Circular slip (Swedish circle) concept: The shearing force acting along the slices of a curved surface of slippage in an earth slope tends to make the soil mass slide in which direction?

Introduction / Context:
Limit equilibrium analysis of slopes often assumes a circular slip surface (Swedish or Fellenius method). The potential sliding mass above this surface is subdivided into vertical slices. The mobilized shear along the base of each slice drives movement toward the slope face and toe. Understanding the direction of this driving component is fundamental to setting up force and moment equilibrium.

Given Data / Assumptions:

• Curved (approximately circular) slip surface with a center of rotation.
• Vertical slices used to compute interslice forces and base reactions.
• Smooth slip surface with mobilized shear aligned tangentially.
• Weight of slices acts vertically; driving component acts downslope.

Concept / Approach:

At every point on the circular arc, the base shear acts in the tangential direction consistent with potential motion of the sliding mass. For typical concave-up slip circles in an earth embankment, the resultant effect of gravity on the mass produces a tangential downslope component that pushes the mass toward the open face, culminating at the toe region. Thus, the net tendency is to slide downward toward the toe, which is why resisting moments are commonly summed about a point near the circle center while checking factor of safety.

Step-by-Step Solution:

Verification / Alternative check:

In Fellenius/Ordinary method of slices, driving moment is the sum of W * r * sin α over all slices (α being the local inclination), which is maximum near the toe side of the slip surface.

Why Other Options Are Wrong:

(a) and (c) suggest centrally directed or upward motion, inconsistent with gravity-driven sliding. (d) and (e) do not represent the tangential downslope nature of the driving shear.

Common Pitfalls:

Confusing local shear direction with the radial line to the circle center; assuming uniform motion everywhere rather than net downslope tendency toward the toe.

Final Answer:

Down at the toe of the slope