What are Soil Nails?
A soil nail wall is a gravity composite soil structure in which an excavated slope or vertical cut is internally reinforced through placement of closely spaced linear reinforcing elements. Reinforcing elements are installed by placing them into the existing soil slope or new excavation.
Construction is performed in vertical steps, with construction starting at the top of the excavation and proceeding down. Once an excavated level is reinforced with soil nails, a permanent or temporary facing is applied to retain the soil. The resulting soil structure has soil nails placed to a depth and of a sufficient density to ensure it can resist the forces imposed by the soil and surcharge loads. The failure modes that are analyzed to ensure stability for a soil nail wall include sliding, bearing, and global stability failure modes.
There are two different types of soil nails available
- Screw anchor soil nails
- Grouted soil nails
Soil Nail Technology
Retaining walls using anchored bars date back to the 1960’s and earlier. Soil nailing technology can be traced back to the use of the New Austrian Tunneling Method (NATM), in which grouted rock bolts and shotcrete were used for supporting tunnels. This technology was reportedly first applied for the permanent support of retaining walls in a cut in soft rock in France in 1961. The use of grouted “soil nails” and driven soil nails, which consist of solid steel bars and steel angle iron, continued to grow in the 1970’s, in France and Germany. The first wall built in France using current soil nail techniques was reported to have been built by Soletanche, in Versailles in 1972, using a high density of grouted soil nails in sand. The wall was on a 21-degree batter, was 60 feet tall, had a reinforced concrete facing and supported an excavation for a railroad track.
In North America, soil nails were first introduced for temporary excavation support in Vancouver, B.C., in the late 1960’s and early 1970’s. The first documented project in the U.S. was in Portland, Oregon for excavation support of a hospital foundation. The maximum excavation depth was 45 feet. The soils consisted of medium dense to dense silty fine sands. The work was reported to have been completed in 50 to 70 percent of the time required for conventional tieback construction and at a 15 percent cost saving.
Two major research programs to study soil nailing were undertaken in the late 1970’s in Germany (University of Karlsruhe and Bauer Construction) and in the 1980’s in France (Clouterre Program). The French program consisted of a $5 million study, jointly funded by the French government and private industry, with the objective of developing a design methodology for soil nail walls. Considering the results of full-scale testing and monitoring of 6 full-scale structures, the Recommendations of Clouterre, published in 1991, represent the basis for soil nail standards in France.
In 1996 the U.S. Federal Highway Administration published its Manual for Design and Construction of Soil Nail Walls. This manual synthesizes the work in Germany, France and current U.S. practice, to form a guideline for soil nail design for highway works.
Today in the United States, the major use of soil nail walls is for temporary and permanent support of building excavations. Walls up to 75 feet tall have been successfully constructed. This application for soil nailing continues to grow due to the economic benefits it has over conventional tieback construction. Soil nailing has been used for highway applications dating back to the 1980’s. Soil nail walls up to 40 feet tall have been used on Federal highway projects. With the development of the FHWA guidelines and promotion of this technique for highway works, the use of soil nailing will continue to grow.
Soil Nails from MDTI
Soil Nails from MDTI are hollow, threaded, steel rods that can extend to any length in all soil and rock conditions to meet your project needs. The MDTI Magnacore® Soil Nail Wall system is utilized to stabilize slopes and structures at shallow or deep depths. This is a quick installation that combines drilling, placing, and grouting in one operation. This allows the system to be suitable for working in a limited space.
Thanks to the full-length thread and extension couplings, this system offers flexibility as it easily adjusts the bar length to the site requirements. This is especially useful if anchoring has to be performed in a confined workspace, for example with limited head room which is usually the case in underpinning.