In Bishop's Simplified Method for slope stability analysis, what is the critical assumption made about the forces on the sides of the slices?

In Bishop's Simplified Method for slope stability analysis, the critical assumption made about the forces on the sides of the slices is that the resultant inter-slice forces are horizontal. This means that any forces acting between adjacent vertical slices, which are used to divide the potential failure mass, are assumed to have no vertical component. While actual inter-slice forces include both normal forces, which are perpendicular to the slice interface, and shear forces, which are parallel to the slice interface, Bishop's Simplified Method simplifies these by assuming their combined resultant force acts purely in the horizontal direction. This simplification is crucial because it reduces the number of unknown variables in the equilibrium equations, making the problem statically determinate and solvable. By ignoring the vertical component of the inter-slice forces, the method can satisfy overall moment equilibrium for the entire sliding mass about a chosen center of rotation, which is essential for calculating the factor of safety. This assumption simplifies the calculations significantly compared to more rigorous methods that consider both horizontal and vertical inter-slice force components, leading to a more tractable solution while generally providing a reasonable and conservative estimate for the factor of safety, particularly for circular failure surfaces.