Retaining & slopes · Anchors, nails & shoring
Ground anchor design calculator
Strand count, fixed (bond) length, free length beyond the active wedge, lock-off and test loads, elongation and grout for a grouted ground anchor.
Standard: FHWA-IF-99-015 · BS 8081 · AS/NZS 4672 strand
Open the live calculator → All retaining & slopes calculators
Inputs in the worked example
The live calculator starts with these values; change any of them and every result, check and diagram updates instantly.
| Input | Value | Unit |
|---|---|---|
| Horizontal load on the wall at the anchor row | 120 | kN/m |
| Anchor horizontal spacing | 2.5 | m |
| Inclination below horizontal | 25 | ° |
| Excavation depth H | 8 | m |
| Anchor head depth below top | 3 | m |
| Soil φ′ (active wedge) | 32 | ° |
| Strand (AS/NZS 4672) | 15.2 mm, 143 mm², 250 kN | |
| Drill-hole diameter | 150 | mm |
| Ultimate grout–ground bond τult | 150 | kPa |
| Factor of safety on bond | 2 | |
| Minimum fixed length | 4.5 | m |
| Maximum effective fixed length | 12 | m |
| Ultimate grout–strand bond | 2 | MPa |
| Lock-off load / design load | 1 | |
| Test load / design load | 1.33 | |
| Strand modulus Es | 195000 | MPa |
Worked example — results
| Result | Value | Unit |
|---|---|---|
| Design (working) load DL | 331.0 | kN |
| Strands required | 3 | |
| Tendon use DL / 0.6 MBL | 74 | % |
| Lock-off load | 331 | kN |
| Test (proof) load | 440 | kN |
| Fixed length required | 9.37 | m |
| Fixed length adopted | 9.5 | m |
| Grout–strand bond length needed | 2.31 | m |
| Distance to active plane along anchor | 2.43 | m |
| Free length adopted | 4.5 | m |
| Total anchor length | 14.0 | m |
| Depth to top of fixed length | 4.90 | m |
| Elastic elongation at test load | 23.7 | mm |
| Grout volume per anchor | 0.247 | m³ |
- ✓ Design load ≤ 0.60 × MBL
- ✓ Lock-off ≤ 0.70 × MBL
- ✓ Test load ≤ 0.80 × MBL
- ✓ Required fixed length ≤ effective maximum — 9.4 m
- ✓ Fixed length ≥ grout–strand bond length
- ✓ Fixed length starts ≥ 4.5 m below ground — 4.9 m
Method, formulas and assumptions
- DL = Fh·s / cos θ = 120 × 2.5 / cos 25° = 331.0 kN
- Strands n = ⌈DL / (0.6 × 250 kN)⌉ = 3 (FHWA: design ≤ 60 %, lock-off ≤ 70 %, test ≤ 80 % of MBL)
- Fixed length Lb = FS·DL / (π·D·τult) = 2 × 331.0 / (π × 0.15 × 150) = 9.37 m → 9.5 m (min 4.5 m)
- Active plane from the excavation base at 45° + φ′/2; distance along the anchor = 2.43 m. Free length = that + max(1.5 m, 0.2H), at least 4.5 m
- Elongation = Ptest·Lf / (Ap·Es) — the apparent free length from the test should be 80–110 % of the design free length
- τult must come from the site investigation or anchor trials: dense sand 70–380 kPa, stiff clay 30–100 kPa, weak rock about UCS/10 (FHWA presumptive values)
Related retaining & slopes calculators
- Soil nail wall (simplified) — Nail forces from active pressure, pull-out beyond the failure plane and tendon tension for each nail row.
- Shotcrete facing (soil nail / anchored wall) — Flexure and punching-shear resistance of a reinforced shotcrete facing against the nail-head load, plus…
- Ground anchor acceptance test — From the elastic extension between alignment and test load: apparent free length against the acceptance…
- Soil nail verification / proof test — Checks that the test reached the required multiple of the design load without pull-out or excessive creep…
- Lateral earth pressure — Active, passive or at-rest pressure diagram with water table, surcharge and cohesion.
- Seismic earth pressure (Mononobe–Okabe) — Static and seismic active coefficients, thrusts and overturning moment on a retaining wall.
For preliminary design and education. Check every result against the current standard and your own engineering judgement before use.