Cantilever retaining wall without a heel slab: Which of the following thumb rules best applies to preliminary sizing when the wall is founded on firm support (e.g., rock) and only a toe slab projects on the earth face?

Difficulty: Medium

Correct Answer: Adopt the overall base slab width at roughly 0.7 times the total wall height as a starting value

Explanation:


Introduction / Context:
Preliminary proportioning of cantilever retaining walls uses practical thumb rules before detailed stability checks (overturning, sliding, bearing) and structural design. When there is no heel slab (e.g., on sound rock), sizing focuses on a toe slab and stem adequate for earth pressure and stability.



Given Data / Assumptions:

  • No heel slab; toe slab only.
  • Firm foundation conditions (e.g., rock) so settlement is small.
  • Rules are preliminary; final sizes come from full analysis.


Concept / Approach:
A common preliminary recommendation is to take the base slab width around 0.6 to 0.7 times the wall height H when starting design. The stem thickness is generally tapered (thicker at base where earth pressure is higher), and uniform thickness is rarely adopted because it is uneconomical. A fixed 10 cm increase of base thickness over stem is not a universal rule either; base thickness should follow structural demand.



Step-by-Step Solution:
Screen each statement against standard preliminary practice.(a) Uniform stem thickness: not a preferred practice; stems are usually tapered.(b) A flat 10 cm thicker base: not a universal sizing rule.(c) Base width ≈ 0.7H is a widely used starting point subject to checks.Therefore select option (c).



Verification / Alternative check:
Trial sections for retaining walls commonly start with base widths of about 0.5H to 0.7H depending on backfill and surcharge; final dimensions are then refined.



Why Other Options Are Wrong:

  • (a) Economically and structurally suboptimal; earth pressure reduces with height.
  • (b) Arbitrary fixed addition; thickness should follow bending and shear design.
  • (d) All of the above: incorrect because (a) and (b) are not reliable rules.
  • (e) None of these: incorrect because (c) is acceptable as a starting rule.


Common Pitfalls:
Using preliminary ratios as final design; ignoring uplift, sliding, and overturning checks; omitting toe key or shear friction where needed.



Final Answer:
Adopt the overall base slab width at roughly 0.7 times the total wall height as a starting value

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