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.htmYou can read the documents using acrobat (pdf file)
Hope that this help.