STEP 1: INITIAL SOIL NAIL WALL DESIGN CONSIDERATIONS

Establish the layout of the soil nail wall, including: (1) wall height; (2) length of the wall; and (3) wall face batter (inclination typically ranges from 0º to 10º). The evaluation of the wall layout also includes developing the wall longitudinal profile, locating wall appurtenances (e.g., traffic barriers, utilities, and drainage systems), and establishing ROW limitations.

Battered wall face can be selected to improve temporary face stability, as a battered face exerts smaller forces on the wall, thus requiring shorter soil nails. The material savings resulting from the use of shorter nails may offset the increased cost of soil excavation incurred to create the batter. A mild batter (i.e., less than 10 degrees) is usually provided for aesthetic reasons, especially around horizontal curves, and may be enough to ensure temporary face stability. A batter greater than 10 degrees can enhance stability.

B. Soil Nail Vertical and Horizontal Spacing

Horizontal nail spacing, SH, is typically the same as vertical nail spacing, SV (Figure 6.1).

Nail spacing ranges from 1.25 to 2 m (4 to 6.5 ft) for conventional drilled and grouted soil nails, and may be as low as 0.5 m (1.5 ft) for driven nails. This reduced spacing for driven nails is required because driven soil nails develop bond strengths that are lower than those for drilled and grouted nails. A soil-nail spacing of 1.5 m (5 ft) is routinely used and is preferred for conventional drilled and grouted soil nails. Soil nail spacing may be affected by the presence of existing underground structures.

Soil nail spacing in horizontal and vertical direction must be such that each nail has an influence area SH × Sv ≤ 4 m² (≤ 40 ft² ft). The design engineer should specify a minimum horizontal soil nail spacing of about 1.0 m (3.3 ft). Design forces from global stability analysis and facing design are affected by soil nail spacing. In general, the larger the spacing, the greater the design forces. The purpose of the minimum nail spacing is to reasonably ensure that group effects between adjacent soil nails are minimized due to potential nail intersection as a result of drilling deviations. Group effects reduce the load-carrying capacity of individual soil nails. The maximum soil nail spacing should also be specified. The purpose of a maximum spacing [usually about 2 m (6.5 ft)] is to provide for a soil nail system that is relatively easy to construct and that effectively supports the lateral earth pressures and imposed surcharge loads.

C. Soil Nail Pattern on Face

The soil nail pattern is commonly one of the following (see Figure 6.1): (1) square (rectangular); (2) staggered in a triangular pattern; and (3) irregular (at limited locations).

A square pattern results in a column of aligned soil nails, and facilitates easier construction of vertical joints in the shotcrete facing (or easier installation of precast concrete panels). Also, a square pattern enables a continuous vertical installation of geocomposite drain strips behind the facing to be easily constructed. In practice, a square pattern is commonly adopted.

GEOCOMPOSITE DRAINAGE STRIPS

SH/ 2

NAIL STAGGERED PATTERN

BOTTOM OF EXCAVATION LIFTS

BOTTOM OF EXCAVATION

NAIL SQUARE PATTERN NAIL

#1

N i

GEOCOMPOSITE DRAINAGE STRIPS

S^VO < S^V

SH/ 2

SVN

S^V S^VO

S^{VN <} S^V S^V

S^H

0.30 M (TYP)

0.30 M (TYP)

A staggered soil nail pattern results in a more uniform distribution of earth pressures in the soil mass. This effect is beneficial because an enhanced soil arching effect is achieved. This method should be considered in cases where marginally stable soils are present because such soils have less margin to redistribute loads. The main disadvantage of the use of a triangular pattern is that it makes installation of geocomposite drain strips more complicated. In particular, it can be difficult to establish a vertically continuous drain system to the footing drain, especially for higher walls.

The use of uniform nail spacing is beneficial because it simplifies construction and quality control. However, due to project-specific geometric constraints, nail spacing may need to be irregular, with reduced spacing at some locations; for instance, in areas where the bottom of the excavation or the top of the wall is not horizontal. In such cases, it is more convenient to install one or two nail rows parallel to the non-horizontal edge and then establish a transition zone where nails have a closer vertical spacing until a horizontal nail row is achieved (Figure 6.2a). It is also customary to reduce horizontal spacing at the vertical edges of the wall to accommodate transition zones (Figure 6.2a).

