By Marilyn Schroeder

When property owners and managers are faced with the arduous task of replacing timber retaining walls, or installing a new retaining wall, they want to know what are the advantages, disadvantages, and pitfalls of the various options and which products are best for my project?

First, retaining walls need to be strong and they need to be resistant against a variety of environmental factors. They need to possess the strength necessary to support the soil and withstand the weight of the earth.

Then the aesthetics and functionality are two characteristics to look for.

Timber walls were installed frequently during in the 1960’s thru the 1980’s, as the only options during this time were timber or poured concrete, poured concrete being cost prohibitive, most of the time. During the 1980’s boulder retaining walls started to be used, as contractors learned how to use them. They quickly became a viable alternative to the timbers, especially in residential use, as they are eco friendly, they are considered timeless in both appealing looks and durability and the cost is not prohibitive. In the late 1980’s segmental concrete modular blocks were designed to take the place of poured-in-place concrete retaining walls, at one-third to one-half the cost of poured-in-place walls. Segmental block walls are still in their infancy, being only 20 years old.

Timber retaining walls

Older timber walls are not only unattractive; they may be rotting and losing structural integrity. There is also an environmental concern, as they may be leeching hazardous chemicals, if they were treated with creosote, which has now been banned by the environmental protection agency. a new chemical compound, known as ACQ reduces the risk. Wood compared to other materials, is not as sturdy nor is it as durable and wood can easily deteriorate under constant rain and snow and they decompose over time. Even though they are less expensive, they have a relatively short life of 10-20 years and are not an environmentally safe as the other alternatives. They are still being used in some situations mainly because of their relatively low cost. The cost for timber walls is $7.00-$10.00 per square foot.

Boulder (field stone) retaining walls

Boulders are highly effective. Their sheer weight makes them great gravity retaining wall as they are installed to lean towards the soil and counteract the effects of gravity. There are few materials sturdier than boulders, and there are even rarer materials that can provide the elegant texture, the smooth colors and the unique ambiance that stones can bring to any environment. Boulder walls are mortar less and this is an advantage as a built in “drainage system” with water able to seep thru the cracks between the stones. Landscape fabric is placed behind the wall to prevent soil from washing through. Another advantage is that no footings are needed beneath the frost line and no compactable base material is required under the boulder walls, thus a cost savings over the alternatives. Boulder walls can be built higher than most are aware. A 20 foot high wall out of boulders is very feasible, however, as in modular block walls, any wall over 4 feet high, needs to be engineered. The structural engineer will determine the size of the boulders needed to be able to sustain the earth at the height the wall is to be built. The boulders should be sized according to the size of the wall for both strength and aesthetic value. Field stone boulders are very popular alternative in Minnesota, as we have an ample supply from the glacier deposits thru central Minnesota. The cost for boulder retaining walls is $19.50-$25.00 per square foot. The price will be higher if granite, limestone or other types of natural stone is used versus field stone. The price will vary also by the type of equipment the site requires to be used for the installation and also the size of the boulders used.

Modular block retaining walls

Segmental blocks are made from concrete. The life of block walls is estimated to be 75 years, due to the geo-grid that is required behind it. ….the real retaining wall is not the facing block (which is merely a veneer) but rather the structure of
geo-rid and compacted soil behind it. The blocks are designed to be set on a compacted gravel base, providing a foundation this allows them to flex slightly with ground movement and still remain strong. However, because of the horizontal appearance of the blocks, it shows fault line cracks and stair-step cracks from movement or settling in the soil, much more than a natural field stone boulder wall. They are available in a wide variety of styles in two basic sizes. They now come in a variety of shapes, textures and colors to choose from, making them look less cold and commercial. They are a good alternative, depending on the look, the particular situation and the location. In a narrow and tight space sometimes the best solution is a block wall, as they are installed by workers physically placing the blocks on top of each other using no machines. However, as long as there is adequate room for a bobcat to turn around or a backhoe can reach from the top, a boulder wall can be used. Segmental blocks are made from concrete, which is susceptible to damage by road salt or salt used for melting snow, which will erode over time. There are some cities, because of potential deterioration, are electing to use boulders for their projects versus modular block. The cost for segmental modular block usually is $20.00-$35.00 per square foot and it could be much more if decorative block is chosen. Additional costs that need to be factored in are the extra excavation, gravel for the base and possible backfill that may be needed, which you do not have in the construction of a boulder wall.

Process

Before you start the process of preparing engineered plans and city permits, have an on-site inspection done by a company that has many years of experience in building the type of walls you need. Many times after our site inspections we find, with some re-grading & re-structuring you may be able to build in a better format, which could mean fewer walls, thus less costly. This would also be the time to correct any drainage issues. Other considerations to be aware of, is if you have plantings close to the top of where the wall will be installed, for instance with a 7 foot high block wall, it would require the soil to be dug back 7 feet from the face of the wall and that may disturb any plantings or they may have to be removed altogether. A boulder wall would be a good alternative in this situation, as no geo-grid is needed with boulders. If the retaining wall is close to a property line, boulders may be the best choice also, as the geo-grid (that is needed behind modular block walls) would be considered an infringement.

Warranty

Make sure you address the warranty issue. Minnesota only requires a one year warranty on boulder walls, so this should be a minimum you accept and we feel a company that is very experienced, should warranty their work for much longer. Modular block walls usually are warranted for 2 years.

Engineered walls

Engineered walls are any type of wall that exceeds the local building code requirements (usually over 4 feet high) and these need to be designed by a structural engineer, who will draft a design and specifications for the Contractor to follow and it will usually mean obtaining a city permit for the project.

Marilyn Schroeder has been involved in the construction trades since 1977 when she started her own home building and remodeling construction company and went on to be involved in the plumbing and kitchen and bath industries. Since 1994 she has been an owner of Creative Landscapes by Pro-Wall and has been involved on a daily basis in all phases of the Landscaping and Retaining Wall business. Her contacts in the building trades and her knowledge of the construction trades helped Pro-Wall grow from a small mom and pop organization to one of the largest installers of boulder retaining walls in the upper Midwest under her direction.