Tools › Civil and Geotechnical › Foundations › Sheet Pile and Braced Excavation › Example
Anchored sheet-pile bulkhead in two soil layers: toe-failure factor, anchor pull and Rowe moment reduction (NAVFAC DM-7.02 Figure 21)
| Tool | Sheet Pile and Braced Excavation |
| Source | Naval Facilities Engineering Command (1986). Foundations and Earth Structures. Design Manual NAVFAC DM-7.02, Department of the Navy, Alexandria, VA. Figure 21, Example of Analysis of Anchored Bulkhead, p. 7.2-93; Figures 18 and 19, pp. 7.2-88 and 7.2-89. |
| Level | Professional |
Problem
A sheet-pile bulkhead retains 30 ft of soil and is driven 14 ft below the dredge line, with one tie 6 ft below the top. The upper 24 ft is a soil with φ = 25°, c = 100 psf and 115 pcf; below it lies a soil with φ = 35°, c = 50 psf and 130 pcf. The water stands 5 ft below the top behind the wall and 7.5 ft below it in front. A 300 psf surcharge and a 5,000 lb/ft line load 3 ft from the wall act on the backfill. Find the factor of safety against toe failure by free earth support, the anchor pull, the maximum moment and the design moment after the reduction for wall flexibility.
Inputs to Enter
- Anchored wall, free earth support; H 30 ft, entered embedment 14 ft, anchor depth 6 ft; water behind 5 ft, in front 7.5 ft, net water pressure tapering to zero at the toe
- Upper layer 24 ft: 115 pcf (saturated 115.5), φ 25°, c 100 psf, K_A 0.41; lower layer: 130 pcf (saturated 130.5), φ 35°, c 50 psf, K_A 0.27, K_P 6.0 (DM-7.02 Figure 5 with wall friction 14°)
- Uniform surcharge 300 psf; line load 5,000 lb/ft at 3 ft; factor on the passive force 2.5
- Sheeting E 30,000 ksi, I 385.7 in⁴/ft, very compact coarse-grained curve; allowable bending 25 ksi; ties at 8 ft, 20 ksi; deadman 15 ft deep
Answers
- Active pressures 231 psf at 5 ft, 644 and 457 psf at the layer boundary, 567 psf at the dredge line, 824 psf at the toe (book 0.23, 0.65, 0.46, 0.57, 0.83 ksf); passive 245 and 5,965 psf (book 0.24 and 5.94 ksf); net water 156 psf (book 0.16 ksf)
- Passive force 43,472 lb/ft (book 43.26 kips), driving forces 29,179 lb/ft (book 29.65 kips from pieces rounded to two or three figures)
- Factor of safety against toe failure, moments about the anchor: 1,441,068 / 544,317 = 2.65 (book 2.61), above the 2.5 required
- Anchor pull by the figure's formula 29,179 − 43,472 / 2.5 = 11,790 lb/ft (book 12.37 kips); in the equilibrium state, with the passive pressure divided by 2.65, 12,759 lb/ft, which the tool uses
- Maximum moment at zero shear: 79,323 ft-lb/ft at 21.12 ft with the figure's anchor pull (book 86.9 ft-kips at 21.1 ft), 94,552 ft-lb/ft in the equilibrium state
- Flexibility number ρ = (528 in)⁴ / (30,000,000 psi ⋅ 385.7 in⁴/ft) = 6.72 in²/lb (book 6.7); M_design / M_max = 0.82 (book 0.83); design moment 77,668 ft-lb/ft, section modulus 37.3 in³/ft at 25 ksi
Notes
The tool's default inputs are this case; press Submit. The tool integrates the pressure diagram, the figure adds forces rounded by hand, one of which (P_A1 = 1.55 kips) is larger than its own diagram gives (1.43); because the anchor pull is a small difference of large numbers, the 1.7 % difference in the driving force becomes 4.6 % in the anchor pull and 9 % in the maximum moment. The moment ratio is read by eye from DM-7.02 Figure 19 (± 0.02).