Calculate Bearing Capacity From Spt n Value

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

Relation between phi and SPT value (N)

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*Df Where, q = Effective surcharge at the base level of foundation, in kgf/cm 2 ϒ = Unit weight of soil, in kgf/cm3 Df = 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. Nq & Nϒ) from the table given below. For the intermediate values of ‘ϕ’, make linear interpolation.

Be aring Capacity Factors

STEP-7

Calculate shape factors (i.e. sq & sϒ) using formula given below.

Where, B = Width of foundation, in cm L = Length of foundation, in cm STEP-8

Calculate depth factors (i.e. dq & dϒ) using following formula. dq=dϒ=1 (for ϕ < 100) dq = dϒ = 1+0.1(Df/B)(Nϕ)1/2 (for ϕ>100) Nϕ is calculated using following formula Nϕ = tan2[(π/4)+(ϕ/2)] STEP-9

Calculate inclination factors (i.e. iq & iϒ) 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[Dw/(Df+B)] Where, W’ = Correction factor for location of water table Dw = Depth of water table, in cm Df = Depth of foundation, in cm

STEP-11

Using the equation given below calculate the net ultimate bearing capacity.

Where, qd = Net ultimate bearing capacity of foundation, kgf/cm2 q = Effective surcharge at base level of foundation, in kgf/cm2 (Refer step-5) Nq & Nϒ = Bearing capacity factors (Refer step-6) sq & sϒ = Shape factors (Refer step-7) dq & dϒ = Depth factors (Refer step-8) iq & iϒ = 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/cm3.