Retaining wall sizing: For a retaining wall of overall height H that retains soil with surcharge, what is the usual preliminary proportion adopted for the total base slab width (B) before detailed stability checks?

Introduction / Context:
Preliminary sizing of cantilever retaining walls is commonly done using proportioning rules that are later refined by detailed checks for overturning, sliding, and allowable bearing pressure. When surcharge exists on the backfill, a wider base is typically required than for the no-surcharge case.

Given Data / Assumptions:

• Overall wall height = H.
• Soil retained with surcharge loading.
• Conventional cantilever RC retaining wall on spread footing.

Concept / Approach:
Rules of thumb often choose total base width B between 0.5 H and 0.7 H depending on soil properties and surcharge. With surcharge present, a value around 0.6 H is a widely used starting point that provides a reasonable lever arm for resisting overturning while keeping bearing pressures within limits after optimization.

Step-by-Step Solution:
Select B preliminarily as 0.6 H to account for surcharge effects.Divide B into toe and heel (e.g., toe ≈ B/3, heel ≈ 2B/3 for trial).Carry out stability and bearing checks; adjust B as needed.

Verification / Alternative check:
Experience-based design charts and example problems commonly converge to B in the 0.55 H–0.65 H range when surcharge is present, making 0.6 H a practical initial choice.

Why Other Options Are Wrong:

• 0.3 H, 0.4 H: Typically too small under surcharge; likely to fail overturning/bearing checks.
• 0.5 H: Common without surcharge; may be marginal with surcharge.
• 0.7 H: Safe but often uneconomical as a starting point.

Common Pitfalls:
Treating preliminary proportions as final; ignoring soil strength and groundwater can lead to unsafe or overly conservative designs.

Final Answer:
0.6 H