Few highway construction projects are more nightmarish – for both workers and drivers–than building a retaining wall on a heavily trafficked commuter road near a busy airport. When the Colorado Department of Transportation (CDOT) faced the inevitable on State Highway 82 near the Aspen Airport, it decided to go with a fairly new technology: soil nail construction. CDOT chose soil nail construction to avoid the large footing necessary below and behind an ordinary concrete retaining wall. Construction of a wall such as this would typically involve the movement of numerous dump trucks on the highway as they hauled excavation and backfill materials – a traffic nightmare on wheels. Instead, CDOT hired local construction companies to build an 18,000-ft. soil nail retaining wall. Once the nails were in place, the structure was sprayed with shotcrete and then covered with precast textured panels. CDOT reports that soil nailing saved the agency more than half a million dollars on this one wall alone. Soil nailing was such a success, the same technique is being used to construct similar walls around the airport. A Brief History of Soil NailingSoil nail technology was first used in France to build a permanent retaining wall cut in soft rock. The project, undertaken in 1961, was the first where steel nails were used to reinforce a retaining wall. The first soil nail wall to use modern soil nailing techniques was built near Versailles in 1972. The technique included installing high-density, grouted soil nails into a 60-ft.-high wall and facing it with reinforced concrete. Since then, this technology has improved to the point where soil nail construction is fast becoming the preferred method of building soil retaining walls. Europe, particularly France and Germany, continues to lead the world in soil nail technology.Soil nail construction is fairly new to North America. The first soil nail techniques are believed to have been applied to a temporary retaining wall in Vancouver, BC, in the late 1960s. The first documented construction project to use soil nailing was in Portland, OR, in 1976, for excavation of a hospital foundation; the wall height was 45 ft. Project managers in Portland reported that soil nailing reduced construction time by up to 50% and yielded a cost savings of 15% over conventional tieback construction.
You can see the top-down layers during the construction of this soil nail wall at Hickman Bluff in Kentucky.Use of soil nail construction is increasing in popularity in the United States, where it is used primarily for temporary and permanent support of building excavations and for highway projects. In the US, excavations of up to 75 ft. in height have been stabilized using this technique. The Federal Highway Administration (FHWA) has implemented this technology on highway projects, such as road widening, since the 1980s. The money saved with soil nail construction is one of the main factors that accounts for its growing use. In 1996, the FHWA published guidelines for soil nail construction in the US based on the extensive European experience. The Manual for the Design and Construction of Soil Nail Walls is available from the agency.Overview of the Soil Nail ProcessInstallation of drainage strips along one construction layer of the Hickman Bluff soil nail wallSoil nailing is a method of construction that reinforces the existing ground. Passive inclusions (the “nails”) are inserted into the soil in a closely spaced pattern to increase its overall shear strength. The nails are called “passive” because they are not pretensioned (as tieback inclusions are); the nails develop tension as the ground deforms laterally in response to ongoing excavation. In most cases, a temporary or permanent facing is added to retain the soil. It should be noted that engineers and other experts familiar with this type of construction must analyze the site and develop a site-specific nail placement design, including their correct depth, angle, and frequency. This ensures that the structure can resist the forces acting upon it and remain stable. Drainage of the site must also be carefully planned and implemented.A distinct feature of soil nailing is its top-down construction. Excavation occurs in layers of about 6 ft., one layer at a time, from the top of the wall. As each soil layer is excavated, nails are installed and facing is added, then the next layer down is similarly treated. Soil nailing is cost-effective, with savings realized mainly from the ease of construction, which relies primarily on small hydraulic, track-mounted, rotary- or percussive-type drill rigs for nail installation. Thus, soil nailing techniques are highly effective for emergency repairs along highways or on other sites with limited maneuvering room. As was the experience in Colorado, soil nail construction limits disruption of traffic flow around highway construction sites. There are three basic steps in the construction