## Soil Bearing Capacity for N-Values of SPT

## Soil Bearing Capacity for N-Values of SPT

(OP)

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?

## RE: Soil Bearing Capacity for N-Values of SPT

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.

## RE: Soil Bearing Capacity for N-Values of SPT

Guest(visitor)## RE: Soil Bearing Capacity for N-Values of SPT

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

## RE: Soil Bearing Capacity for N-Values of SPT

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