HOW TO CALCULATE BEARING CAPACITY OF SHALLOW FOUNDATION IN SAND FROM SPT VALUES OR N-VALUES

 

STEP-BY-STEP PROCEDURE FOR CALCULATION OF BEARING CAPACITY FROM STANDARD PENETRATION TEST VALUES OR N-VALUES

STEP-1

Perform a standard penetration test on the location for which you want to calculate bearing capacity. This is done as per the standard procedure given in IS-2131. A standard penetration test must be done at every 75 cm in the vertical direction.

STEP-2

Decide the depth, width, and length of the 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.

STEP-3

Apply necessary corrections to the standard penetration test values. Calculate the cumulative average value of corrected SPT values from the base level of the foundation to a depth equal to 2 times the width of the foundation.

STEP-4

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)

STEP-5

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

q = ϒ*Df

Where,

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

ϒ = Unit weight of soil, in kgf/cm3

Df = Depth of foundation, in cm

STEP-6

For the angle of shearing resistance value (ϕ) as calculated in step-4, find out the corresponding values of bearing capacity factors (i.e. Nq & Nϒ) from the table given below. For the intermediate values of ‘ϕ’, make linear interpolation.

Bearing Capacity Factors

STEP-7

Calculate shape factors (i.e. sq & sϒ) using the formula given below.

Where,

B = Width of foundation, in cm

L = Length of foundation, in cm

STEP-8

Calculate depth factors (i.e. dq & dϒ) using the following formula.

dq=dϒ=1 (for ϕ < 100)

dq = dϒ = 1+0.1(Df/B)(Nϕ)1/2 (for ϕ>100)

Nϕ is calculated using the following formula

Nϕ = tan2[(π/4)+(ϕ/2)]

STEP-9

Calculate inclination factors (i.e. iq & iϒ) using the formula given below

Where,

α = Inclination of the load to the vertical in degrees

ϕ = Angles of shearing resistance in degrees

STEP-10

Calculate the correction factor for the location of the water table using the following formula

W’ = 0.5+0.5[Dw/(Df+B)]

Where,

W’ = Correction factor for the location of the water table

Dw = Depth of water table, in cm

Df = Depth of foundation, in cm

STEP-11

Using the equation given below calculate the net ultimate bearing capacity.

Where,

qd = Net ultimate bearing capacity of the 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 the location of the water table (Refer step-10)

B = Width of foundation, in cm

ϒ = Bulk unit weight of foundation soil, in kgf/cm3.