Retaining walls can be large or small, but their purpose remains the same. They are meant to hold back soil from a building or structure, but they can also provide aesthetic and landscape design functions as well.
Retaining walls can be constructed from railroad ties, treated timber, wall stone, natural stone, brick, or concrete block. Although wood is an inexpensive option, its lifespan is rather short and makes it a poor choice of material. Stone may seem like the better alternative, but this material can be extremely expensive and labor intensive, and the maintenance can become a headache as the stone erodes and rodents and weeds move in. Another option is cast-in place or mortared retaining walls. This unfortunately is also not much better, as these structures are rigid and do not move and flex with climate changes, earthquakes, and other natural forces. They are also equally expensive to build.
Truly the best material for a retaining wall is concrete, which was used on the retaining wall built at our Whittaker’s Mill project recently.
Precast concrete has many advantages over other materials. For one, the strength of precast actually increases over time, while other materials deteriorate, experiences stress, lose strength, or are unable to withstand impact. It is an extremely durable material, with easy installation and overall great aesthetics, as it is visually appealing and can be stained any natural color. Perhaps its greatest advantage over competing materials – with the exception of wood – is that it is nontoxic and environmentally safe.
Alfred Yee (1925-2017), an accomplished engineer from Hawaii and a pioneer in the design of prestressed precast concrete structures, wrote for PCI Journal in 2001:
For the past half century, precast/prestressed concrete construction has been marketed on the basis of savings in materials and labor, improved quality of product and workmanship, and speed in construction. For purely economic reasons, contractors knowledgeable in precast technology have frequently relied on this method of construction with a high degree of success. In recent years, however, precast concrete technology has taken an important new perspective in terms of its impact on social and environmental issues.
Riprap is another environmentally friendly tool for reshaping and restructuring landscapes, and once our retaining wall was completed in Whittaker’s Mill, we installed some around it for that very reason.
Historically, riprap was the standard method of protection against wave damage along shorelines. In the 1982 publication Rock Riprap Design Methods and Their Applicability to Long-Term Protection of Uranium Mill Tailings Impoundments, W. H. Walters writes:
Wave erosion can be severe along unprotected shoreline embankments, particularly for material containing some quantity of sand. The design high water is determined by the mean sea level elevation and the maximum tidal range (Figure 3.2). The height of the riprap protection above design high water and the sizing of the stone usually are determined by estimating a design wave height. Although the riprap surface is rough, which tends to reduce wave runup on the slope face, some freeboard must be allowed to prevent continual overtopping of the rock layer.
However, riprap is used on many projects to avoid soil erosion; not simply along shorelines.
Overall, the use of retaining walls and riprap is an environmentally-conscious method of reshaping landscapes and creating structure for vulnerable soil while limiting degradation from storms and other damaging forces.
References
Walters, W. H. (1982). Rock riprap design methods and their applicability to long-term protection of uranium mill tailings impoundments (No. NUREG/CR–2684). Pacific Northwest Lab.
Yee, A. A. (2001). Social and environmental benefits of precast concrete technology. PCI Journal, 46(3).
The Importance of Bioretention
