STEP 2: PRELIMINARY DESIGN USING SIMPLIFIED CHARTS

Nail length, diameter, and spacing typically control external and internal stability of a soil nail wall.

Therefore, these parameters may be adjusted during design until all external and internal stability requirements are satisfied [i.e., calculated factors of safety for each failure mode are larger than the minimum values selected for the design (e.g., Table 5.3)]. A series of charts was developed in this document as a design aid to provide preliminary nail length and maximum tensile forces (see Appendix B). The charts were developed using the computer program SNAIL, which was selected because it is public domain software, readily available, and free of charge. In preparing these charts, the following main assumptions were made:

• homogenous soil;

no surcharge;

• no seismic forces;

• uniform length, spacing and inclination of nails; and

• no groundwater.

When the conditions of a new analysis case, do not match the assumptions listed above, it is recommended that interpolations or extrapolations be made to estimate the soil lengths from these charts. Alternatively, the use of a preliminary nail length between 0.7 to 1.0 times the wall height can be made. The upper range of soil nail length is used for less favorable soil conditions, wall heights greater than 10 m (30 ft), and where large surcharge loads need to be resisted by the wall.

The charts were developed for different values of face batter (α), backslope (β), effective friction angle (φ′), and ultimate bond strength (qu). Table 6.2 presents the set of values used for the development of the design charts.

Table 6.2: Variable Parameters.

Parameter Units Values

Face Batter Degrees 0, 10

Backslope Degrees 0, 10, 20, 30

Effective Friction Angle Degrees 27, 31, 35, 39 Ultimate Bond Strength KPa 52, 104, 172, 276, 689

These values encompass a wide range of soil nail wall geometries and ground conditions. If intermediate parameter values are required for a particular project application, then it is acceptable to evaluate preliminary values of nail length and maximum nail tensile force by interpolating between values on the charts.

The first type of charts was developed to evaluate the nail length (Figures B.1a through B.6a in Appendix B) for combinations of α and β. Using these charts, the required nail length, L, (normalized with respect to the wall height, H) to achieve a global safety factor FSG = 1.35 is obtained as a function of the normalized allowable pullout resistance (µ). The normalized allowable pullout resistance is defined as:

V H P

DH u

S S γ FS

D

µ = q (Equation 6.1)

where FSP is the factor of safety against pullout (typically 2.0); DDHis the drillhole diameter; γ is the total unit weight of the soil behind the wall; and SH and SV are the horizontal and vertical nail spacing, respectively. The nail lengths in these charts were computed based on the most critical failure surface (i.e., considering base and toe failures) for the selected geometry and material properties, and assuming that failure of the nail (i.e., tensile breakage) and/or failure of the facing would not take place. Therefore, the pullout failure is implicitly assumed. Equation 6.1 is based on a drillhole diameter of 100 mm (4 in.). Also, the use of Equation 6.1 inherently assumes that the soil has a cohesion intercept c′ such that c* = c/γH = 0.02. If the drillhole diameter or cohesion

intercept values being considered are different than the assumptions stated here, then adjustments to the calculated nail length and maximum tensile forces are made in the final step. These adjustments are discussed subsequently.

The second type of charts (Figures B.1b through B.6b in Appendix B) provides the corresponding maximum normalized design tensile force of all nails (tmax-s) as a function of µ calculated for a global safety factor of 1.0. The maximum normalized design tensile force in the bar is defined as:

V H

s s max

max HS S

t T

= γ ⁻

− (Equation 6.2)

With tmax-s read from the design charts, the maximum nail tensile force, Tmax-s can be calculated using Equation 6.2. These design charts are developed for the case in which all nail bars are the same length. These design charts do not provide information on the distribution of tensile load in individual soil nails or the maximum load in any particular nail. A preliminary design with the charts is not necessary if a full, final design using computer programs will be used. However, even a final design is performed, the charts can still provide preliminary values.

6.3.2 Preliminary Design Procedure

A step-by-step procedure for preliminary design using the charts provided in Appendix B is presented in this section. An example of the use of the charts is provided in the example presented in Section 6.7.

1. For a specific project application, evaluate batter (α), backslope (β), effective friction angle (φ′), and ultimate bond strength (qu). Calculate normalized pullout resistance (µ) using Equation 6.1.

2. Obtain normalized length (L/H) from the first set of charts (Figures B.1a through B.6a in Appendix B).

3. Obtain normalized force (tmax-s ) from the second set of charts (Figures B.1b through B.6b in Appendix B).

4. Using Figure B.7, evaluate correction factors for: (a) normalized length to account for a drillhole diameter other than 100 mm (4 in.) (correction factor C1L), (b) a c* value other 0.02 (correction factor C2L), and (c) a global factor of safety other than 1.35 (correction factor C3L).

5. Using Figure B.7, evaluate correction factors for normalized maximum nail force to account for: (a) a drillhole diameter other than 100 mm (4 in.) (correction factor C1F), and (b) a c* value other 0.02 (correction factor C2F).

6. Apply correction factors to normalized length and/or normalized force. Calculation method is provided on Figure B.7.

7. Multiply the normalized length by the wall height to obtain the soil nail length.

8. Calculate the maximum design load in the nail Tmax-s using the value of tmax-s and Equation 6.2.

9. Calculate the required cross-sectional area (At) of the nail bar according to:

y T s t max

f FS

A = T ⁻ (Equation 6.3)

where fy is the steel yield strength and FST is the factor of safety for nail bar tensile strength (see Table 5.3).

10. Select closest commercially available bar size using Table A.1 that has a cross-sectional area of at least that evaluated in the previous step.

11. Verify that selected bar size fits in the drillhole with a minimum grout cover thickness of 25 mm (1 in.).

12. If the length and/or nail diameter are not feasible, select another nail spacing and/or drillhole diameter, recalculate the normalized pullout resistance, and start the process again.