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Soil Bearing Capacity for N-Values of SPTHelpful Member!(3) 

artnkman (Structural) (OP)
11 May 00 18:08
Is there any reference material that lists values of soil bearing capacities for various soil types given the N-values taken from Standard Penetration Tests?
Helpful Member!(2)  georam (Geotechnical)
12 May 00 13:46
The bearing capacity of soils for shallow foundations generally follows the acceptable Terzaghi-Meyerhof equation.

Qult = 1/2*gamma*B*Ny + c*Nc + (Pq +gamma*Df)*Nq  where
 
Q ult is the ultimate bearing capacity. To get the acceptable, you may use a factor of safety of 3.

Gamma is the overburden pressure
c is the undrained shear strength or 0.5 of unconfined strength (for clay), c=o for sandy soils
Df = depth to foundation level
Pq = additional surcharge load above ground, if required.

Ny,Nc and Nq are capacity factors, and need to be looked from a chart. It is a function of friction angle phi
Correction is required for  for size and depth of footing.

From here there are correlation from N values to estimated undrained shear strength (for cohesive soils).

For sandy soil, the above equation can be estimated to be :
Qa = 0.11 Cn * N
Qa = allowable bearing pressure = Q ult/Factor of safety = Q ult/3
Cn is overburden factor  Cn=1 if overbudern is about 1 tsf, Cn=2 if overburden is 0 tsf, Cn=0.5 if overbudern is 4.5 tsf.

If you want details, I suggest that you look at one of the soil mechanics or Foundation Engrg book, or visit the web site of US Army and read the text on bearing capacity of soils  Document number EM 1110-1-1905
Address is
http://www.usace.army.mil/inet/usace-docs/eng-manuals/em.htm
You can read the documents using acrobat (pdf file)

Hope that this help.
Guest (Visitor)
29 Jan 01 21:17
How to relate California Bearing Ratio to Bearing Capacity
Helpful Member!  OSMEL (Coastal)
2 Feb 01 16:38
First to start, it is important that you know that predictions on bearing capacity using SPT are only moderately reliable, principally due to some inconsistencies in SPT test procedures.
Furthermore, the standard penetration test are not too good for predict pile capacities in cohessive soils.
By the way, your question is for shallow or deep (pile)foundations?
For the second of them you could use these formulas:

MEYERHOF
End bearing:
qe = minor of 0,40*N'60(at toe)*D/B*sigma.r and 4*N'60(at toe)*sigma.r (Sands and gravels)
qe = minor of 0,40*N'60(at toe)*D/B*sigma.r and 3*N'60(at toe)*sigma.r (Non plastic silts)

Skin Friction:
fs = sigma.r/50*N60 (Large displacement piles)
fs = sigma.r/100*N60 (Small displacement piles)

Where:
qe= unit point resistance
fs= unit lateral resistance
sigma.r=100 KPa =2000 lb/ft2
D/B is the ratio pile embedment depth- diameter
N'60 is the normalized value for the SPT, corrected for overburden pressures and field procedures.

N'60= CN*N60
CN= 2/(1+sigma.v/sigma.r)
sigma v= Overburden pressure

N60 is the SPT value corrected for field procedures. It assumes an standard of 60% of efficiency of the penetration hammer. If you are not sure of the efficiency of the hammer, you can assume your N= N60. Off course, you can mislead if your efficiency is less than 60 %, because it will need the next transformation:

N60= N* % Efficiency/60

BRIAUD:
qe= 19,7*sigma.r*(N60)^0,36
fs= 0,224*sigma.r*(N60)^0,29

BOWLES:
fs= Xm*N55 (or N60)
Xm= 2,0 for large volume displacement
Xm= 1,0 for small volume displacement

SHIOI and FUKUI:
fs= 10 N55 (KPa) Driven piles in clay
Riz (Structural)
12 Mar 01 8:56
Art,
Yes, u can find relevant soil type according to the N values u get. Once the soil type is established u can than  read a suitable allowable bearing capacity.
Look for:
A. Hodgkinson, 1986,
Foundation Design,
Architectural Press, UK
However, I would point out that u carry out from 1st principles the allowable bearing capacity based on N values u have. Its easy once u get going!
Riz

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