Select the incorrect statement.

Answer: Option A

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.