Introduction:
In the design of a retaining wall, a shear key is a crucial element that helps to enhance the sliding resistance of the wall. It is designed to resist the horizontal forces acting on the wall and prevent its movement.

Explanation:
The functions of a shear key in the design of a retaining wall are as follows:

1. Increasing the sliding resistance:
- The primary function of a shear key is to increase the sliding resistance of the retaining wall.
- By providing a horizontal key or projection at the base of the wall, the shear key increases the frictional resistance between the wall and the foundation.
- This helps to prevent the wall from sliding horizontally, especially when there are significant lateral forces acting on the wall, such as soil pressure.

2. Preventing horizontal movement:
- The shear key acts as a mechanical interlock between the retaining wall and the foundation.
- It helps to prevent the horizontal movement or sliding of the wall by providing a positive connection between the two.
- This is particularly important in situations where the soil pressure or other external forces can cause the wall to move horizontally.

3. Enhancing stability:
- By increasing the sliding resistance and preventing horizontal movement, the shear key contributes to the overall stability of the retaining wall.
- It helps to ensure that the wall remains in its intended position and does not experience excessive deformation or failure.

4. Reducing the risk of overturning:
- The shear key also helps to reduce the risk of overturning of the retaining wall.
- By increasing the sliding resistance and providing a positive connection between the wall and the foundation, it helps to counteract the overturning moments generated by the lateral forces acting on the wall.

5. Increasing durability:
- Another advantage of incorporating a shear key in the design of a retaining wall is that it enhances the durability of the structure.
- By preventing horizontal movement and reducing the risk of failure, the shear key helps to ensure that the wall remains stable and intact over its design life.

In conclusion, the function of a shear key in the design of a retaining wall is primarily to increase the sliding resistance, prevent horizontal movement, enhance stability, reduce the risk of overturning, and improve durability.