MECHANICAL STABILIZED EARTH WALLS
Mechanical stabilized earth (MSE) walls (Figure ) are used for a variety of retaining structures. Metal strips (Figure , geotextiles (Figure ), or geogrids (Figure ) reinforce the soil mass. Other names used are geosynthetics and geocomposities. Geotextile is any permeable textile material used in geotechnical applications. Geosynthetics are polymeric, planar materials (polypropylene, polyethylene, polyester, polyamide, and nylon). Geocomposite is a product made from a combination of geosynthetics.
A geogrid is a polymeric product formed by joining intersecting ribs. Geotextiles are manufactured from fi laments and yarns that are combined to form planar products called fabrics. In geotechnical applications, nonwoven and woven facrics are popular. Nonwoven geotextiles are manufactured from continuous fi laments formed by extruding polymers through spinnerets or staple fi bers (2 cm to 10 cm of fi laments). Woven geotextiles are made from yarns knitted using various weave patterns such as plain, twill, satin, or various combinations of these.
The American Society for Testing and Materials (ASTM) has a number of standard tests to determine the physical and mechanical properties of geotextiles. For retaining walls, the key properties are tensile strength, creep, and durability. The ASTM tests for tensile strength and creep are:
1. Grab tensile strength test (ASTM D 4632). This test gives the ultimate tensile load (N) of the geotextile.
2. Wide-width tensile test (ASTM D 4595). This test gives the load per unit width (N/m) of geotextile. You can also obtain the wide-width modulus from this test.
3. Tensile creep test (ASTM D 5262). This test gives the percentage of the ultimate tensile load at which the rate of creep strain approaches zero with time.
Geotextiles suffer strength reduction from ultraviolet light degradation. Consequently, during construction it is essential not to expose the geotextile unduly to sunlight. Specifi cations normally limit the exposure time to less than 14 days.
(a) A geogrid reinforced wall, (b) metal strip,
(c) geotextile, and (d) geogrid
Effects of increasing the lateral soil
resistance by reinforcement