The construction sequence is typically to excavate, nail, and shotcrete the face in increments from the top down. Figure 8.56 shows a schematic of a sequence for underpass widening. Where face stability is a concern, a flashcoat of shotcrete may be applied before nail installation. The most common method of nail installation in

FIGURE 8.56 Construction sequence for soil nailed wall used in widening of underpass. (From Oregon Department of Transportation, with permission)

STEP 7: Place second structural application to full height and architectural finish

FIGURE 8.56 (Continued)

Europe, as in the United States, is the drill-and-grout method. Most commonly, the steel tendon is installed prior to grouting, although this sequence is sometimes reversed. In France, however, very significant use is also made of driven nails without grouting. Other specialty techniques for installing nails include jet grout nailing (France), driven nails with an oversize head and subsequent grouting of the annulus (Germany), and compressed-air explosive injection of nails (United Kingdom).

Small hydraulic, track-mounted drill rigs of the rotary-percussive type are most com­monly used to install nails. These rigs can work in relatively confined surroundings and are therefore compatible with many of the constraints associated with crowded urban

environments. Open-hole drilling methods are predominant, with cased-hole methods used in particularly difficult ground conditions. The most common grouting method used with open-hole drilling is the low-pressure tremie method. Where extensive use of casing is required, alternative methods of construction are often more cost-competitive.

Steel tendons typically used for drill-and-grout soil nails usually consist of /4- to 2-in­diameter (20- to 50-mm) bars with a yield strength in the range of 60 to 70 kips/in2 (420 to 500 N/mm2). These steels exhibit ductile behavior under bending action. The driven nails used commonly in France are typically steel angle sections, which show better ability to deal with subsurface obstructions such as cobbles and small boulders than do circular steel sections.

Drainage is a critical aspect of soil nail wall construction. Face drainage is virtually always used with permanent walls, and very commonly used with temporary walls. Face drainage usually consists of synthetic drainage elements placed between the shotcrete and the retained soil, and may be typically 8- to 12-in-wide (200- to 300-mm) synthetic strips or perforated pipes. Depending on the site groundwater conditions, face drainage may be supplemented with weep holes through the facing and longer horizontal perforated drain pipes. Control of surface water is also an important element of the drainage system.

Temporary soil nail wall facings generally consist of shotcrete 3 to 4 in (80 to 100 mm) thick and a single layer of wire mesh. Permanent shotcrete walls 6 to 10 in (150 to 250 mm) thick are very common in Germany, and these walls typically include a second layer of wire mesh. For architectural reasons, permanent walls of precast panels and cast-in­place concrete are also commonly used in France and Germany.

Testing and monitoring during construction are an important aspect of soil nail wall construction in Europe. Nail bond testing is almost universally performed, to confirm the assumptions made during design or to enable redesign in the event that the design assumptions cannot be realized. For relatively homogeneous sites, typically 3 to 5 percent of the nails will be tested, depending on the size of the job. Testing is also undertaken whenever changed geologic or construction conditions occur. Wall performance moni­toring usually consists of measuring horizontal wall movements during construction. Some contractors make more use of inclinometers for displacement monitoring. Maximum horizontal displacements are typically in the range of 0.1 to 0.3 percent of the height of the wall, depending on ground conditions. Strain gauging of nails, together with the use of load cells at the nail head, is usually reserved for experimental walls.

The level of quality assurance and control monitoring varies significantly. In Germany, for example, the QA-QC inspector might be on the job from 10 percent to full time.

Overall, soil nail wall performance in Europe has been very good. Problems during construction have typically been associated with encountering loose fill, granular soil with no apparent cohesion, water, and constructed obstructions such as utility trenches. Other problems have been associated with a contractor’s failing to construct the wall in accordance with the plans and specifications (e. g., eliminating nails, overexcavation of lifts). Frost action on fully bonded nails has also resulted in development of very large loads near the head of the nail, where no insulating protection has been provided in the wall design.