of soil nail walls:1. cutting to the shallow depth (3—6.5 ft.) of the desired nailing layer,2. installation of the metal nails,3. adding shotcrete (or reinforced shotcrete) facing.For permanent walls, a decorative stone or other facing can be added atop the shotcrete.Soil and WaterA TRICORE rotary-bit drill, typically used for drilling into the soil prior to installation of nailsCost-efficient soil nail walls should be constructed in ground where a 3- to 6.5-ft. vertical slope can stand without support for up to two days during construction and is stable for the few hours it takes to drill and insert the nails. The depth of the cut layer depends on the soil’s ability to stand unsupported while the nails are being inserted. Weathered rock, talus slope deposits, silts, clays with low plasticity that are not prone to creep, naturally cemented sands and gravels, heterogeneous and stratified soils, and some kinds of fine-to-medium homogeneous sand are suitable for soil nail construction. Soils not conducive to soil nail technology are soft plastic clays; peat/organic soils; loose, low-density, and/or saturated soils; and coarse sand and gravels that are uncemented or lack capillary cohesion.Soil analysis is essential prior to soil nail construction. Among other considerations, experts must determine if the soil is “aggressive”; if it is, the nails need to be specially treated to prevent corrosion (see below). Drainage is a critical element in planning and construction. Most commonly, face drainage is used: a drainage element is placed behind the shotcrete wall covering the nailed structure. The drainage elements are installed from the top down as construction proceeds. Typically, synthetic strips or perforated pipes (8-12 in.) are installed, usually spaced about 5—6.5 ft. apart. The water is collected at the wall base and channeled away. Alternatively, weep holes can be made through the face of the wall, used with or without perforated drainpipes. Whichever method is used, it’s vital to channel the water away from the wall so it doesn’t collect behind it.NailsSoil nails are installed in a pattern designed to ensure both internal and external stability of the wall. A relatively large number of nails are placed so they can resist the tensile, compressive, and shear stresses within the wall and transfer them into the ground. Engineers use a method of equilibrium analysis to make certain that the number and placement of nails guard against sliding and guarantee stability. The nails used in construction are generally steel bars that resist tensile and shear stresses and bending moment. Therefore, ductile steel is preferred over brittle. Most projects are designed to use nails with a uniform length and cross-sectional area. Nail length is usually about 60-80% of the height of the wall, depending on soil conditions (e.g., rocklike material may get shorter nails). Prior to construction, nails are tested to determine nail-soil adhesion and their resistance to pullout failure. Types of NailsSeveral types of soil nails are currently in use:Driven nails: Generally small-diameter nails (15-46 mm) with a relatively limited length (to about 20 m) made of mild steel (about 50 ksi) that are closely spaced in the wall (two to four nails per square meter). Nails with an axial channel can be used to permit the addition of grout sealing. Driven nails are the quickest (four to six per hour) and most economical to install (with a pneumatic or hydraulic hammer). Grouted nails: Steel bars, with diameters ranging from 15 to 46 mm, stronger than driven nails (about 60 ksi). Grouted nails are inserted into boreholes of 10-15 cm and then cement-grouted. Ribbed bars are also used to increase soil adhesion.Corrosion-protected nails: For aggressive soils as well as for permanent structures.Jet-grouted nails: A composite of grouted soil and a central steel rod, up to 40 cm thick. Nails are installed using a high-frequency vibropercussion hammer, and cement grouting is injected during installation. This method has been shown to increase the pullout resistance of the composite, and the nails are corrosion-resistant.Launched nails: Nails between 25 and 38 mm in diameter and up to 6 m or longer are fired directly into the soil with a compressed-air launcher. Used primarily for slope stabilization, this technique involves the least site disturbance.Nail PlacementThe equilibrium design is site-specific and determines nail placement. The commonsense rule of thumb is that greater performance results from more nails closely spaced, rather than fewer nails widely spaced. Typically, nails of equal length and cross-sectional area are uniformly spaced. In general, for drilled and grouted nails, spacing is one nail per 3—6.5 ft., both vertically and horizontally. Driven nails require higher densities of as much as one and a half to two nails per square meter. Nail rows are often staggered to increase face stability. The