Soil nailed walls have some similarities with MSE walls but also some fundamental
differences. The main similarities are:
1. The major mechanism in both MSE and soil nailed retaining structures is the development of tensile forces in the reinforcements due to frictional interaction and, consequently, restrainment of lateral deformations of the structures.
2. The reinforced soil mass is separated into two zones based on the points of maximum tension in the reinforcement (Fig. 8.54): an “active” zone close to the facing, where the shear stresses exerted on the surface of the reinforcement are directed outward and have a tendency to pull out the reinforcements, and a “resistant” zone, where the shear stresses are directed inward and prevent the sliding of the reinforcements.
3. The reinforcement forces are sustained by a frictional bond between the soil and the reinforcing element; the reinforced zone is stable and resists the thrust from the unreinforced soil it supports, much like a gravity retaining structure.
4. The facing of the retained structure is relatively thin, with prefabricated elements used for MSE walls and, usually, shotcrete for soil nailed walls.
The main differences are:
1. The construction procedure. Although at the end of construction the two structures may look similar, the construction sequence is radically different. Soil nailed walls are constructed “top down” by staged excavations, while MSE walls are constructed “bottom up.” Thus, the wall deformation pattern is different during construction. This also results in differences in the distribution of the forces that develop in the reinforcement, particularly during the construction period. In an MSE structure (built bottom up), the working forces that develop in the reinforcement layers generally increase from top to bottom. In a nailed structure (built top down), the working loads that develop in the reinforcement layers are generally of uniform magnitude, similar to those in a braced excavation.
2. Nature of the soil. Soil nailing is an in situ reinforcement technique exploiting natural ground, the properties of which cannot be preselected and controlled as they are for MSE fills. MSE walls usually utilize clean, low-water-content granular backfills, which have a known friction angle and little to no cohesion. On the contrary, nails are installed into soil and rock (natural ground) whose strength properties (friction angle and cohesion) and water content can vary through a wide range.
3. Soil-reinforcement bond. Grouting techniques are usually employed to bond the nail reinforcement to the surrounding ground, with the load transferred along the grout to the soil interface. In MSE structures, friction is generated directly along the reinforcement-soil interface.
