PDF- -Terzaghi's Bearing Capacity Equations - Calculate Bearing Capacity From Spt n Value



Perform standard penetration test on the location for which you want to calculate bearing capacity

This is done as per standard procedure given in IS-2131

Standard penetration test must be done at every 75 cm in vertical direction

Decide the depth,

width and length of foundation for initial calculation

This is a trial and error process

In the first attempt,

you can never get the exact size of foundation which will satisfy all of your needs

Apply necessary corrections to the standard penetration test values

Calculate the cumulative average value of corrected SPT values from the base level of foundation to a depth equal to 2 times the width of foundation

Correlate the above cumulative average SPT value with the fig given below to find out the corresponding angle of shearing resistance (ϕ)

Relation between phi and SPT value (N)

Calculate effective surcharge at the base level of foundation by multiplying the effective unit weight of soil with the depth of the foundation i

q = Effective surcharge at the base level of foundation,

in kgf/cm 2 ϒ = Unit weight of soil,

in kgf/cm3 Df = Depth of foundation,

For the angle of shearing resistance value (ϕ) as calculated in step-4,

find out the corresponding values of bearing capacity factors (i

Nq & Nϒ) from the table given below

For the intermediate values of ‘ϕ’,

Be aring Capacity Factors

Calculate shape factors (i

sq & sϒ) using formula given below

B = Width of foundation,

in cm L'= Length of foundation,

Calculate depth factors (i

dq & dϒ) using following formula

dq=dϒ=1 (for ϕ < 100) dq = dϒ = 1+0

Calculate inclination factors (i

iq & iϒ) using the formula given below

α = Inclination of the load to the vertical in degrees ϕ = Angles of shearing resistance in degrees STEP-10

Calculate the correction factor for location of water table using the following formula W’ = 0

W’ = Correction factor for location of water table Dw = Depth of water table,

in cm Df = Depth of foundation,


Using the equation given below calculate the net ultimate bearing capacity

qd = Net ultimate bearing capacity of foundation,

kgf/cm2 q = Effective surcharge at base level of foundation,

in kgf/cm2 (Refer step-5) Nq & Nϒ = Bearing capacity factors (Refer step-6) sq & sϒ = Shape factors (Refer step-7) dq & dϒ = Depth factors (Refer step-8) iq & iϒ = Inclination factors (Refer step-9) W’ = Correction factor for location of water table (Refer step-10) B = Width of foundation,

in cm ϒ = Bulk unit weight of foundation soil,