ANS:D - all the above.

When constructing structures like dams, embankments, or retaining walls, ensuring stability against sliding is crucial to prevent catastrophic failure and maintain structural integrity. If the factor of safety against sliding is found to be less than 1.5, additional measures are required to enhance stability and prevent potential sliding of the structure. One such measure involves constructing a portion of the slab downwards at the end of the heel slab. This construction technique serves to increase the resistance against sliding forces and improve overall stability. Here's a detailed explanation of the terms associated with this construction technique:

1. Key:
   • A key is a structural feature constructed at the toe or downstream end of a dam or embankment to resist sliding. It is typically formed by extending the concrete slab downwards into a keyway excavated into the foundation material.
   • The purpose of a key is to increase the effective weight of the structure and provide additional frictional resistance against sliding forces. By interlocking with the foundation material, the key helps anchor the structure in place and prevent downstream movement.
   • Keys are essential components in the design of dams and embankments, where stability against sliding is critical for long-term performance and safety.
2. Cut-off Wall:
   • A cut-off wall, also known as a seepage barrier or impermeable barrier, is a vertical or inclined barrier constructed below ground level to prevent the seepage of water or the movement of soil.
   • In the context of enhancing stability against sliding, a cut-off wall may be installed to intercept potential sliding surfaces and prevent the migration of water or soil that could compromise the stability of the structure.
   • By creating a barrier that extends into stable soil or rock layers, a cut-off wall effectively blocks the path of sliding forces and reinforces the foundation of the structure.
3. Rib:
   • In the context of stability against sliding, a rib refers to a structural element incorporated into the foundation or base of a structure to provide additional support and resistance against lateral forces.
   • Ribs are commonly used in bridge piers, retaining walls, and other structures where stability is a concern. They may be constructed as part of the foundation or added later during rehabilitation or reinforcement projects.
   • By increasing the contact area with the foundation material and providing additional bearing capacity, ribs help distribute loads more evenly and resist sliding forces more effectively.

In summary, constructing a portion of the slab downwards at the end of the heel slab to improve stability against sliding involves incorporating features such as keys, cut-off walls, or ribs. These structural elements serve to increase resistance against sliding forces, enhance frictional stability, and ensure the long-term performance and safety of the structure.