How does a cantilever retaining wall work?
A cantilever retaining wall works by using its T-shaped structure and the weight of the soil behind it to resist the lateral pressure of the earth.
A cantilever retaining wall is typically constructed from reinforced concrete and consists of two main parts:
- The Stem: This is the vertical section of the wall that stands up to hold back the soil. It acts like a cantilever beam that is fixed at the base.
- The Base Slab (or Footing): This is the horizontal section that the stem sits on. A critical aspect is that the base slab extends under the backfill soil it is retaining.
The Working Principle
The primary function of a cantilever retaining wall is to counteract the significant lateral earth pressure exerted by the soil behind it, which constantly tries to push the wall over. Here's how it achieves this stability:
- Lateral Earth Pressure: The soil retained behind the wall exerts a horizontal force on the stem. This force increases with depth, creating an overturning moment that attempts to rotate the wall about its toe (the front edge of the base slab).
- Stem Action: The stem, being fixed to the robust base slab, bends like a cantilever beam under this lateral load. Reinforcement within the concrete stem handles the bending stresses.
- Role of the Base Slab and Backfill: This is where the cantilever design is particularly effective. The base slab extends under a substantial portion of the soil mass it is retaining.
- The weight of the soil sitting directly on top of the base slab provides a significant vertical load.
- According to the reference, "The vertical stress behind the wall is transferred onto the foundation [via the base slab], preventing toppling due to lateral earth pressure from the same soil mass, allowing cantilever walls to stand unobstructed."
- This vertical load (from the backfill's weight) pressing down on the base slab, combined with the wall's own weight, creates a large stabilizing moment that directly opposes the overturning moment from the lateral earth pressure. The base slab effectively uses the weight of the problem (the soil) to provide the solution (stability).
- Sliding Resistance: The weight of the soil on the base slab also increases the friction between the base slab and the ground beneath it, helping to resist sliding forward. A shear key (a small projection) is often added under the base slab to further enhance sliding resistance.
In essence, a cantilever retaining wall leverages its shape and the weight of the soil it holds back to create a large counteracting force and moment, keeping the wall upright and stable against the lateral pressure.
Key Forces at Play
- Lateral Earth Pressure: Horizontal force pushing on the stem.
- Weight of Soil on Base Slab: Downward vertical force providing stability.
- Weight of the Wall: Downward vertical force contributing to stability.
- Friction/Passive Pressure: Resistance at the base and toe preventing sliding.
By strategically using the weight of the retained soil on the base slab, the cantilever design efficiently transforms the potential instability created by lateral pressure into a source of stability.
