Episode 4 – Mechanically Stabilized Earth Walls: Importance of Proper Drainage Design

One of the most underestimated aspects of MSE wall performance is the drainage system. Poor drainage can compromise wall stability long before the geogrid or facing shows any sign of distress.

 Drainage design for MSE walls is not one-size-fits-all. It depends heavily on the groundwater level, seepage patterns, and proximity to water bodies.

 In practice, we encounter four main scenarios:

1. Case A: Groundwater well below the wall base. Minimal external drainage needed and internal drainage may be enough.

2. Case B: Water near the base, but no significant horizontal seepage. Blanket drain is typically advised.

3. Case C: Water above the base and/or horizontal seepage. Chimney drain at the interface becomes critical.

4. Case D: Wall adjacent to a water body with risk of flooding. Buoyancy and rapid drawdown must be addressed in the design.

 Good practice includes:

• Using clean, free-draining material (less than 5% fines) for backfill material and chimney and blanket drains.

• Designing blanket drains and geotextile filters with proper gradation compatibility.

• Ensuring all drains have a direct and reliable outlet.

• Considering global stability and potential slip surfaces when using synthetic drainage products.

 When water is mismanaged, the reinforced soil zone can lose effective weight, reduce friction between soil and reinforcement, and destabilize the entire structure (especially during rapid drawdown).