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