualified geotech engineer and use a SF of not less than 2.0.<br><br> <p>William Ford<br><a href=mailto:sesi@mindspring.com>sesi@mindspring.com</a><br><a href=

http://www.spiraleng.com>Spiral

Engineering Services, Inc.</a><br>