Our June 15 Soils Matter blog post described the various types of retaining walls and how they work.
When not built properly, retaining walls may cave under the pressure of the materials they are supporting. This can occur gradually, which allows you time to fix the wall yourself or call for assistance. Although it rarely occurs, retaining walls may collapse rapidly under the lateral forces.
A retaining wall failure could be a complete collapse of the wall. But there may be signs that the wall is not functioning properly that you can see in advance. These signs may lead to weakened walls and seepage of the retained material or collapse. Preventing your wall from failing can be done by addressing common failure issues:
- Saturated soils/backfill: Water can exert significant pressure on the wall. Drainage tiles or channels, grading the site, and using a backfill material that drains well can alleviate excess water build-up.
- Poor engineering: Design flaws should almost never happen. Always choose a reputable, certified and trained contractor to help design and construct your wall to the specific conditions at the site.
- Foundation settlement: Properly compact the soil underneath the wall prior to building. This will prevent settlement that can lead to your wall cracking or collapsing.
Several factors may cause a retaining wall to fail. Often they result from poor engineering or not understanding the conditions at the location.1 We recommend inspecting your wall at least once a year to make sure that it is not starting to fail. A spring inspection will allow you to determine if your wall is capable of withstanding snowmelt and spring rainstorms. You should look to see if any of the blocks or other wall materials are sticking out. Do you see a lot of sediment at the base of the wall? If so, you may have a drainage problem. A yearly inspection will help keep any problems minor.
When you do have a failure in your retaining walls, there are a few approaches that may allow you to avoid excavating the wall.
- Regrade the area maintained by the wall to redirect water flow away from the wall. This will reduce some of the water pressure that may be building up behind the wall.
- Drill additional weep holes into the wall to allow for increased surface drainage.
- Reduce the height of the retained material by regrading. Sometimes changing the landscape design is an acceptable method to an allowable limit based on the wall design.
- Transfer some of the shear force at the location where the wall connects with the ground. This increases the overall strength of the wall. There are two methods:
- Extending the footing at the base; or,
- Placing concrete to thicken the base.
If your wall failure is too great to be fixed by one of the methods above, you may need to excavate. How much will depend on the extent of the failure. Inserting tiebacks will secure the wall and add additional support.1 However, this will require additional planning and could require extensive excavation to secure the tiebacks in the correct location. Tiebacks add strength to retaining walls. Adding a gravel bed behind and beneath the wall or perforated drain tiles lining the base of the wall can substantially improve drainage. This reduces trapped water and freezing behind the wall that can exert pressure, causing failure.
The best way to prevent a retaining wall failure is to design the wall for the site conditions, taking into account the terrain, climate, and soil properties, so that the wall will be able to perform its function safely. Rebuilding and correcting the failures is always an option. But it may be costly and require excavation. Typically, the wall can be salvaged and starting over rarely occurs. When rebuilt or fortified, and designed correctly, retaining walls can have a long life span for improving the aesthetic appeal and functionality on your site.
Answered by Christina Hebb, Duraroot
- For more information about the conditions at your retaining wall site, and structural elements that add strength, visit our first retaining wall post from June 1st.