Tools › Civil and Geotechnical › Foundations › Soil Nail and Anchored Walls › Example
25 ft soil nail wall: nail and facing resistances, capacity-to-demand ratios and overall stability (FHWA GEC 7 Appendix C)
| Tool | Soil Nail and Anchored Walls |
| Source | Lazarte, C. A., Robinson, H., Gómez, J. E., Baxter, A., Cadden, A., and Berg, R. (2015). Soil Nail Walls Reference Manual. Geotechnical Engineering Circular No. 7, FHWA-NHI-14-007, Federal Highway Administration, Washington, DC, Appendix C, pp. 301–317. |
| Level | Professional |
Problem
A permanent 25 ft vertical soil nail wall is cut in medium dense silty sand (125 pcf, friction angle 32°, cohesion 100 psf, bond strength 14.5 psi) with level ground and a 150 psf live load over the 25 ft behind the crest. Five rows of No. 7 Grade 75 bars, 18 ft long, inclined 15° in 6 in drill holes, are spaced 5 ft both ways with the top row 3 ft down. The initial facing is 4 in of 4,000 psi shotcrete with 6 × 6 W2.9 mesh, two No. 5 bars each way at the nail heads and 9 in bearing plates; the final facing is 8 in of concrete with No. 4 bars at 12 in each way on four 1/2 in headed studs. Find the nail and facing resistances, the LRFD capacity-to-demand ratios and the factor of safety of overall stability.
Inputs to Enter
- Wall 25 ft, vertical, level backslope; surcharge 150 psf over 25 ft; S_H = 5 ft; rows at 3, 8, 13, 18, 23 ft, each 18 ft long at 15°
- Soil 125 pcf, φ = 32°, c = 100 psf; bond strength 14.5 psi; drill hole 6 in; bar 0.60 in², 75 ksi
- Initial facing 4 in, 4,000 psi, mesh 0.029 in² wires at 6 in, 0.62 in² of bars each way at the head, C_F = 2.0, plate 9 in × 1 in
- Final facing 8 in, 4,000 psi, 0.20 in² bars at 12 in; four studs 1/2 in shaft, 1 in head 0.31 in thick, 4.125 in long, at 6 in, 60 ksi
- Factors of safety 1.5 / 2.0 / 1.8 / 1.5 / 1.5 / 2.0; load factor 1.35; resistance factors 0.65 / 0.75 / 0.90 / 0.90 / 0.70
Answers
- Pullout π q_u D_DH = 3.280 kip/ft (book: 51.2 kip over 15.6 ft); tendon R_T = 45.0 kip (book 45)
- Initial facing: flexure 36.3 kip by equations 6.9 to 6.14 and 36.5 kip by the simplified equation 6.15 (book 36); punching shear 41.6 kip (book 41.6); ρ_tot 1.00 % between 0.25 % and 2 % (book 0.99 %)
- Final facing: flexure 61.6 kip, 60.8 kip by equation 6.15 (book 60.8); punching 37.1 kip (book 36.9 with h_c rounded to 4.8 in); headed studs 47.1 kip (book 47.1)
- With the book's loads T_max = 24.7 kip and T_o = 17.3 kip the test script returns the printed ratios: pullout 1.0, tendon 1.01, flexure 1.39 and 2.34, punching 1.60 (printed 1.58) and 1.42, studs 1.41 (printed 1.42)
- Tool's own stability analysis: FS = 1.24 on the critical Bishop circle (1.32 without the surcharge), T_max = 25.0 kip in row 5 limited by the tendon, T_o = 0.714 T_max = 17.9 kip; ratios 1.04, 1.00, 1.35, 1.55, 2.30, 1.39, 1.37
- Book (program SNAP-2): FS = 1.51 with the surcharge and 1.62 without; not reproduced. On the circle through the book's printed nail crossings the tool gives 1.31 and nail forces of 3.2, 7.5, 12.7, 18.9 and 25.0 kip against the printed 3.2, 7.2, 12.5, 19.0 and 24.7
- Displacement at the top H/500 = 0.60 in (book 0.6 in); ground movement to 1.25 H = 31.3 ft behind the wall (the book takes 0.8 H = 20 ft, the coefficient of weathered rock in its Table 5.12)
Notes
The tool's default inputs are this case; press Submit. The resistances and the verifications follow the manual to its printed digits; the overall factor of safety does not. Both analyses use Bishop's simplified method, but SNAP-2's manual does not say how the nail forces enter its equations, and the tool, which follows the manual's own wedge equations 5.6 to 5.9 with allowable nail forces, is about 18 % lower, so the page shows the example wall as short of the 1.5 it asks for. Choosing the ground anchor wall fills Design Example 1 of FHWA GEC 4, which the tool reproduces throughout (43.6 kPa, 168 and 172 kN/m, 435 and 445 kN, 8.9 m of bond, the embedment table of Figure A-4).