Bearing Capacity of Purely Cohesive Soil

Purely cohesive soil refers to soil that has no cohesion, but only adhesion due to the presence of clay minerals. The bearing capacity of purely cohesive soil can be determined using the following methods:

Terzaghi's Bearing Capacity Theory

Terzaghi's bearing capacity theory is based on the assumption that the soil below the foundation behaves as an elastic material. According to this theory, the bearing capacity of purely cohesive soil can be calculated using the following formula:

Qc = cNc

Where Qc is the bearing capacity of the soil in kN/m², c is the cohesion of the soil in kN/m², and Nc is the bearing capacity factor.

The value of Nc can be determined using the following formula:

Nc = [(Nq - 1) tan² φ] / [tan² φ + 1]

Where φ is the angle of internal friction of the soil and Nq is the bearing capacity factor for the soil.

Meyerhof's Bearing Capacity Theory

Meyerhof's bearing capacity theory takes into account the shape and depth of the foundation, as well as the shear strength of the soil. According to this theory, the bearing capacity of purely cohesive soil can be calculated using the following formula:

Qc = cNc + qNq + 0.5γBNγ

Where Qc is the bearing capacity of the soil in kN/m², c is the cohesion of the soil in kN/m², q is the vertical stress at foundation level in kN/m², γ is the unit weight of the soil in kN/m³, B is the width of the foundation in meters, and Nγ is the bearing capacity factor for the soil.

The value of Nc and Nq can be determined using the same formulas as in Terzaghi's theory.

Skempton's Bearing Capacity Theory

Skempton's bearing capacity theory takes into account the effect of the water table on the soil. According to this theory, the bearing capacity of purely cohesive soil can be calculated using the following formula:

Qc = cNc + (1 - B/Bf)γDNγ

Where Qc is the bearing capacity of the soil in kN/m², c is the cohesion of the soil in kN/m², B is the width of the foundation in meters, Bf is the width of the foundation at the level of the water table in meters, γ is the unit weight of the soil in kN/m³, D is the depth of the foundation in meters, and Nγ is the bearing capacity factor for the soil.

The value of Nc can be determined using the same formula as in Terzaghi's theory, and the value of Nγ can be determined using the following formula:

Nγ = [(Nq - 1) / (tan φ)] + 2

Where φ is the angle of internal friction of the soil and Nq is the bearing capacity factor for the soil.

In conclusion, the bearing capacity of purely cohesive soil can be determined using various theories and formulas, which take into account different factors such as the shear strength of the soil, the shape and depth of the foundation, and the effect of the water table on the soil.