Government agencies, landowners and property developers increasingly are turning to soil nail wall solutions from structural and geotechnical engineers to construct retaining walls with lower expense and a higher level of aesthetic appeal. Cost savings vary depending on a wall’s height – the higher, the better.
A soil nail wall generally consists of a steel tendon, similar to a reinforcement bar, that is inserted into a drilled hole measuring about six inches in diameter and then surrounded by cement grout. Once the cement grout has set and the soil nails are integral with the earth behind the wall, one or two layers of reinforced shotcrete are applied to the excavated face to form one uniform retaining wall. The tendons can be anywhere from 15 feet to 50 feet in length depending on the overall height of the wall and the geological conditions of the project area.
The strength of a soil nail wall comes from the friction interaction between the tendon/cement grout and the retained soil which in turn holds the reinforced shotcrete wall in place. The Pennsylvania Turnpike Commission, U.S. Army Corps of Engineers, energy companies, railroads, utilities and developers, among others, have utilized this seemingly simple but innovative technology to add strength to slopes to prevent slides and erosion and degradation of weather-sensitive bedrock.
“Soil nail walls produce a solid, stable block of soil,” says LDG geotechnical project engineer Steven Lowden. One of LDG’s top current soil nail wall projects is for a natural gas power plant site in Pennsylvania. That wall measures 700 feet long by 40 feet high and has required the installation of 487 soil nails with lengths ranging from 20 feet to 45 feet.
Previously, soil nail walls were considered a temporary solution to provide support for more conventional types of retaining walls such as reinforced concrete cantilever retaining walls. Today, Lowden says, they’re accepted as long-term solutions at a fraction of the cost. One of the other benefits of soil nail walls is their visual enhancement through a variety of facings, paints and dyes. Often, they are finished to resemble rock strata along a highway, thus blending nicely into the surrounding topography.
“These walls reduce the need to excavate higher volumes of soil necessary for many other types of walls such as gravity, cantilever or mechanically stabilized earth walls,” says Kevin Altman Jr., LDG structural design engineer. “They also limit the headaches and costs of right-of-way takes, permitting and disturbances for neighboring properties and structures. Soil nail walls are built from the top-down which adds to the efficiency and ease of installation. The economics work best for walls of 15 feet or higher.”
Lowden has 28 years of experience in geotechnical projects, while Altman has nine years of structural design experience. For these projects, LDG provides engineering design, construction cost analysis, client coordination, preliminary and final structural design, quality control of geotechnical and construction inspection tasks, as well as construction consultation to handle any issues that may arise during the soil nail wall construction.