The stability of a soil nailed structure relies on (1) transfer of resisting tensile forces generated in the inclusions in the active zone into the ground in the resistant zone, through friction or adhesion mobilized at the soil-nail interface, and (2) passive resistance developed against the face of the nail. Ground nailing using closely spaced inclusions produces a composite coherent material. As shown in Fig. 8.54, the tensile forces gen­erated in the nails are considerably greater than those transmitted to the facing.

The design procedure for a nailed retaining structure includes (1) estimation of nail forces and location of the potential sliding surface, (2) selection of the reinforcement type, cross-sectional area, length, inclination, and spacing, and (3) verification that stability is maintained during and after excavation with an adequate factor of safety. Methods for determining tensile, bending, and shear stresses in the nails are given by FHWA based on a limit equilibrium analysis.

The majority of soil nailed retaining structures constructed in France are based on two distinct technologies: (1) the method of Hurpin, with nails driven into the ground on close spacing, i. e., vertical and horizontal spacing equal to or less than 3 ft (1 m), and (2) widely spaced grouted nails. With the method of Hurpin, the nails (generally reinforcing bars) are relatively short and are driven into the ground by percussion or vibratory methods. The relatively high nail density permits thinner wall facings. In walls with widely spaced nails, the nails are generally longer. Typical data for soil nailed walls with a vertical facing and a horizontal earth pressure are shown in Table 8.11. The “nailing density” listed in Table 8.11 is a dimensionless parameter representing soil nails placed in a uniform pattern. It is defined as

(8.25) where T = ultimate tensile force that can be mobilized at head of nail

Г

Sh = horizontal spacing between nails Sv = vertical spacing between nails L = length of nails у = total unit weight of soil

This parameter represents the maximum tensile force in a nail as it relates to the weight of the soil reinforced with a chosen grid spacing.

A full set of preliminary design charts is included in the FHWA translation of Recommendations Clouterre, 1991. Diagrams for an angle of installation of the nails of i = 20° are shown in Fig. 8.55 for illustration.

Figure 8.55 provides a preliminary chart for a soil nailed wall. It seeks to define in approximate terms the lengths, spacings, and resistance values of the nails to ensure

internal and external stability. It may be used in an early evaluation stage based on macro assumptions such as homogeneous soil, identical and evenly spaced loads in the nails, and pure tension in the nails; the bending stiffness of the nails is neglected, regardless of the angle of incidence of the potential failure surface. This approach is based on the classic method of vertical slices with circular potential failure surfaces. The charts are based on a system of coordinates that characterize the shear resistance of the soil.

c 20

N = ————— = —————————- = 0.10 tan ф = tan 35° = 0.70

7H 20 X 10 v

Draw a line from the origin O to M. The safety factor F is the ratio OM/OA. Therefore, for a safety factor of 3/2, locate point A two-thirds of the distance along the line OM. Interpolation gives the required nailing density d as 0.33. Thus:

TL

7—

0. 33 X 20 X 0.8 X 10 = 52.8 kPa

Thus, for a nail tensile force TL, the spacings Sh and Sv can be determined. The result from the chart should be generally conservative and used only for preliminary evaluation.

The final design for stability of a soil nailed wall is analyzed either by calculating the deformations or by using limit equilibrium design. The first method uses finite ele­ment calculation and has not been refined to the point where there is an “acceptable” procedure. In Europe to date, there has been considerable diversity in some details among the various design approaches, both within and across national boundaries. A most significant factor is the postulated mechanism by which nails are considered to reinforce a soil mass. For nails installed nearly parallel to the direction of maximum soil tensile strain (e. g., near-horizontal nails and a near-vertical excavation face), the prevailing opinion is that the reinforcing action is predominantly related to tensile loading within the nails. Under service load conditions, the contribution of shear or bending is considered negligible. As failure conditions are approached, the contribution of shear or bending action is more significant but still small. From a practical point of view, however, it is recognized that the soil nails should exhibit ductile behavior in response to bending in order to minimize the potential for sudden failures related to brittleness. Where reinforcing elements are used as dowels and are oriented nearly perpendicular to the direction of maximum shear strain, the shearing, bending, and tensile action of the reinforcement should be considered.

