If soil conditions are right and space is limited, soil nail construction might be your best bet for building a retaining wall.
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 Nailing
Soil 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.
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| 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 Process
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| Installation of drainage strips along one construction layer of the Hickman Bluff soil nail wall |
Soil 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 Water
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| A TRICORE rotary-bit drill, typically used for drilling into the soil prior to installation of nails |
Cost-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.
Nails
Soil 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 Nails
Several 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 Placement
The 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 Corrosion
Corrosion 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.
Grouting
Neat 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.
Facing
Once 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.
Applications
Zion National Park
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| The landslide along the Virgin River in Zion National Park |
In 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.
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| The slide mass has been excavated and replaced with buttress material to stabilize the slope prior to wall construction. |
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| 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.
It was decided to build the wall of precast concrete units. The soil nail shoring system used about 345 nails at 5-ft. intervals. Holes were drilled and grouted, and a 5.8-m threaded bar was installed. A bearing plate was attached to each nail/bar at the surface of the slope. A facing of 8,000-ft.2 wire mesh and 2,400-ft.3 shotcrete covered the wall. This ended the first phase of construction.
The second phase involved construction of a permanent wall. Water at the bottom of the temporary wall was drained off using a cofferdam and pumps. Soil conditions at the base were good for construction of a permanent wall, and a concrete leveling pad was installed. The permanent wall was then built in a total of 90 days.
Hickman Bluff
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| A collapsed portion of Hickman Bluff in Kentucky |
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| X marks the spot where the soil nails are to be inserted at Hickman Bluff |
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| Drilling at Hickman Bluff prior to nail installation |
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| The nearly completed soil nail wall at Hickman Bluff in Kentucky |
Hickman Bluff is located in the extreme southwestern part of Kentucky, only 1 mi. from the Mississippi River. The bluffs were found to be caving, threatening adjacent public buildings and residences. Prior to construction in the mid-1990s, a section of the bluff collapsed near an adjacent street. The soil nail project, conducted by the US Army Corps of Engineers, addressed and corrected these problems.
Costs
As mentioned above, soil nail walls are highly cost-effective, especially when using the open-hole drilling method and as long as the site is suitable and no short-term face stability problems are encountered. Based on the experience in France, the estimated cost of a permanent soil nail wall is about $13-$18/ft.2 for shotcrete and between $12 and $19/ft.2 for soil nails (though some estimates run as high as $38/ft.2 for both in a permanent wall). In Germany, estimates for temporary soil nail walls (shotcrete and nails) ranged between $9 and $22/ft.2; for permanent walls, between $19 and $38/ft.2
Pluses and Minuses
Soil nail walls compare favorably with other soil-retention construction systems. Tieback walls, for example, require structural facing elements that are pretensioned and anchored to the ground with steel that is strong and stiff enough to hold the soil structure behind it. The anchors must be tensioned with enough load to support the facing without creep or other failure. Unlike tiebacks, soil nail walls are not tensioned; they are passive reinforcements that, once placed within the soil of the wall, create a coherent gravity mass. Thus, soil nails create a condition of internal stability within the wall; stability does not depend on the strength of the outer facing but is generated within the structure itself. Further, soil nail construction eliminates the need for placing H-piles, timber lagging, or sheet piling, as well as the need for costly facing systems. The nail length is shorter than that used in tiebacks, improving traffic flow around highway construction projects.
In mechanically stabilized earth (MSE) walls, which are somewhat akin to soil nail walls, the greatest amount of stress accumulates at the bottom of the wall through compaction. Thus, the lower parts of an MSE wall are most likely to deform or fail. In a soil nail wall, the greatest stress is initially contained within the upper layers of the wall, then passes downward as construction proceeds. Unlike MSE walls, however, the placement of more and/or longer nails in the upper portion of the soil nail wall seems to limit the transfer of tension to the lower wall. Studies have shown that, over time, the stress within the soil nail wall reaches an equilibrium at all levels.
Soil nail construction has many advantages:
- Soil nail walls can be built to follow curved or zigzagged outlines.
- The equipment used is highly portable and can fit easily into small spaces.
- The process is flexible and makes modifications easy to carry out (e.g., nails can be moved as needed during construction).
- Construction causes less noise and traffic obstruction on highways.
- The process creates less impact on adjacent or nearby properties than do other construction methods.
- It generally requires less space and manpower.
Like everything, soil nail construction also has its drawbacks. Among the disadvantages:
- The method cannot be used at sites where groundwater is a problem.
- It is inappropriate for sites with soils having very low shear strength, in sand and gravels that lack cohesion, and on sites with other unsuitable soils.
- Soil must be able to stand unsupported while it is being nailed and before shotcrete application.
- Good drainage is essential, especially for permanent structures and in places prone to freeze-thaw cycles.
Finally, it is imperative that projects be designed, constructed, and monitored by specialists with experience in soil nail wall construction.