HOW TO CALCULATE BEARING CAPACITY OF SHALLOW FOUNDATION IN SAND FROM SPT VALUES OR N-VALUES (IS-6403)

Source: http://civilblog.org

Step-by-Step Procedure for Calculation of Bearing Capacity from Standard Penetration Test Values or N-values

Step-1

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.

Step-2

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.

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 foundation to a depth equal to 2 times the width of 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 foundation by multiplying the effective unit weight of soil with the depth of the foundation i.e.
q = ϒ*D_f
Where,
q = Effective surcharge at the base level of foundation, in kgf/cm²
ϒ = Unit weight of soil, in kgf/cm³
D_f = 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. N_q & N_ϒ) from the table given below. For the intermediate values of ‘ϕ’, make linear interpolation.

Bearing Capacity Factors

Step-7

Calculate shape factors (i.e. s_q & s_ϒ) using formula given below.

Where,
B = Width of foundation, in cm
L = Length of foundation, in cm

Step-8

Calculate depth factors (i.e. d_q & d_ϒ) using following formula.
d_q=d_ϒ=1 (for ϕ < 10⁰)
d_q = d_ϒ = 1+0.1(D_f/B)(N_ϕ)^1/2 (for ϕ>10⁰)
N_ϕ is calculated using following formula
N_ϕ = tan²[(π/4)+(ϕ/2)]

Step-9

Calculate inclination factors (i.e. i_q & i_ϒ) 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 location of water table using the following formula
W’ = 0.5+0.5[D_w/(D_f+B)]
Where,
W’ = Correction factor for location of water table
D_w = Depth of water table, in cm
D_f = Depth of foundation, in cm

Step-11

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

Where,
q_d = Net ultimate bearing capacity of foundation, kgf/cm²
q = Effective surcharge at base level of foundation, in kgf/cm2 (Refer step-5)
N_q & N_ϒ = Bearing capacity factors (Refer step-6)
s_q & s_ϒ = Shape factors (Refer step-7)
d_q & d_ϒ = Depth factors (Refer step-8)
i_q & i_ϒ = 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, in kgf/cm³.