Ground anchor bolts in construction are used to connect structural and non-structural elements to the concrete. The connection is made by an assembling of different components such as: anchor bolts, steel plates, stiffeners. Anchor bolts transfer different types of load: tension forces and shear forces. A connection between structural elements can be represented by a steel column attached to reinforced concrete foundation. Whereas, a common case of non-structural element attached to a structural one is represented by the connection between a facade system and a reinforced concrete wall.
Ground anchors are effectively restraining devices used in many different types of structures including retaining walls, dams, wharves, bridge abutments and foundations for buildings. Anchors are stressed (active anchorage) to prevent structural movement and they typically transfer their load over a fixed length. These are commonly referred to as tension anchors and are suited for strong rock conditions. For anchors founded in soil or weak rock, load distributive compressive (and tension) anchors are used as they rely on the succession of small successive bond lengths rather than one unique longer bond length.
Types of ground anchor bolts used in construction
- Straight shaft gravity-grouted / low-pressure-grouted; these are typically installed in rock, very stiff to hard cohesive soils, and sandy or gravelly soils using either rotary drilling or hollow-stem auger methods.
- Straight shaft pressure-grouted; these are most suitable for coarse granular soils and weak fissured rock, and are also used in fine grained cohesionless soils. Grout is injected into the bond zone with pressure greater than 150 psi. A variety of drilling and grouting techniques can be used.
- Single-under-reamed or multi-under-reamed; these are primarily installed with large uncased drilled holes in cohesive soils and grout is placed with no pressure.
- Post-grouted; these are installed using delayed multiple grout injections and are used to enlarge the surface area of the grout body of the anchors listed above to increase the load transfer capacity
The components of ground anchors
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.
Grouted ground anchors, referenced simply as ground anchors, are installed in grout filled drill holes. Grouted ground anchors are also referred to as “tiebacks”. The basic components of a grouted ground anchor include the:
- anchorage;
- free stressing (unbonded) length;
- bond length.
The anchorage is the combined system of anchor head, bearing plate, and trumpet that is capable of transmitting the prestressing force from the prestressing steel (bar or strand) to the ground surface or the supported structure. Anchorage components for a bar tendon and a strand tendon respectively. The unbonded length is that portion of the prestressing steel that is free to elongate elastically and transfer the resisting force from the bond length to the structure. A bond breaker is a smooth plastic sleeve that is placed over the tendon in the unbonded length to prevent the prestressing steel from bonding to the surrounding grout. It enables the prestressing steel in the unbonded length to elongate without obstruction during testing and stressing and leaves the prestressing steel unbonded after lock-off. The tendon bond length is that length of the prestressing steel that is bonded to the grout and is capable of transmitting the applied tensile load into the ground. The anchor bond length should be located behind the critical failure surface.
A portion of the complete ground anchor assembly is referred to as the tendon. The tendon includes the prestressing steel element (strands or bars), corrosion protection, sheaths (also referred to as sheathings), centralizers, and spacers, but specifically excludes the grout The sheath is a smooth or corrugated pipe or tube that protects the prestressing steel in the unbonded length from corrosion. Centralizers position the tendon in the drill hole such that the specified minimum grout cover is achieved around the tendon. For multiple element tendons, spacers are used to separate the strands or bars of the tendons so that each element is adequately bonded to the anchor grout. The grout is a Portland cement based mixture that provides load transfer from the tendon to the ground and provides corrosion protection for the tendon.
Design and installation of ground anchors
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:
- A steel bar or wire strand
- GRP
- Aluminum alloy – 30 years+
- Hard anodized aluminum 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.