Installing Difficult Retaining Walls

1 Mar, 2006 By: Cynthia Kinman, Gary Kinman LDB Solutions

If retaining walls are not difficult to install, then you're probably not retaining anything. Today's landscape design-build contractors install retaining walls for two reasons: for structural reasons to retain slopes and modify the site, or for aesthetic reasons.

Adding structure

Gravity is why we need retaining walls. Typical gravity structures, such as rocks, walls or crib walls, rely on their own weight to counter slope movement. Cantilevered structures are anchored to stable soils that underlie slide material. Other types of retaining structures include tie-back walls and reinforced earth embankments. Retaining structures can be placed at the base, middle or near the top of a slope, depending on the nature of the potential landslide and the type of wall.

THIS SITE had a severe slope that didn't allow for construction accessibility. A temporary road was built so cranes, dozers and concrete trucks could get in. This was the slope previous to the temporary road being built.

If you really need a retaining wall, then the site itself will be difficult to work on. Steep grades or drop-offs make site access hazardous, and may require a temporary plateau to be built up with soils so equipment can get to the area and maneuver. Don't underestimate how lack of site preparation will exponentially impact your profits and quality of the work your installation crew can do. Make the work area easy and safe to work in.

Good engineering is critical

Skilled installation and proper design is paramount. Retaining wall structures can be expensive, not from the materials and labor for installation, but because of the preparation and planning. Most landscape contractors cost out a job based on what their industry supplier suggests materials cost, plus labor. Some suppliers will even help you calculate how much of their product you will need for the wall based on what your dimensions are. Your supplier will not be held liable for the construction warranty, however. Many suppliers can become a fair-weather friend the minute a project goes sour and their product was used on the installation.

THIS RETAINING WALL had a high level of difficulty because the backyard depth offered limited access. The extreme slope, and the existing mature white pine trees that the client requested remain, required the patio to be floated over the root systems so as not to compact the soil and eventually kill them.

Good engineers and competent landscape architects can be your best allies. Their fees may be the best investment when designing retaining walls. They are a reliable source for making good decisions and recommending what materials to use, because they are not making a commission on a product sale. Their fee the first couple of times may seem like unnecessary expenses to a project, but what you will learn from their expertise you will carry with you on every wall you build after that. Remember, failures of retaining walls are relatively common and repairs can be far more expensive than the original work.

Dealing with drainage

Problems can occur when water builds up behind improperly drained structures, putting excessive loads on the wall or weakening it. Drainage systems are relatively simple. The goal is to prevent the build-up of water accumulating behind the wall. Areas where freeze/thaw occurs can be catastrophic if water freezes and expands between the structure and a frozen hill of soil. Something has to give, and it will be the wall.

There are many standard drainage systems that can be found in manuals, books and online. You don't have to reinvent drainage systems. They are already designed — and sometimes prepackaged for the specific job you are doing.

Enhancing the site

Designing enhancement walls can be just as important as the structural needs of retaining walls, but for different reasons. Enhancement walls can offer benefits to the site or user just as much as structural retaining walls can secure the site's use.

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