Contractors should be aware that water is a major factor in the failure of mse walls. Major storm events where flash flooding is involved can cause damage in multiple ways.
- Washing away the soil in front of the wall.
- Washing away the soil behind the wall.
- Saturation of the backfill soil over time.
Toe restraint of an mse wall is important because it has a limiting effect on displacements of the modular block facing. Washing away the toe and exposing the leveling pad puts the wall at risk. It is important to include enough embedment and use toe protection.
Directing water over the mse walls is not uncommon, but not preferred. Typically a low permeable cap (8” min) is proposed on top of the wall to limit infiltration. An additional measure is to direct the water to drain away from the face or around the ends of the wall using a swale. The use of a swale is especially needed if a large volume of water from a parking lot or hillside is expected to pass over the wall. IN ANY CASE, directing water to one point behind the wall is NEVER a good idea and could result in the backfill soils washing away. The geogrid does not work without soil between it and the facing will fail.
Saturation of the backfill over time can occur from seepage or from water from ponds in front of the wall. Saturated soil puts more pressure on the wall face than anticipated. Drains are recommended to provide a path for water to outlet through the wall face. Also, a free draining reinforced backfill, in the form of chimney drains, blanket drains, or the entire reinforced zone up to the high water level is recommended.
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