Retaining structures — up to what retained height are cantilever (stem) reinforced concrete retaining walls generally considered safe and economical without counterforts?

Introduction / Context:
Cantilever reinforced concrete retaining walls (with a base slab and a vertical stem) are popular for moderate heights. As retained height increases, bending moments and shear grow rapidly, and counterfort or buttress walls become more economical and structurally efficient.

Given Data / Assumptions:

• Conventional site conditions and normal backfill properties.
• No special ground anchors or relief measures.
• Seeking a common practical height limit before counterforts are considered.

Concept / Approach:

Beyond about 6 m retained height, member sizes and reinforcement for a simple cantilever wall become heavy, sliding and overturning checks become more demanding, and base width increases significantly. Counterforts reduce the span of the stem and redistribute forces, improving economy for higher walls.

Step-by-Step Solution:

Verification / Alternative check (if short method exists):

Preliminary sizing exercises show section demands beyond ~6 m become uneconomical versus counterfort solutions, confirming the rule of thumb.

Why Other Options Are Wrong:

3–5 m are conservative but do not represent the typical upper economical limit; 8 m is generally high for a pure cantilever without special design measures.

Common Pitfalls (misconceptions, mistakes):

Ignoring sliding/overturning checks; neglecting drainage and backfill surcharge; relying solely on stem thickness instead of adopting counterforts when appropriate.

Final Answer:

6 m