ANS:D - 6 m

Cantilever retaining walls are a type of structural system commonly used in civil engineering to retain soil or other materials. They are characterized by a stem (or vertical portion) that extends below the ground surface and a base (or footing) that extends away from the retained soil. The stem of the wall acts as a cantilever, with the soil pressure exerted on it resisting the retained soil's tendency to collapse. The maximum height at which a cantilever retaining wall can be safely used depends on several factors:

1. Soil Properties: The properties of the retained soil, including its type, cohesion, angle of internal friction, and density, influence the lateral pressure exerted on the wall. Cohesive soils and densely compacted granular soils can exert higher lateral pressures, affecting the stability of the wall.
2. Wall Design and Geometry: The design of the retaining wall, including its height, thickness, reinforcement, and toe length (the distance between the wall base and the front face of the wall), plays a crucial role in its stability and load-bearing capacity. Taller walls require thicker sections and additional reinforcement to resist bending moments and shear forces.
3. Loading Conditions: The loads imposed on the retaining wall, such as surcharge loads from adjacent structures or traffic, hydrostatic pressure from groundwater, and seismic forces, can affect its stability and structural performance. Proper consideration of these loads is essential in the design process.
4. Global Stability: Beyond resisting overturning and sliding, the overall global stability of the wall system must be considered, including factors such as foundation bearing capacity, settlement, and potential for slope instability.

While cantilever retaining walls can be designed for various heights depending on site-specific conditions and engineering requirements, a maximum height of around 6 meters may be considered as a practical limit in many cases. This limitation is based on common engineering guidelines and standards, which aim to ensure adequate safety and stability while balancing factors such as cost and constructability. For taller retaining walls exceeding this height limitation, alternative solutions such as reinforced soil walls, anchored walls, or the incorporation of additional stabilizing elements like counterforts or buttresses may be necessary to provide the required stability and performance. Therefore, the option "6 m" as a maximum height for cantilever retaining walls reflects a practical guideline based on engineering considerations and industry standards, but it's essential to evaluate each project's specific requirements and conditions to determine the appropriate wall design and height.