Retaining Walls

FAILED OR FAILING RETAINING WALLS

Failed, or failing retaining walls can lead to property damage, or even cause serious injury.

Catastrophic Failure (walls tipping over completely) is unusual; partly because the process often progresses slowly, and corrective action is usually taken before the catastrophe can occur. However, this is not always the case, and this type of failure has been known to occur.

How does Catastrophic Failure occur?

This type of failure is most likely to occur when the retaining wall is poorly designed; having very small footings and no reinforcing steel.

During  a  period  of  prolonged  heavy  rains,  the soil behind the wall may soak up a great deal of water,   and become heavier and more fluid; increasing the force behind  the  wall.  Simultaneously,  the  ground  beneath, and in front of the wall will become soft and slippery.

This undermines the stability of the footing and offers little resistance to the tipping wall.

Prevention: Soils with more than a minimum of clay content (5% fines by weight) are generally water retentive, and should not be used as backfill. Instead, place gravel or clean sand behind the wall; with footing drains similar to common foundation footing drains.

Good practice is to separate the backfill from the native soil face of the excavation with geotextile fabric, often nothing more unusual than “weed mat.”

Freeze Damage

In Northern climates, the freeze-thaw cycle is the primary cause of retaining wall failure. Most retaining wall problems result from the expansion of wet soil when it freezes during cold weather.

This type of failure is still an indirect result of backfilling with water-retentive soil and /or the lack of good drainage; but it is frost action that causes the damage.

Frozen solid causes the wall to tip over.  This kind of damage often occurs over a period of years. With each freeze the soil expands and pushes the wall outward a little bit. Then, as the soil thaws, it settles into the space against the wall and  the cycle is set to begin again with the next frost.

This is sometimes referred to as “frost ratcheting”, as it progresses in small increments and never reverses itself.  With poured concrete walls, the entire wall often tips as a whole. In the case of block walls, or mortared stone walls, cracks and bulges may appear as sections of the wall tip.

Solution:  The best solution for frost damage is to use gravel or sand backfill, and footing drains. In some cases, it may be possible to use 2-inch extruded polystyrene foam insulation to help retain ground heat and thereby prevent freezing. This strategy is appropriate where a large amount of granular backfill cannot be accommodated, such as a tight site where excavation is limited. Consider foam insulation anywhere you don’t have room to place the backfill all the way to frost depth.

The forces exerted by freezing ground are too great to be resisted by any practical structure design. The only prevention is to eliminate the conditions which lead to freezing;  water and cold.

Shear failure is another relatively unusual condition. It occurs when the upper part of the wall manages to slide forward, without tipping, along a failed mortar line. Mortar is not an effective glue; it usually fails under a load, unless there is a large compressive force such as the weight of a house sitting on it.

Sometimes the slippage occurs between the footing and the soil beneath (usually with clay soils)in which case the shear failure is also called a slipping failure.

Prevention: The footing should always set 12 to 18 inches below grade, and compact any fill between the toe of the footing and the footing excavation.