Reinforced soil, or mechanically stabilized soil, is a construction technique that consists of soil that has been strengthened by tensile elements such as metal strips, geotextiles, or geogrids. In the 1960's the French Road Research Laboratory conducted extensive research to evaluate the beneficial effects of using reinforced soil as a construction technique. Results of the early work were well documented by Vidal . During the last thirty years many retaining walls and embankments were constructed all over the world using reinforced soil, and they have performed very well.
The metallic strips that are used for reinforced soil are usually galvanized steel strips. However, the galvanized steel strips are subject to corrosion at the rate of about 0.025 to 0.05 mm/year. Hence, depending on the projected service life of a given structure, allowances must be made for the rate of corrosion.
Geotextiles and geogrids are non-biodegradable materials. They are made from petroleum products such as polyester, polyethylene, and polypropylene.
Geotextiles perform four major functions: (a) allow drainage from the soil; (b) keep the soil layers separated; (c) provide reinforcement to the soil; and (d) allow free seepage from one layer of soil to the other; however, they protect fine-grained soil from being washed into the coarse-grained soil.
Geogrids are made by tensile drawing of polymer materials such as poly-ethylene and polypropylene. They are relatively stiff material compared to geo-textiles. They have large apertures which allow interlocking with surrounding soil to perform the functions of reinforcement and/or segregation. Commercially available geogrids are generally of two types—uniaxial and biaxial.
Figures 7.1a and 7.1b show these two types of geogrids. Geogrids are manufactured so that the open areas of the grids are greater than 50% of the total area. They develop reinforcing strength at low strain levels such as 2%.
FIGURE 7.1 Geogrids: (a) Uniaxial; (b) Biaxial