D. Soil Nail Inclination

Soil nails are typically installed at an inclination ranging from 10 to 20 degrees from horizontal with a typical inclination of 15 degrees. This recommended range of soil nail inclination assures that grout will flow readily from the bottom of the hole toward the nail head for typical borehole and soil nail dimensions and conventional grout mixtures. Steeper nail inclinations may be required, particularly for the upper row of nails, if a significantly stronger soil zone is located at a greater depth and a more effective anchorage in the stiffer layer is desired. Such evaluations can be readily made during design. Nail inclination smaller than about 10 degrees should not be used because the potential for creating voids in the grout increases significantly. Voids in the grout will affect the load capacity of the nail and reduce the overall corrosion protection provided by the grout.

Project conditions may, however, require that other nail inclinations be used. For example, Figure 6.2b shows a case in which utilities or other underground structures are located within the proposed soil nail zone. In most cases, this situation only occurs for the upper first and second rows of nails. Another situation where different nail inclinations may be used is at exterior wall corners. To avoid intersecting nails behind exterior corners of a wall, nail inclination on one side of the corner could be installed with a different inclination. An alternative layout for exterior corners is to splay the nails on a plan view (Figure 6.2c).

Overhead space restrictions may require that the nail inclination be smaller than 15 degrees.

This might be the case for road widening at embankment bridge abutments. Logistical limitations due to location of nailing equipment (i.e., operating at the bottom of a narrow excavation) may require a steeper nail inclination.

The effect of nail inclination should be considered in global and local stability analyses of the soil nail wall system because stability factors of safety for the system, particularly for sliding wedge analyses in the upper portion of the wall, can decrease significantly as the nail inclination increases below the horizontal.

E. Soil Nail Length and Distribution

The distribution of soil nail lengths in a soil nail wall can be selected as either uniform (i.e., only one nail length is used for the entire wall), or variable, where different nail lengths may be used for individual soil nail levels within a wall cross section. Additional information on nail distribution is provided below.

15°

SV

(b) CHANGE OF NAIL DECLINATION AROUND UTILITIES UTILITIES

(c) NAIL SPLAYING IN CORNERS SH

SH SH SH SH

<15°

>15°

TOP OF WALL

BOTTOM OF EXCAVATION

(a) EXAMPLE OF NAIL ARRANGEMENT FOR NONHORIZONTAL GROUND

ALL S AND S MUST BE SMALLER THAT MAXIMUM SPECIFIED

v h

NAIL LOCATION (TYP)

CROSS-SECTION PLAN VIEW (NOT TO SCALE)

SH NAILS 1 AND 2 ARE

OFFSET HORIZONTALLY TO AVOID INTERSECTION

1

2

SH

SH

SV

SVO

SH

MANHOLE

Figure 6.2: Varying Nail Patterns.

• Uniform Nail Length: When the potential for excessive wall deformation is not a concern (e.g., soil nail walls constructed in competent ground or in an area without nearby structures), it is beneficial to select a uniform length distribution because it simplifies construction and quality control. Additionally, a slightly smaller total length of nails is obtained with a uniform soil length pattern. This pattern provides commonly a high sliding stability safety factor. Uniform patterns should be used in most projects.

• Variable Nail Lengths: Occasionally, a variable nail length distribution may be used if wall deformations need to be controlled. The global equilibrium and deformation pattern of a system with different nail lengths will be different from a system with uniform nail lengths. Field measurement data from constructed soil nail walls indicate that wall displacements can be significantly reduced if the nail lengths in the upper two-thirds to three-quarters of the wall height are greater than those in the lower portion. Placing additional reinforcement (i.e., length of soil nail) near the top of the wall will provide more resistance to wall movement at the critical areas near the top of the wall. As lower nails are shorter in non-uniform length patterns, this distribution tends to produce a lower sliding stability safety factor.

