What is the role of active earth pressure in relation to a retaining wall?

Active earth pressure refers to the lateral pressure exerted by soil on a retaining wall when the wall moves slightly away from the soil. This movement allows the soil to expand or release tension. In the context of retaining walls, active earth pressure develops when the wall is allowed to move, typically as a result of factors like soil swelling, structure settlement, or thermal expansion.

Option C accurately demonstrates this principle by stating that active earth pressure is present behind the wall and it influences the soil in a way that creates tension in the remaining soil. When a retaining wall moves away from the soil, the soil behind it experiences reduced lateral pressure, leading to an extension or tension in that soil mass. This is crucial in the design of retaining walls, as it affects stability and load calculations.

The other options do not accurately describe the nature or role of active earth pressure. While the movement of the wall does relate to lateral forces, the specific function of active earth pressure is best characterized by its presence behind the wall and its effect on the tension in the soil. The understanding of this concept is fundamental for civil engineers when designing retaining structures to ensure they can effectively manage soil pressures.