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

## 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 = *DF

Where,

q = effective overload at the base level of the foundation, kgf/cm. In2

= unit weight of soil, kgf/cm. In3

DF = 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. NWhy & n?) from the table below. For intermediate values ​​of ‘ϕ’, perform linear interpolation.

### Step-7

Calculate the size factors (ie sWhy & s?) using the formula given below. Where,

B = foundation width, cm. In

L = foundation length, cm. In

### Step-8

Calculate depth factors (ie dWhy & D?) using the following formula.

DWhy=D?=1 (for < 10 .)0,

DWhy =D? = 1+0.1 (dF/b)(n?,1/2 (for >10 .)0,

n? is calculated using the following formula

n? = body2[(π/4)+(ϕ/2)]

### Step-9

Calculate tilt factors (ie iWhy & 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[Dw/(Df+B)]

Where,

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

Dwoo = depth of water table, cm. In

DF = depth of foundation, cm. In

### Step-11

Calculate the net final bearing capacity using the equation below. Where,

WhyD = net final bearing capacity of the foundation, kgf/cm2

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

nWhy & n? = bearing capacity factor (see step rate 6,

sWhy & s? = size factor (ref) STEP-7,

DWhy & D? = depth factor (see step-8,

IWhy & 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. In3,

Er. Mukesh Kumar Er. Mukesh Kumar is Editor in Chief and Co-Funder at ProCivilEngineer.com Civil Engineering Website. Mukesh Kumar is a Bachelor in Civil Engineering From MIT. He has work experience in Highway Construction, Bridge Construction, Railway Steel Girder work, Under box culvert construction, Retaining wall construction. He was a lecturer in a Engineering college for more than 6 years.