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
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TABLE 8.11 Typical Characteristics of Soil Nailed Walls with Vertical Facing and Horizontal Earth Pressure
Source: From Recommendations Clouterre, French National Research, 1991, (English translation by Federal Highway Administration, 1993), with permission. |
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FIGURE 8.55 Preliminary design charts for soil nailed walls. (From Recommendations Clouterre, French National Research, 1991, translation by Federal Highway Administration, 1993, with permission) |
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.
