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

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)
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*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
Bearing Capacity Factors

Step-7

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

Shape Factors

Where,

B = Width of foundation, in cm

L = Length of foundation, in cm

Step-8

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

Nϕ = tan2[(π/4)+(ϕ/2)]

Step-9

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

Inclination factor

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[Dw/(Df+B)]

Where,

W’ = Correction factor for location of 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.

Bearing capacity equation1

Where,

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, in kgf/cm3.

 

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