Soil Mechanics And Foundation MCQ Questions & Answers | Civil Engineering

The correct answer is Option D: Unconfined compression test can be carried out on all types of soils.
Here's why:
Option A: In a direct shear box test, the plane of shear failure is predetermined - This is correct. The shear force is applied along a specific horizontal plane, so the failure is forced to occur along that plane.
Option B: Better control is achieved on the drainage of the soil in a tri-axial compression test - This is also correct. In a triaxial test, you can control whether the soil is allowed to drain or not, giving you better control over drainage conditions compared to, say, a direct shear test.
Option C: Stress distribution on the failure plane in the case of tri-axial compression test is uniform - This is correct. Triaxial test are designed to applied uniform stresses.
Option D: Unconfined compression test can be carried out on all types of soils - This is incorrect. An unconfined compression test is a special case of a triaxial test where the confining pressure is zero. It's only suitable for cohesive soils (like clays) that can stand on their own without lateral support. Granular soils (like sands) and very loose soils will collapse under their own weight in an unconfined state, so you can't perform this test on them.

The bearing capacity of soil is influenced by various factors:

Option A: Bearing capacity of a soil depends upon the amount and direction of load. This is correct because different loads and directions can affect the soil's ability to support structures.

Option B: Bearing capacity of a soil depends on the type of soil. This is correct because different soil types have different strength and stability characteristics.

Option C: Bearing capacity of a soil depends upon shape and size of footing. This is correct because the distribution of load through different shapes and sizes of footings impacts the soil's capacity.

Option D: Bearing capacity of a soil is independent of rate of loading. This is incorrect because the rate of loading can affect the soil's strength and stability, influencing its bearing capacity.

Therefore, the incorrect statement is Option D: Bearing capacity of a soil is independent of rate of loading.

Option A: Effective cohesion of a soil can never have a negative value. This statement is incorrect. While effective cohesion typically has a positive value, it can be negative in certain conditions. For example, in unsaturated soils where negative pore pressure exists (matric suction), the effective cohesion can become negative. Therefore, it is not correct to say that effective cohesion can never be negative.

Option B: Effective angle of internal friction for coarse-grained soils is rarely below 30°. This statement is correct. Coarse-grained soils such as sand and gravel generally have an effective angle of internal friction that is above 30°. It would be rare to find coarse-grained soils with an angle of internal friction below this value, as the frictional resistance is typically quite high in such soils.

Option C: Effective angle of internal friction for a soil increases as state of compactness increases. This statement is correct. As the state of compactness increases, the soil particles interlock more tightly, making it harder to shear the soil. This results in an increase in the effective angle of internal friction, which reflects the improved resistance to shear forces.

Option D: Effective angle of internal friction is a complicated function of mineralogy and clay size content. This statement is correct. The effective angle of internal friction is indeed influenced by the mineralogy of the soil (the type and texture of the particles) and the clay size content. The properties of the soil particles and their interactions significantly affect the angle of internal friction.

Therefore, the correct answer is Option A: Effective cohesion of a soil can never have a negative value as it is the only incorrect statement in this case.