The following sections provide a summary of SPT-based methods used for estimation of bearing capacity of shallow foundations. Please note that these methods are developed for footings supported on granular soils, based on a settlement of 25 mm (1 inch) and with a level subgrade. The effect of groundwater level on bearing capacity is not considered in these equations.

Meyerhof, 1976 (based on 25mm settlement)

The allowable bearing capacity based on the SPT test according to Meyerhof is:

qa=N60.Kd/F1                      B≤F4

qa=N60.Kd/F2.{(B+F3)/B}2       B>F4

where Kd=1+D/(3B)≤1.33, and F1 to F4 are defined as below for SI units:

F1=0.05 , F2=0.08 , F3=0.3 , F4=1.2 m

 

and N60 is the average SPT blow counts from 0.5B above to 2B below the foundation level. We note that the above equation includes the 50% adjustment per Bowles (see Ref 1).

 

Parry, 1977 (based on 25mm settlement)

The allowable bearing capacity according to Parry for cohesionless soil is (kPa):

qa_1=30N60                          Df≤B

Where N60 is the average SPT blow counts below 0.75B underneath the footing. In addition, Parry provided a second equation which is also a function of footing width (B):

qa_2=1000N60/15B                     for 20 mm settlement


NovoSPT adjusts this second equation for 25 mm (1 inch) of settlement. The minimum value of qa_1 and qa_2 will be provided as allowable bearing capacity (see Ref 1).


Burland and Burbidge, 1985 (based on 25 mm settlement)

They collected more than 200 records of structures founded on sands and gravels. They started with the premise that the settlement could be represented by an equation of the form:

qa=2540.N601.4/(10T.B0.75)


original equation  where af = S(mm)/qa(kPa)

where N60 is the average SPT blow counts to a depth of B0.75 below footing and T~2.23

 

Peck, Hansen and Thornburn, 1974

The original form of equation (see Ref 6) is in the following format:

Where Se is in inches, and q is in tsf (note 1 tsf = 96 kPa). Cw is for groundwater correction which is not taken into account in NovoSPT. The following formula is used in NovoSPT for this method (using 25.4 mm of settlement):

qa=10.6N1(60)

General Terzaghi Formula

The following Terzaghi equation is used for indirect estimation of bearing capacity of shallow footing on cohesionless soil (see Ref 2):

where:

 = is the overburden stress at foundation level (Df).

Nq = e[π.tan(ϕ)] [tan(π/4+ϕ/2)]2             Bowles, 1996

Nγ = 1.5(Nq-1).tan(ϕ)                          Brinch & Hansen, 1970

ϕ = friction angle correlated from the equation proposed by Hatanaka and Uchida, 1996 based on SPT at the foundation level (see Ref 2):