Final answer:
Water pressure within rock joints reduces the effective normal stress on the joint surfaces, which can decrease friction but does not directly increase or decrease cohesion. The correct answer to the question is C) Reduces the effective normal stress acting on the solid area of joint contact.
Step-by-step explanation:
When considering water pressure acting on a joint plane in a mass of rock, it does not increase cohesion or decrease cohesion directly. Instead, water pressure within rock joints influences the effective normal stress on the joint surfaces. The effective normal stress is the total normal stress minus the pore water pressure. When water pressure increases within a joint, it effectively 'pushes' the two sides of the joint apart slightly. This reduction in effective normal stress in turn can lead to a decrease in friction between the two surfaces, since friction is partially dependent on the normal stress pushing the two surfaces together. This concept is critical to understanding soil mechanics and rock stability, particularly in the context of engineering geology and civil engineering.
So, the correct answer to the question is C) Reduces the effective normal stress acting on the solid area of joint contact. This reduction in effective normal stress indirectly affects the stability of the joint by potentially decreasing the frictional resistance to sliding, which is a different concept from cohesion. Cohesion refers to the internal molecular attraction that holds a material together.
In reference to surface contact and friction, which provides context to our main question, it is important to note that the material friction coefficient typically remains constant unless the properties of the materials in contact change or there are other external factors at play. For example, water or other fluids can act as lubricants and reduce the coefficient of friction between two surfaces.