ESTIMATING SOIL CAPACITY

[danmccarthy]

Can anyone recommend (or supply) a reference to get an approximation of the bearing capacity of soil based on blow count or other parameters?
Most projects will include a geotechnical report done by a soil expert, but I would like some help at the estimate phase. Often a soil profile is known or can be conjectured based on an old soil report but bearing pressure may not be given.
Thank you
Dan

 

[ishvaaag]

Correlations you have at

Foundation Analysis and Design 5th edition
Bowles
Mc Graw Hill

To just a guess suffices a general assesment of the bearing capacity of the soils

Soil allowable foundation compression
at service level
very bad under 0.5 kgf/cm2
bad 0.5 to 1 kgf/cm2
moderate strength 1 to 2 kgf/cm2
sound soil 2 to 3 kgf/cm2
very hard soil 3 to 5 kgf/cm2


Normally one uses special measures for bad soils. If capacity is enough one knows to use about 1.5 kg/cm2 is reasonable and if the soil is definitely good 2.5 kgf/cm2 shouldn't give any problem on bearing capacity.

Total and differential settlements need be additionally considered.

 

[DRC1]

Check your state building code. They usually give presumptive bearing pressures that you can use for fairly straight forward work or preliminary design with out geotech investigation.

 

[Zeccos]

The ultimate bearing capacity is given by the equation:
qult=N*B*(1+D/B)/10. Where B is the B is the width of the foundation and N is the N60 value of the soil averaged over a depth of B to 1.5B of the foundation. D is the embedment depth. The ultimate bearing capacity is given in terms of tsf and the D and B in ft.

I hope that helps,

Dimitris

*************************************************
Dimitris P. Zeccos
Graduate Student
Geotechnical Engineering
Department of Civil and Environmental Engineering
University of California, Berkeley

Moderator "Geotechnical Engineer" Group

http://groups.yahoo.com/group/TheGeotechnicalEngineer/

Webmaster of "Geoengineer" Website

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[RG88]

I know I am several months behind on this one.

To get a general assesment of the soil you conduct a DCP tetst. Humboldt sells the DCP equipment. You can contact them about obtaining a copy of ASTM STP 399 (I think that is the number). It outlines the correlation of DCP testing to blow count (N) and then to bearing capacity.

Try searching the USACE site also. The have several manuals on 'Bearing Capacity of Soils'. I cannot recall the exact address, but I believe that to be the title. It provides a rather in depth discussion on the blow count and associated bearing capacity.

 

[BigH]

For USACE - got to

http://www.rediff.com,

type in "US Army Engineering" and look for third reference - this is a listing of the manuals. Follow the path and you can download any or all of the US Army civil engineering manuals.