Ground anchor

[edit] Introduction

Ground anchors, otherwise known as an earth, percussion driven or mechanical anchors, are versatile devices used to hold, restrain and support building, civil engineering and other structures, either permanently or temporarily.

They come in a wide range of sizes and capacities, up to 70 m in length, with a capacity of more than 3,000 kN. They are lightweight, corrosion-resistant anchors that can be installed from ground level, either by hand or using portable equipment, depending on size and ground conditions. When loaded, they exert pressure on a cone of the ground that surrounds their length, providing very good resistance to movement.

As they create minimal soil disturbance during installation and can be stressed to an exact holding capacity, they offer a popular technique for anchoring a wide range of structures into place:

• Agricultural fixings.
• Bridges.
• Buoyancy control.
• Drainage.
• Erosion control measures.
• Foundations.
• Gabion support.
• Geomesh or geogrids.
• Guyed structures such as radio masts.
• Landfill capping.
• Marine applications such as floating docks and pipelines.
• Pipelines.
• Portable buildings/structures.
• Retaining walls.
• Rock retention.
• Scaffolding.
• Security fixings.
• Sheet piling.
• Slope stabilization measures.
• Temporary works.
• Tie backs for watercourse walls.
• Tunnel linings.
• Utility poles.

[edit] Design and installation

The life expectancy of an anchor is dependent upon the corrosivity of the soil in which it is placed and the materials used. The main component of the anchor, sometimes describes as a ‘tendon’ can be made from a wide range of materials:

• Steel bar or wire strand.
• GRP.
• Alumimium alloy - 30 years+.
• Hard anodised aluminium alloy - 40 years+.

Permanent anchors may include additional corrosion resistant protection. Temporary anchors may be removed after use.

The method of installation will vary according to the situation; drive rods, spiral sockets and impact hammers are commonly used to push or screw the anchor into the ground, as well as simple hand tools. Depending on the ground conditions, it may be necessary to bore a hole first for the installation of the anchor, and sometimes it may be necessary to use a casing to support the hole before the anchor is installed.

The hole may be pre-grouted hole or post-grouted after installation. Typically, the anchor is then tensioned and locked off against a head plate.

Care must be taken to ensure that no services or other obstructions in the ground are damaged during installation.

The ultimate performance of the anchor is dependent upon:

• The shear angle of the soil.
• The size of the anchor.
• The depth of the installation.
• The load applied to the anchor.

Anchors can perform very well in granular soils as well as stiff, cohesive soils. Soft alluvial clays which are weaker may require a larger anchor size and a deeper driven depth.

The pullout capability of anchors can be tested in similar ground conditions before installation.

[edit] Benefits

There are several benefits to using ground anchors. These include:

• They are lightweight and corrosion-resistant which makes them suitable for a range of design life requirements and soil conditions.
• They are quick and easy to install.
• Installation allows for minimal disturbance, making them suited to complex and sensitive sites.
• They can be installed with minimum disruption to surface finishes.
• They can achieve a holding capacity of up to 3,000 kN.
• They have low environmental impact.

[edit] Find out more

[edit] Related articles on Designing Buildings Wiki

• Bored piles.
• Building foundations.
• Compensated foundation.
• Continuous flight auger piles.
• Diaphragm wall.
• Driven piles.
• Footings.
• Geothermal pile foundations.
• Ground heave.
• Groundworks.
• Micropiles.
• Pad foundations.
• Pile foundations.
• Prestressed concrete.
• Raft foundation.
• Retaining walls.
• Screw pile foundations.
• Shoring.
• Socket piles.
• Soil nailing.
• Temporary works.
• Tension cable and rod connectors.
• Tension piles.
• Types of fixings.
• Types of nails.
• Underpinning.

[edit] External references

• Platipus - Anchors
• BS 8081:2015 Code of practice for grouted anchors