The correct answer is: A. Effective cohesion of a soil can never have a negative value.
Cohesion is the property of a soil that causes it to stick together. It is caused by the attraction between the particles of the soil. The effective cohesion of a soil is the cohesion that remains after the water has been drained from the soil. The effective cohesion of a soil can be negative if the water pressure in the soil is greater than the effective stress.
The effective angle of internal friction is the angle between the major principal stress and the direction of the shear stress at failure. It is a measure of the resistance of a soil to shearing. The effective angle of internal friction is affected by the type of soil, the state of compaction, and the mineralogy and clay size content of the soil.
The effective angle of internal friction for coarse grained soils is rarely below 30°. This is because coarse grained soils have a high degree of interlocking between the particles. This interlocking makes it difficult for the particles to slide past each other, and so the soil has a high resistance to shearing.
The effective angle of internal friction for a soil increases as state of compactness increases. This is because as the soil is compacted, the particles are forced closer together. This makes it more difficult for the particles to slide past each other, and so the soil has a higher resistance to shearing.
The effective angle of internal friction is a complicated function of mineralogy and clay size content. This is because the mineralogy and clay size content of the soil affect the way in which the particles interact with each other. The effective angle of internal friction is also affected by the water content of the soil.