# 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}.

I appreciate your knowledge in geotechnical issues.

through your comment i have learn much and now am a bit comfortable

thanks sir,

Regards,

Eng. Kahema

In this case, cohesion is taken as zero for being sand. What if we need to use cohesion to calculate net ultimate bearing capacity, how do we find that.

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