Table of Contents

**Step-by-Step Procedure for Calculating Bearing Capacity from Standard Penetration Test Values or N-Values**

**step 1**

Perform a standard penetration test at the location for which you wish to calculate the bearing capacity. This is done according to the standard procedure given in IS-2131. The standard penetration test should be performed every 75 cm in a vertical direction.

**step 2**

For preliminary calculations decide the depth, width and length of the foundation. It is a trial and error process. In the first attempt, you may never find the exact size of foundation that will meet all your needs.

**step 3**

Apply necessary corrections to standard entrance exam values. Calculate the cumulative average value of the true 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 figure below to find the corresponding angle of shear resistance (ϕ).

**Step-5**

Calculate the effective overload at the base level of the foundation by multiplying the effective unit weight of the soil with the depth of the foundation.

Q = *D_{F}

Where,

q = effective overload at the base level of the foundation, kgf/cm. In^{2}

= unit weight of soil, kgf/cm. In^{3}

D_{F} = depth of foundation, cm. In

**step rate 6**

For the angle of shear resistance value (ϕ) as calculated in **step 4**, find the corresponding values of the bearing capacity factors (i.e. N_{Why} & n_{?}) from the table below. For intermediate values of ‘ϕ’, perform linear interpolation.

**Step-7**

Calculate the size factors (ie s_{Why} & s_{?}) using the formula given below.

Where,

B = foundation width, cm. In

L = foundation length, cm. In

**Step-8**

Calculate depth factors (ie d_{Why} & D_{?}) using the following formula.

D_{Why}=D_{?}=1 (for < 10 .)^{0},

D_{Why} =D_{?} = 1+0.1 (d_{F}/b)(n_{?},^{1/2} (for >10 .)^{0},

n_{?} is calculated using the following formula

n_{?} = body^{2}[(π/4)+(ϕ/2)]

**Step-9**

Calculate tilt factors (ie i_{Why} & I_{?}) using the formula given below

Where,

α = Inclination of load to perpendicular in degrees

= angle of shear 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[D_{w}/(D_{f}+B)]

Where,

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

D_{woo} = depth of water table, cm. In

D_{F} = depth of foundation, cm. In

**Step-11**

Calculate the net final bearing capacity using the equation below.

Where,

Why_{D} = net final bearing capacity of the foundation, kgf/cm^{2}

q = effective overload at the base level of the foundation, in kgf/cm2 (see **Step-5**,

n_{Why} & n_{?} = bearing capacity factor (see **step rate 6**,

s_{Why} & s_{?} = size factor (ref) **STEP-7**,

D_{Why} & D_{?} = depth factor (see **step-8**,

I_{Why} & I_{?} = tilt factor (see **STEP-9**,

W’ = correction factor for the location of the water table (see.) **STEP-10**,

B = foundation width, cm. In

= Bulk unit weight of foundation soil, kgf/cm. In^{3},

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