Carl Brahe
In most places the earth is in constant motion. It usually moves too slowly for us to see until it reaches critical mass and slides to the lowest point. Places where the landscape is uneven in height like hills, cliffs and waterways can present challenges when building. Gravity will eventually pull higher ground to the lowest level, usually with the aid of water. To protect property from earth movement retaining walls are built from a variety of materials and designs.
Retaining walls control the movement of earth and water by creating a barrier, or wall, to stop rocks, dirt, mud and water from sliding to a lower level. Some retaining walls are built to hold back water in the soil. Most are built to allow water to drain from the soil and be channeled away from structure.
Basement walls are a type of retaining wall that prevents gravity and water flow from undermining the house. An improperly prepared foundation can allow tremendous forces of water and earth to push against basement walls. When basement walls fail the whole structure may collapse. Proper preparations of the ground being held back by a retaining wall can make the difference between a one season retaining wall and one that lasts the life of the structure. Most residential retaining walls are less than 4’ high and usually require no permits to build. In my experience as an inspector most are poorly built.
These retaining walls lean outward and by definition of a retaining wall have failed.
Basement walls are a type of retaining wall that prevents gravity and water flow from undermining the house. An improperly prepared foundation can allow tremendous forces of water and earth to push against basement walls. When basement walls fail the whole structure may collapse. Proper preparations of the ground being held back by a retaining wall can make the difference between a one season retaining wall and one that lasts the life of the structure. Most residential retaining walls are less than 4’ high and usually require no permits to build. In my experience as an inspector most are poorly built.
Expansive soil is very common. Some expansive soils can swell by 15% when wet. Most residential landscaping retaining walls try to directly control soil movement without regard to drainage or buffering the soil expansion with non-expansive soil. Trying to control forces that can break your house into pieces by a timber or piece of rock or masonry is futile. Proper preparation is absolutely necessary to prevent frequent repair and replacement. This can be especially costly for commercial properties.
This retaining wall is the foundation for a commercial building. Note the many drain holes. This wall is cracking from improper construction.
The soil facing the retaining wall should be replaced with a porous material that will allow water to flow through rather than building up behind the wall adding stress to the structure. The porous material also serves to absorb pressure from soil expansion rather than pushing against the wall itself. A drain is placed underneath the porous buffer to collect drainage flow and direct it to a proper exit point.
Building a common backyard retaining wall may cost around $50/sq ft. This includes using common materials like timbers, rocks, masonry, concrete or wire cages filled with rock. These walls usually last 12-15 years before needing total replacement. If built properly they may last much longer, if poorly built they will fail sooner. Replacing a wall requires removing the existing wall and will add substantially to the cost.
Retaining walls are built with an inward slant toward the material being held back. When they are plumb, or lean outward, they have technically already failed. Collapse is considered just a matter of time at this point. The most common areas to fail in retaining walls are the corners. These are usually the weakest spots. Most residential retaining walls are built without reinforcement to tie the corners together.
In most cases where a retaining wall fails you can easily identify the cause by digging a small amount of dirt from behind the wall. If the soil forms into a ball it is probably expansive soil in direct contact with wall. A wall built this way is certain to fail. Consider the cost of replacing poorly built retaining walls when buying a property.
Window wells are another kind of retaining walls often overlooked. Improper soil preparation along with poor drainage doom these window wells to a short life. This window well has been repaired where the bottom was rusted and bulged. This window well is only 7 years old and needs to be replaced. The rust spots show patterns of corrosion from water trapped behind the window well.
Drainage runs toward this window well hidden by landscape rock. This is a landscape defect that I see often. I am always puzzled by the reasons. Is it from ignorance, laziness or passive aggressive behavior of poorly paid labor? When looking at a property with landscape rock or bark check next to the foundation and other structure like sidewalks and driveways for drainage problems covered up rather than fixed.
Retaining walls can add unforeseen expense if poorly built. Proper preparation and construction should never be assumed with retaining walls that are often built by homeowners or unskilled landscapers. No permits required means no knowledge or experience required.