A Ground Anchor, also known as Earth Anchor, is a structural member which transmits an applied tensile force to capable ground. The tensile force is resisted by shear strength of the surrounding ground. An earth anchor may comprise of following three main components
- Tension pile
- Rock bolt
Nowadays, a High strength steel tendons are installed at an inclination to resist the applied tensile load efficiently.
Anchor should have enough capacity required to keep the movement of the structure and the ground mass are kept to tolerable levels.
Characteristics of Anchor Tendons
- The tendon transmits the tensile force to the surrounding ground via the grout annulus.
- The tie back is usually a high strength steel member surrounded by grout.
- The tendon must be adequately protected against corrosion.
Anchor & Anchor Length
The fixed anchor length is that length of the anchor over which the tensile force is transmitted to the surrounding ground, while over the free anchor length no tensile force is transmitted to the surrounding ground mass.
Anchor is dead or prestressed. Large movements are necessary to mobilize the full load-carrying capacity, to lessen such movements to more acceptable levels, ground anchors are usually pre-stressed by tensioning. The level of prestressing is usually a percentage of the design working load.
Types of Ground Anchors
Types of anchor commercially available may be divided into three main groups
- Cylinder filled with grout
- Enlarged by grout injected under a high but controlled pressure.
- Cylinder mechanically enlarged at one or more positions along its length, to enable a larger load to be mobilized.
Type 1 Earth Anchor
This anchor type is not usually used in soils. It is mainly used for rocks, where the rock strata are stable. These are often referred as Rock Anchors.
- A percussive-type drill rig is used.
- If collapsible soils overlying rock are encountered, a rotary percussive rig is normally used.
Type 2 Earth Anchor
- This type of anchor can be in use in both cohesive and cohesionless soils.
- Rotary or rotary percussive rig can be used to drill anchor hole.
- Bit on hollow rods working within and just ahead of an outer casing is widely used.
Type 3 Ground Anchor
- This type of anchor is used in with clay strata.
- The load carrying capacity of the anchor depends on the strength of the clay available at the anchor/clay interface.
- The most successful anchor unit comprises the drilling a cylindrical shaft and the mechanical enlargement of this shaft at predetermined positions.
Ground anchors are used in many applications where soil nails are not an option. Ground anchors can carry very high loads to hold down / up/ back structures /slopes.