Soil Bearing Capacity from N-values of SPT for Design Purposes
Soil Bearing Capacity from N-values of SPT for Design Purposes
(OP)
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?






RE: Soil Bearing Capacity from N-values of SPT for Design Purposes
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.
A bearing capacity should only be calculated when the following information is known, or can be reasonably estimated by a qualified person:
1. stratification of soil above and below the proposed foundation.
2. soil classification through lab testing or visual identification by a qualified individual.
3. depth of groundwater.
4. size and location (depth) of proposed foundation.
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). The next step is to indentify the strength properties of the soil(s).
The best option is obviously to get the appropriate lab tests of the samples that were retrieved during the SPT sampling. 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. 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).
There are plenty of "rule of thumb" 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.
Sorry for the lengthy commentary, but I hope this was helpful. If there are any geotech's out there, please let me know if I missed anything.
Regards,
TTK
RE: Soil Bearing Capacity from N-values of SPT for Design Purposes
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)
RE: Soil Bearing Capacity from N-values of SPT for Design Purposes
RE: Soil Bearing Capacity from N-values of SPT for Design Purposes
Just out of interest, if you provide me with the relevant data I could see what I could do. ( I have some geotech. experience)
RE: Soil Bearing Capacity from N-values of SPT for Design Purposes
RE: Soil Bearing Capacity from N-values of SPT for Design Purposes
However, if it's lateral passive pressure you need (such as lateral loads on direct buried poles), then you might try these conversion estimates:
For sand, phi angle = 27.5 + N/4. From phi,of course, you can get the Rankine value.
For clay, C = N/7.5 in ksf.
In both cases, always consider the soil in a submerged condition for applying minimum density (well, maybe not in Arizona or west Texas!).
And last but not least, run it by a qualified geotech engineer and use a SF of not less than 2.0.
William Ford
sesi@mindspring.com
Spiral Engineering Services, Inc.