Ground Anchor Calculator
Anchor count, length and grout volume - the prestressed upgrade to soil nails.
Also known as ground anchors · tieback anchor · soil anchor
With the default inputs: Wall length 40 m · Number of rows 3 rows · Horizontal spacing 2 m etc.Number of anchors 60 bars
Use your own dimensions ↓Inputs
Result
| Per row | 20bars |
| Length per anchor | 14.0m |
| Total length | 840m |
| Bond grout volume | 6.4m³ |
| Strand (3 × Φ15.2) | 2520m |
How this is worked out
An anchor is the prestressed upgrade to a soil nail: nails are grouted full length and work passively, while an anchor is grouted only over its bond, sleeved over the free length and tensioned to prestress the wall. The free length must cross the active slip surface and the bond must reach stable ground with enough pull-out capacity (normally ≥ 3 m, shorter in rock, longer in soft ground). Grout follows the bond hole volume and runs slightly over in porous ground. Strand is estimated as a common 3 × Φ15.2; strand count and prestress follow the design.
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Reverse: known result, work out one input
Enter a target result; the other inputs stay fixed and this one is worked out.
Formula
| Per row | n = ⌈length ÷ spacing⌉ |
| Total | N = per row × rows |
| Length | L = free + bond |
| Grout | V = N × π×(d/2)² × bond length |
How to work it out
- Anchor count = rows × anchors per row, per row = ⌈retention length ÷ horizontal spacing⌉.
- The free length must cross the active slip surface into stable ground; bond length is usually at least 3 m (shorter in rock, longer in soft soil).
- Total length = free + bond length; grout volume is the bond-hole volume plus an overfill allowance.
- Strands are usually 3 × Φ15.2; the number and prestress level follow design, not more-is-better.
- Tension in stages, lock off at the design prestress, and protect the anchor head against corrosion.
Where it goes wrong
- Free length too short - the bond length ends up inside the sliding mass and the anchor slides with it; useless.
- Incomplete grouting - pull-out relies on grout-to-soil bond, and any void cuts the capacity badly.
- Tensioning before the grout has strength - the grout cracks, the anchor relaxes, and the prestress is lost.
Material and cost
Anchors are priced per metre (drilling, strand, grouting, tensioning all in), from a few hundred to a thousand-plus each. With long retention and many rows the anchors can out-cost the piles, so lay them out carefully rather than blindly densifying.
On site
- Run pull-out tests first to confirm bond length and grout quality before full production.
- Grout the bond zone fully with a second pass and treat the overfill at the top.
- Tension in stages, hold each stage to watch displacement, and check anchor-head movement after lock-off.
Basis
- EN 1537:2013 Execution of special geotechnical work — Ground anchors
- BS 8081:2015 Code of practice for grouted anchors
- EN 1997-1:2004, Section 8 (anchors and tiebacks)
- IS 10270:1982 Guidelines for Design and Construction of Prestressed Rock Anchors
FAQ
What is the difference between an anchor and a soil nail?
How long should the bond be?
What to calculate next
These jobs are done back to back on site — follow the steps to work through the whole sequence.
Retaining: pressure to supportStep 4 of 4
- 1Earth Pressure Calculator
- 2Retaining Wall Stability Calculator
- 3Soldier Pile Calculator
- 4Ground Anchor Calculator
Cite this page
Use this format in reports, method statements or coursework. The link is permanent.
Building Calculator. Ground Anchor Calculator. 2026-10-25. https://jianzhujisuan.com/en/ground-anchor