In Rankine's theory, what does the angle alpha represent?

In Rankine's theory, the angle alpha represents the angle of inclination of the wall. This theory is often used in geotechnical engineering to analyze earth pressures on retaining walls when the wall is either vertical or inclined. When a wall is inclined, the angle alpha gives insight into how the soil pressure reacts against the wall's surface.

The walls can be vertical (0 degrees) or can have varying inclinations, which impacts the distribution of lateral earth pressures. The relationship defined by Rankine’s theory assumes that the wall exerting lateral earth pressure is at an angle, and understanding this angle helps in calibrating the design to prevent failures due to excessive pressures.

In this context, the other options refer to different concepts associated with soil mechanics and earth pressure theories. The angle of friction between soil and the wall and the angles of passive and active earth pressure pertain to the behavior of soils under different loading conditions and the forces when the wall is moving outward or inward. However, in the context of this specific question, the angle of the wall’s inclination is the aspect represented by alpha.