8.8.1 Development and General Considerations
Figure 8.52 shows a cross-section of the first soil nailed wall, which was a temporary wall built in France (1972-1973) for a railroad project. Such walls are constructed from the top down during excavation. Reinforcing bars are either inserted in drilled holes and grouted into place, or driven into place. Then a facing of cast-in-place concrete or shotcrete is installed as the work progresses.
The wall in France was built in Fontainebleau sand using a high density of short nails of two different lengths: 13 ft (4 m) for nails in the upper portion of the wall and 20 ft (6 m) for those in the lower portion. The first full-size experimental wall was constructed in Germany in 1979 using grouted nails and loaded to failure. In 1981, a prefabricated — concrete-facing soil nailed wall was used in a commercial application in France. An extensive national research project conducted in France during the years 1986-1990 resulted in a noted publication titled Recommendations Clouterre.
A soil nailed wall is constructed as an integral part of the construction of an excavation as illustrated in Fig. 8.53. (See Art. 8.8.7.) The soil is reinforced as the slope excavation progresses. Reinforcement generally consists of bars inserted parallel to one another and placed at a downward-sloping angle. The bars are inserted in a passive state; however, as the skin friction between the soil and the nails is mobilized, the nails are placed into tension. Figure 8.54 compares the action of soil nails and ground anchors.
The work is carried out from the top downward in increments, gradually building up a reinforced soil mass. Some type of facing is generally necessary to keep the soil from caving between the soil nails. In the case of the Fontainebleau sand (effective friction angle of 38° and some cohesion), the distance for stability of the excavation
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2 FIGURE 8.52 First soil nailed wall, constructed at Versailles, France, 1972-1973. (F. Schlosser, Behavior and Design of Soil Nailing, Proceedings of Symposium on Soil and Rock Improvement Techniques, Bangkok, 1982, with permission) |
Excavate unsupported cut — 3 to 6 ft high
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STEP 4: Place initial shotcrete layer and install bearing plates and nuts
between soil nails was about 6.6 ft (2 m) with failure occurring at about 9.8 ft (3 m). The sand between the nails will slough until an “arching” action occurs within the soil. The point where this action can no longer occur because the internal friction capacity of the soil has been exceeded defines the temporary facing limit.
The concept of soil nailing has not been patented, nor is it patentable; however, numerous technologies have been patented. For soil nail walls to be cost-effective, the ground must be capable of standing unsupported while the nails and shotcrete are installed. The success of soil nail walls depends upon:
1. Selection of good applications in ground suitable for nailing
2. Ability to quickly respond to changed ground conditions
3. Use of a rational design procedure for the wall and each of its components
4. Use of good construction specifications
5. Ability of the owner and contractor to work together in a partnering concept
6. Handling of work performance on-site by knowledgeable personnel representing each of the parties, including the owner