angle of inclination is generally between 10 and 20°.Nail length depends on several factors, including soil strength, soil nail adhesion, and the overall loading of the system. In general, minimum nail length is considered to be about 0.6 times the wall height for vertical walls with no backslope. Shorter nails have been used in walls with more rocklike soils. The vast experience in Europe indicates that it might be preferable in some cases to install longer and higher-capacity nails in the upper two-thirds of the wall, as research shows that this reduces wall displacement. Though arguments have been made to the contrary, longer and heavier nails in the upper part of the wall seem to be more effective in preventing failure than reinforcements in the lower wall. Overall, though, uniform length, strength, and placement yield good results.Aggressive Soils and CorrosionCorrosion prevention is necessary in permanent structures and in “aggressive” soils, which are defined as having a pH below 4.5, a resistivity below 2,000 ohm-cm, sulfate levels above 200 ppm, and chloride levels above 100 ppm. If these conditions exist, corrosion-protected nails must be used. The German approach to corrosion protection is considered conservative and is preferred. It involves using nails with “double corrosion protection,” in which the steel nail is encapsulated in a corrugated plastic sheath (> 40 mil) and cement grout annulus. The double coating prevents damage even if small cracks occur in the cement grout. This double corrosion protection is required for permanent structures and for temporary structures in aggressive ground intended to last more than 30 years. Epoxy coatings or grouts are not recommended and are far more expensive than the double corrosion system described above. Further, research indicates that under no circumstances should stainless steel reinforcing strips be used in aggressive ground. In France, a structure less than 10 years old failed using this method of reinforcement. GroutingNeat cement grout with a water-to-cement ratio of about 0.4:0.5 is usually used. In many cases for open-hole drilling, the low-pressure tremie method works well. In Germany, the nail may be installed with a regrout pipe attached, and the grout is added under pressure, fracturing the initial grout and creating a better bond between the grout and the soil. In general, grout may be added either before or after installation of the nail. FacingOnce the nails are installed and grouted, a shotcrete facing between 3 and 6 in. thick is applied, with a wire mesh at midthickness. This is generally used for temporary wall facings. Permanent walls may receive a shotcrete cover of up to 10 in. thick, usually with a second layer of wire mesh. In both of these cases, the facing is not considered to be a structurally significant supporting part of the wall.The experience in France indicates that nail loads at the facing generally do not exceed 30-40% of the maximum loads in the nail, so they recommend a facing designed for a uniform wall pressure equal to 60% of the maximum nail load on a nail spacing of 3 ft. For walls with greater nail spacing (e.g., 10 ft.), the facing should be designed for 100% of the maximum nail load. Permanent structures can be made more pleasing to the eye with the addition of cast-in-place concrete facings with a minimum of 8-in. thickness. Precast decorative panels may also be attached directly to the shotcrete facing. ApplicationsZion National ParkThe landslide along the Virgin River in Zion National ParkIn 1995, the rain-swollen Virgin River touched off a landslide at Zion National Park in Utah. Flash floods and the landslide succeeded in washing out 590 ft. of road within two hours. After evaluation by the FHWA, emergency funds were allocated to begin repairs. Several constraints affected the repair plans, including the assessment by the National Park Service that the slide was a significant geologic event in the park and should be at least partially preserved for interpretive purposes, though of course the slide had to be stabilized. The agreed-upon solution involved construction of a buttress of solid rock to retain and stabilize the slide, which consisted primarily of sandstone rock. Though black basalt was chosen for the buttress, it was used only in its interior; the face was constructed of sandstone to blend in with the native rock.The slide mass has been excavated and replaced withbuttress material to stabilize the slope prior to wall construction.The completed soil nail wall in Zion National Park before masonry facing added
Because the site was in a national park, wall construction presented out-of-the-ordinary constraints and problems. The road had to remain open, and construction had to be completed in about six weeks (when tourism in the park increased). The wall had to follow the curve of the roadway, look good, and withstand the erosional effects of the river.
