What is one assumption regarding the resultant force in Rankine's Theory?

In Rankine's Theory, one key assumption is that the resultant force is parallel to the backfill slope. This theory analyzes the lateral earth pressure exerted on a retaining wall by soil. It posits that the pressure distribution in the soil behind the wall is linear and that the resultant lateral earth pressure acts at a particular angle in relation to the surface of the backfill.

By assuming that the resultant force is parallel to the backfill slope, Rankine's Theory simplifies the calculations needed for determining the stability and design of retaining structures. This alignment is a direct consequence of the failure plane that develops in the backfill soil and reflects the state of static equilibrium in a two-dimensional failure wedge model. The assumption aids in determining the magnitude and location of the resultant force, which is critical for ensuring the safety and effectiveness of the wall under various load conditions.

The other options refer to forces that do not align with the core assumptions of Rankine's Theory. For instance, the resultant force is not aligned with the wall or perpendicular to the soil in typical scenarios considered by this theory, nor does it vary based on the soil type, as the fundamental assumptions are based on homogeneous, isotropic conditions. Hence, focusing on the backfill slope as the basis for