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).

Relationship between phi and spt value (n)
Relationship between phi and spt value (n)

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.

bearing capacity factor
bearing capacity factor

Step-7

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

shape factor

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

tilt factor

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.

Bearing Capacity Equation 1

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

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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.