All design methods are based on concepts of limiting equilibrium or ultimate limit states. Various types of potential slip surface are considered, including circular, log spiral, and bilinear wedge. In general, each of the methods appears to provide a satis­factory representation for design purposes. Consistent with the above, most design computer codes consider only the tensile action of the nails, but some also permit con­sideration of the shear or bending action of the nails. Almost all of the design methods do not explicitly consider the potential for pullout of the reinforcing nails within the active block between the facing and the slip surface. It is implicitly assumed that the nail – soil adhesion within this zone, together with the structural capacity of the facing, will be sufficient to prevent this type of failure. Some design approaches offer strict guide­lines for the required structural wall capacity to prevent such active-zone failures, but others appear to rely on experience and do not directly address this issue.

On the basis of the overall reinforcing requirements determined from the limiting equilibrium design calculations, the reinforcing steel is empirically proportioned. In general, designers use nails of uniform length and cross-sectional area, on a uni­form spacing. For drilled and grouted nails, the nail spacing is typically in the 3- to 6-ft (1- to 2-m) range. For driven nails, much higher densities (typically 1.5 to 2 nails per
square meter) are used. The nail lengths are typically in the range of 60 to 80 percent of the height of the wall, but may be shorter in very competent rocklike materials and longer for heavy surface surcharge or high seismic or other operational loading.

As noted above, facing design requirements are empirically determined using a variety of techniques. German practice requires the use of a uniform facing pressure equivalent to 75 to 85 percent of the active Coulomb loading. The Clouterre recom­mendations require designing the facing and connectors to support between 60 and 100 percent of the maximum nail loading (for both ultimate and serviceability limit states), depending on the nail spacing.

Almost all shotcrete failures in slope stabilization applications have resulted from

inadequate drainage. Therefore, drainage is a critical design and construction element.

Drainage from behind the shotcrete face can be provided by the following methods:

1. Surface interceptor ditch. Excavate a shallow ditch along the crest of the excavation to lead away surface water. Drainage gutters or lined ditches are recommended immediately behind the top of the wall.

2. Prefab geotextile drains. Place 12-in-wide (300-mm) prefabricated geotextile drain strips (Miradrain 6000, Amerdrain 200, etc.) vertically prior to applying the shotcrete. Typical spacings are the same as the horizontal nail spacing. Extend the drain mats down the full height of the excavation and discharge into a collector pipe at the base.

3. Weep holes and horizontal drains. Install 2-in-diameter (50-mm) PVC pipe weep holes on approximately 10-ft (3-m) centers through the shotcrete face where heavier seepage is encountered. Plug the pipe temporarily when shotcrete is applied. Longer PVC horizontal drain pipes can also be installed in heavy seepage areas.

8.8.3 Wall Facing Systems

Temporary walls are typically left with a rough shotcrete face—“gun” finish—with weep holes and protruding nail heads. For permanent walls, where the rough finish is aesthetically unacceptable, the following face options are available:

1. Separate fascia wall. As an alternative to the exposed shotcrete finish, the shotcrete can be covered with a separate concrete fascia wall, either cast in place (CIP) or constructed of precast panels. The CIP section is typically a minimum of 6 to 8 in (150 to 200 mm) thick. Precast face panels can be smaller modular panels or full – height fascia panels such as those used to cover permanent slurry walls. A disadvantage of the smaller modular face panels is difficulty of attaching the face panels to the nail heads and some proprietary patent restrictions. A disadvantage of full-height precast panels is that, because of practical constructibility weight and handling limitations, their use is limited to wall heights less than 25 ft (8 m).

2. Permanent exposed shotcrete facing. Present technology for shotcrete place­ment is such that the final shotcrete layer can be controlled to close tolerances, and with nominal hand finishing, an appearance similar to a CIP wall can be obtained (if desired). The shotcrete, whether left in the natural gun finish or hand-textured, can also be colored either by adding coloring agent to the mix or by applying a pigmented sealer or stain over the shotcrete surface. Only experienced, well-qualified structural shotcrete specialty contractors should place and finish the permanent structural shot – crete. “Wet-mix” shotcrete should be specified instead of dry-mix because good quality control is easier with wet-mix. Also, wet-mix shotcrete can be air-entrained for improved freeze-thaw durability, whereas dry-mix cannot.

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.