Explanation:

As per Terzaghi's method, ultimate bearing capacity of strip footing is given by equation

q_u = CN_c + γD_f.N_q + 0.5.B.γN_γ 

Where, B = Width of footing (Least lateral dimension)

Df = Depth of footing below G.L.

γD_f = Surcharge at foundation level.

N_c, N_q, N_γ are bearing capacity factors which depend upon frictional angle of soil.

For purely cohesive soils, ? = 0° 

N_c = 5.7, N_q = 1 and N_γ = 0

Additional Information

Value of N_c, N_q, and N_γ as per Prandtles for purely cohesive soil,

Nc = 5.14, Nq = 1 and Nγ = 0

Concept;

Dilatancy correction:

It is to be applied when N_o obtained after overburden correction, exceeds 15 in saturated fine sands and silts. IS: 2131-1981 incorporates Terzaghi and Peck recommended dilatancy correction (when N_o > 15) using equation

$\mathrm{N}=15+\frac{1}{2}\left({\mathrm{N}}_0-15\right)$

N₀ - SPT value after overburden correction

Calculation:

Given: N₀ = 21

$\mathrm{N}=15+\frac{1}{2}\left(21-15\right)$

⇒ N = 18

For cohesionless soils (c=0), Indian standard code recommends that bearing capacity can be calculated i) based on relative density or ii) based on standard penetration resistance value, and iii) based on static cone penetration test.

The split tube sampler, used in a test for standard penetration is commonly known as split spoon sampler resting on bottom of bore hole which is allowed to sink under its own weight.

For purely cohesive soil, local shear failure may be assumed to occur when soil is soft to medium, with an unconfined compressive strength qu ? 100 kN/m².

The bearing plate used varies in size from 300 to 750 mm with a chequered or grooved bottom.

For simple spread footing on clayey soils, allowable pressure should be such that differential settlement does not exceed 1/300.

In local shear stress re is no tilting of footing and refore failure is not sudden.

According to Brinch Hansen?s equation, i represent inclination factor to account for both horizontal and vertical components of foundation loads.