The types of facing elements used in the different reinforced soil systems control their aesthetics, since they are the only visible parts of the completed structure. A wide range of finishes and colors can be provided in the facing. In addition, the facing provides protection against backfill sloughing and erosion, and provides drainage paths. The type of facing influences settlement tolerances. In multianchored structures, the facing is a major structural element. Major facing types include the following:
1. Segmental precast-concrete panels. Examples of these are found in Reinforced Earth, the Georgia stabilized embankment system, the California mechanically stabilized embankment system, the VSL Retained Earth system, the Hilfiker
TYPE TYPICAL MECHANICAL PROPERTIES
J represents the modulus in terms of force per unit width of the reinforcement.
* J = E(Ac/b) where: Ac = total cross section of reinforcement material
b = width of reinforcement E = modulus of material
* Allowable values with no reduction for durability considerations
* Confined
FIGURE 8.26 Types of reinforcement and mechanical properties. (From J. K. Mitchell and B. R. Christopher, “North American Practice in Reinforced Soil Systems," Proceedings, Specialty Conference on Design and Performance of Earth Retaining Structures, Geotechnical Division, American Society of Civil Engineers, 1990, with permission)
Reinforced Soil Embankment, Tensar GeoWall, the American Geo-Tech system, the Stress Wall systems, the TRES system, the WEBSOL system, the Tensar system, and the York system of the Department of Environment, United Kingdom. (See Fig. 8.27.)
2. Cast-in-place concrete, shotcrete, or full-height precast panels. This type of facing is available in the Hilfiker and Tensar systems. Shotcrete is the most frequently used system for permanent soil nailed retaining structures. (See Fig. 8.28.)
3. Metallic facings. The original Reinforced Earth system had facing elements of galvanized steel sheet formed into half cylinders. Although precast concrete panels are now usually used in Reinforced Earth walls, metallic facings are still used in structures where difficult access or difficult handling requires lighter facing elements. Preformed metallic facings are also used in some soil nailing systems.
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FIGURE 8.27 Sloping or vertical wall with reinforcement attached to precast-concrete facing elements. |
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FIGURE 8.28 Vertical wall with cast-in-place concrete facing. Reinforcement is wrapped around fill used for drainage. |
4. Welded wire grids. Wire grid can be bent up at the front of the wall to form the wall face. This type of facing is used in the Hilfiker and Tensar retaining wall systems. Welded wire grid facing is also commonly used with soil nailing in fragmented rocks or intermediate soils (chalk, marl, shales).
5. Gabion facing. Gabions (rock-filled wire baskets) can be used as facing with reinforcing strips consisting of welded wire mesh, welded bar mats, polymer geogrids, or the double-twisted woven mesh used for gabions placed between the gabion baskets.
6. Fabric facing. Various types of geotextile reinforcement are looped around at the facing to form the exposed face of the retaining wall. These faces are susceptible to ultraviolet light degradation, vandalism (e. g., target practice), and damage due to fire.
7. Plastic grids. A plastic grid used for the reinforcement of the soil can be looped around to form the face of the completed retaining structure in a similar manner to welded wire mesh and fabric facing. Vegetation can grow through the grid structure and can provide both ultraviolet light protection for the polymer and a pleasing appearance.
8. Postconstruction facing. For wrapped faced walls, whether geotextiles, geogrids, or wire mesh, a facing can be attached after construction of the wall by shot — creting, guniting, or attaching prefabricated facing panels made of concrete, wood, or other materials. Shotcrete is the most frequently used system for permanent soil nailed retaining structures.
Precast elements can be cast in several shapes and provided with facing textures to match environmental requirements and to blend aesthetically into the environment. Retaining structures using precast-concrete elements as the facings can have surface finishes similar to any reinforced concrete structure. In addition, the use of separate panels provides the flexibility to absorb differential movements, both vertically and horizontally, without undesirable cracking, which could occur in a rigid structure.
Retaining structures with metal facings have the disadvantage of shorter life because of corrosion unless provision is made to compensate for it.
Facings using welded wire or gabions have the disadvantages of an uneven surface, exposed backfill materials, more tendency for erosion of the retained soil, possible shorter life from corrosion of the wires, and more susceptibility to vandalism. These can, of course, be countered by providing shotcrete or hanging facing panels on the exposed face and compensating for possible corrosion. The greatest advantages of such facings are low cost; ease of installation; design flexibility; good drainage (depending on the type of backfill), which provides increased stability; and possible treatment of the face for vegetative and other architectural effects. The facing can easily be adapted and well blended with the natural environment in the countryside. These facings, as well as geosynthetic wrapped facings, are especially advantageous for construction of temporary or other short-term design life structures.
