# 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).

**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/cm^{2}

ϒ = Unit weight of soil, in kgf/cm^{3}

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.

**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 < 10^{0})

d_{q} = d_{ϒ} = 1+0.1(D_{f}/B)(N_{ϕ})^{1/2} (for ϕ>10^{0})

N_{ϕ} is calculated using following formula

N_{ϕ} = tan^{2}[(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/cm^{2}

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/cm^{3}.

Thank you very much sir, you have made it very simple and effective.

Please how do I deduce the bearing capacity of this area? I did 5 SPTs for it and the bearing pressures at 2m where I intend to put my foundation are 20, 112, 340, and 400 kN/m^2 for BH1, BH2, BH3, BH4 and BH5 respectively.

1) how to calculate sbc based on n=Value kindly reply

Foundation is Safe against allowable settlement of 40 mm

U can do the sbc caliculation using n value by graph which is there in IS:6403. by that u can get angle of internal friction ….

Thanks very much for such an explicit note.

Thanks shrikant for sharing info..

Thanks Suryakanta for u r wonderful work. The site is quite handy and useful. Although i had not gone thru entire site, but i was browsing geo-tech blog to refresh my basiscs and i found that u had made concepts in a quick and simple steps.

Thanks for sharing u r knowledge , time and thoughts.

Keep up good work.

Sudhakar

good help