What is a strand anchor?
A strand anchor is a grouted ground anchor whose tendon is made of several high-strength steel prestressing strands rather than one solid bar. The grout body at the far end bonds to the ground, and the load is transferred by friction between that grout body and the surrounding soil. The number of strands is chosen to suit the required load; manufacturers offer from one or two strands up to a dozen or more per anchor.
The anchor is installed in a cased borehole. The strand bundle is inserted, and as the casing is withdrawn the hole is filled with grout under pressure. Like every grouted anchor it has a free length, which lets the tendon stretch when it is stressed, and a fixed (bonded) length that passes the load into the ground. Once the grout has hardened, the anchor is tested and locked off at its working load.
Types
- Temporary anchors: under EN 1537 a short service life of up to two years, with minimal corrosion protection.
- Permanent anchors: double corrosion protection, typically individually greased and sheathed strands plus a grouted corrugated sheath around the bond length.
- Removable anchors: temporary strand anchors whose strands can be pulled out once the works are finished.
In practice
- Weigh strand against bar. Consider temporary or permanent use, aggressive ground, load level, soil layering and construction sequence. Strand anchors suit very high loads; bar anchors are stiffer and more robust.
- Think about what stays behind. In towns, removable anchors are often the better choice so that strands are not left under neighbouring land or service routes.
- Take care with corrosion protection. Poorly installed protection is a known cause of corrosion in permanent anchors.
- Leave design and testing to specialists. Length, inclination and load come from the geotechnical design, and every anchor should be tested.
Common mistake: leaving temporary anchors in the ground without recording them. They can obstruct later excavations or boreholes, so decide at the design stage whether they must be removable.
Related terms
- Grouted anchor
- Sheet pile wall
- Excavation pit
- Deadman anchor
- Grout
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