What factors of safety apply to soil nail walls?

FHWA GEC 7 (FHWA-NHI-14-007, 2015) Table 5.1 gives minimum ASD factors of safety for static loads: overall stability 1.5 (1.35 may be accepted for non-critical permanent walls), pullout 2.0, lateral sliding 1.5, bar tension 1.8 for Grade 60 and 75 bars (2.0 for Grade 95 and 150), facing flexure 1.5, facing punching shear 1.5, and basal heave 2.0 short-term / 2.5 long-term.

What bond strength should I use for a soil nail?

For preliminary design, GEC 7 Tables 4.4a and 4.4b give presumptive nominal bond strengths by drilling method and soil type — for example 15–26 psi for rotary-drilled sand/gravel, 6–17 psi for Piedmont residual soil, and 5–7 psi for rotary-drilled silty clay. Final design bond strengths must be verified by load tests.

How large is the maximum load in a soil nail?

GEC 7 §5.4 summarizes field measurements: the maximum nail force in the upper two-thirds of a wall ranges from about 0.5 to 1.1 Ka·γ·H·Sv·Sh, averaging about 0.75 Ka·γ·H·Sv·Sh; nails in the lower third carry roughly half of that. This calculator reports the 0.75 estimate as a check on the bar size.

How long should soil nails be?

Long enough that the nails anchor past every potential slip surface with enough pullout capacity to give the required overall factor of safety. Length depends on bond strength, drill hole diameter, spacing, soil strength and surcharge. This calculator searches for the shortest uniform nail length that gives an overall-stability FoS of 1.5 in its planar-wedge model; a final design uses slip-surface software such as SNAP-2.

Is a single-wedge analysis enough to design a soil nail wall?

No. GEC 7 presents the simple wedge (Eq. 5.9) to show how the forces interact and to spot-check computer results. Final design searches many slip-surface shapes — through, behind and beneath the nails — and adds facing, drainage, corrosion protection and load-test requirements.

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Soil Nail Wall Calculator (FHWA GEC 7)

Preliminary sizing of a soil nail wall in a vertical cut with level ground above. Overall stability is found by searching planar slip surfaces through the toe with the GEC 7 simple-wedge equation, taking each nail's resistance as the lesser of its allowable pullout behind the slip surface and its allowable bar tension. The calculator also reports the GEC 7 estimate of maximum nail load, lateral sliding of the nailed block, and the shortest nail length that reaches FoS 1.5.

Units

Wall height, Hft

Soil unit weight, γpcf

Friction angle, φdegrees

Cohesion, c (0 is conservative)psf

Uniform surcharge, qpsf

Horizontal spacing, S_hft

Vertical spacing, S_vft

Depth of top nail rowft

Nail length, Lft

Nail inclination below horizontal, idegrees

Drill hole diameter, D_DHin

Soil / drilling method (GEC 7 Table 4.4 — fills in the low end of the q_u range)

Nominal bond strength, q_upsi

Nail bar

Bar grade

Number of nail rows——

Allowable bar tension, f_yA_t/1.8—kips

Allowable pullout per unit length, πD_DHq_u/2.0—kips/ft

Overall stability FoS, critical wedge (≥ 1.5)——

Critical slip plane angle from horizontal—degrees

FoS of the unreinforced cut (same search)——

Estimated max nail load, 0.75K_aγHS_vS_h / allowable bar tension—kips

Lateral sliding of the nailed block (≥ 1.5)——

Shortest uniform nail length for FoS 1.5—ft

Summary——

Rows start at the top-row depth and repeat every S_v while at least S_v/2 remains above the toe. Slip planes pass through the toe at 15°–89°; each nail contributes the lesser of its allowable pullout on the length behind the plane and its allowable bar tension, divided by S_h. Nail-head and facing strength are assumed to develop the nail force — the facing must be designed for it. Vertical face and level ground only; no water table, seismic load, or basal heave check.

$$ FS = \frac{c\,L_f + \left[(W+Q)\cos\alpha + T\sin\beta\right]\tan\varphi}{(W+Q)\sin\alpha - T\cos\beta},\qquad W = \frac{\gamma H^2}{2\tan\alpha},\quad Q = \frac{qH}{\tan\alpha},\quad L_f = \frac{H}{\sin\alpha} $$

$$ T = \frac{1}{S_h}\sum_j \min\!\left(\frac{\pi D_{DH}\,q_u\,L_{p,j}}{FS_{PO}},\ \frac{f_y A_t}{FS_T}\right),\qquad L_{p,j} = L - \frac{H - z_j}{\cos i\,\tan\alpha + \sin i} $$

α slip-plane angle · β = α + i, angle between nail and plane · z_j nail row depth · L_p,j nail length behind the plane · FS_PO = 2.0, FS_T = 1.8 (GEC 7 Table 5.1). This is GEC 7 Eq. 5.9 rearranged for FS, with a cohesion term added. Sliding: nailed block of width L cos i, FS = (W_block tan φ + c·L cos i) / (½K_aγH² + K_aqH).