Figure 6.3 shows different nail length distributions for the same height wall and the required total nail length to obtain a factor of safety of 1.35. The maximum calculated total length (corresponding to Figure 6.3d) is 12 percent greater than that required for the uniform length pattern (base case, Figure 6.3a). For this particular set of examples, the comparison indicates that factors of safety are not very sensitive to nail distribution with depth. However, certain nail length distributions may result in less wall deformation than other nail layouts despite having similar factors of safety. In addition, some nail length distributions may have too short nails in the lower portion of the wall; this unfavorable condition may lead to a sliding stability failure.

Performance of soil nail walls has shown that larger displacements are observed when the upper nails are too short. The deformations in soil nail walls can be significantly reduced when nails at the top of the structure are longer than required by stability analysis. In general, the higher the global factor of safety of a soil nail wall, the smaller the wall deformations.

Therefore, all other variables being equal, the nail layouts shown in Figures 6.3c and 6.3d are likely to result in smaller wall deformations, especially near the top of the wall.

Nail lengths have been installed successfully with a uniform nail length in the upper two-thirds to three-quarters of the wall, with progressively shorter nails to a minimum value, not smaller than 0.5 H (H is the wall height), at the bottom of the wall in dense cohesionless soils that provide relatively large sliding stability. In general practice, nail length in the lower rows should never be shorter than 0.5 H. Nail lengths less than 0.5 H will not likely satisfy sliding stability requirements. As an example, the nail distribution shown in Figure 6.3d may not meet sliding stability requirements. In all cases, and especially where reducing the nail lengths in the lower reaches of the wall are considered, stability analysis considering sliding need to be performed as part of a detailed design.

In general, variable nail lengths result in a more complicated installation and require more nail materials. Nevertheless, as many soil nail projects are specified based on performance criteria, contractors may prefer to use longer nails in the upper rows to reduce deflections. Project specifications must provide ROW constraints, locations of underground utilities and substructures (or requirements that the contractor locate these), and specific deformation criteria (i.e., maximum wall deflection and location where this deflection is to be measured).

ri = Length of Nail i, Li Length of Nail 1, L1

Note: Nails and critical slip surfaces are drawn to scale

Figure 6.3: Effect of Different Nail-Length Patterns.

Based on the discussion presented in this section, the following recommendations are made concerning soil nail length and distribution.

• Select uniform length pattern whenever possible.

• Select longer nails than required by the target factor of safety as a means to reduce wall deformations in the upper portions of the wall.

• Avoid the use of “short” nails in top portion of wall.

• Avoid the use of too “short” nails in lower portion of wall. Evaluate if shorter nails in bottom rows installed in competent ground satisfy sliding stability requirements. Shorter nails at the bottom should be not smaller than 0.5 H.

• Non-uniform nail length patterns may be used if soil layers with very dissimilar conditions are encountered.

For feasibility evaluations, soil nail length can be initially assumed to be 0.7 H, where H is the height of the wall. The length of the nails may be greater than 0.7 H if large surcharge loads are expected or if the wall is very high [greater than 10 m (approximately 30 ft) high]. In Step 2 of the design method presented herein (see Table 6.1), simplified design charts are used to select the length of the nails.

F. Soil Nail Materials

Select appropriate grade of steel for the soil nail bar. Information on the selection of steel grade is presented in Chapter 4; however, for most applications Grade 420 MPa (Grade 60) steel is used.

G. Soil Properties

The procedures and methods used to select soil properties for the analysis and design of soil nail walls is provided in Chapter 3. The ultimate bond strength for the grout-ground interface can be selected using Table 3.10.

H. Other Initial Considerations

• Evaluate corrosion potential (see Section 3.9 and Appendix C).

• Evaluate drilling methods likely to be used by prospective contractors for the project. This information is used to select a design ultimate bond strength value.

• Estimate drillhole diameter based on previous experience in similar ground and diameter restrictions imposed by selected level of corrosion protection.

• Select factors of safety (see Section 5.9) for the different failure modes (e.g., global stability, sliding, tensile strength, pullout).

• Define loads

6.3 STEP 2: PRELIMINARY DESIGN USING SIMPLIFIED CHARTS