Presumptive bond strength (FHWA GEC 7)

Estimated nominal bond strength q_u of soil nails — GEC 7 (2015) Tables 4.4a and 4.4b, modified after Elias and Juran (1991)
Drilling method	Soil type	q_u (psi)	q_u (kPa)
Rotary drilled	Sand/gravel	15–26	103–179
Rotary drilled	Silty sand	15–22	103–152
Rotary drilled	Silt	9–11	62–76
Rotary drilled	Piedmont residual	6–17	41–117
Rotary drilled	Fine colluvium	11–22	76–152
Driven casing	Sand/gravel, low overburden	28–35	193–241
Driven casing	Sand/gravel, high overburden	41–62	283–427
Driven casing	Dense moraine	55–70	379–483
Driven casing	Colluvium	15–26	103–179
Augered	Silty sand fill	3–6	21–41
Augered	Silty fine sand	8–13	55–90
Augered	Silty clayey sand	9–20	62–138
Rotary drilled	Silty clay	5–7	34–48
Driven casing	Clayey silt	13–20	90–138
Augered	Loess	4–11	28–76
Augered	Soft clay	3–4	21–28
Augered	Stiff clay	6–9	41–62
Augered	Stiff clayey silt	6–15	41–103
Augered	Calcareous sandy clay	13–20	90–138

Low and high overburden for driven casing in sand/gravel mean effective overburden pressure below and above 1.5 tsf. These values are for preliminary design; GEC 7 requires verification of design bond strengths by load tests.

Minimum factors of safety (GEC 7 Table 5.1, ASD, static)

Limit state	Static	Seismic	In this calculator
Overall stability	1.5*	1.1	Planar wedges through the toe
Overall, excavation (short term)	1.25–1.33	—	No
Basal heave	2.0 / 2.5	2.3	No
Pullout	2.0	1.5	Yes, inside the wedge search
Lateral sliding	1.5	1.1	Yes
Bar tension, Grade 60 / 75	1.8	1.35	Yes
Bar tension, Grade 95 / 150	2.0	1.50	Not offered
Facing flexure / punching shear	1.5	1.1	No — design the facing

* GEC 7 note 2: some owners accept 1.35 for non-critical permanent walls when geotechnical information is sufficient and there is successful local soil-nailing experience.

Worked example

Example — 20-ft cut in sand (the calculator defaults)

Given: H = 20 ft, γ = 120 pcf, φ = 32°, c = 0, q = 100 psf; nails on a 5 × 5 ft grid starting 2.5 ft down, L = 16 ft, 15° below horizontal, 6-in drill holes, q_u = 15 psi (lower bound for rotary-drilled sand/gravel), #8 Grade 75 bars.

Allowable bar tension = 75 × 0.79 / 1.8 = 32.9 kips · allowable pullout = π(6)(15)(12)/2.0 = 1,696 lb per ft of nail · four rows at 2.5, 7.5, 12.5 and 17.5 ft

Critical plane 42.5°: W + Q = 26,191 + 2,183 = 28,374 lb/ft. Nail lengths behind the plane: 0.70, 5.07, 9.44, 13.81 ft → pullout-limited forces 1.2, 8.6, 16.0, 23.4 kips (all under the 32.9-kip bar limit) → T = 49.3 / 5 = 9.85 kips/ft

β = 42.5 + 15 = 57.5° · FS = [(28,374 cos 42.5° + 9,851 sin 57.5°) tan 32°] / [28,374 sin 42.5° − 9,851 cos 57.5°] = 18,263 / 13,877 = 1.32 ✗ (needs 1.5). Without nails the same search gives FS ≈ 0 — clean sand will not stand vertical.

Bond, not steel, governs: every nail is pullout-limited, and the estimated maximum nail load 0.75(0.307)(120)(20)(5)(5) = 13.8 kips is well under the bar's 32.9 kips. The length search returns L = 17.5 ft (0.88H) for FoS 1.5; closer spacing or larger holes are the alternatives. Lateral sliding FoS = 2.90 ✓.

References: Lazarte, C.A., Robinson, H., Gómez, J.E., Baxter, A., Cadden, A. & Berg, R. (2015), Soil Nail Walls Reference Manual, FHWA-NHI-14-007, Geotechnical Engineering Circular No. 7 — Tables 4.4a, 4.4b and 5.1, Eq. 5.9, §5.4 (maximum nail forces), §5.7 (lateral sliding). Elias, V. & Juran, I. (1991), Soil Nailing for Stabilization of Highway Slopes and Excavations, FHWA-RD-89-198.

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