Ground anchors are used for tensile stress. They transmit the forces from a structure (wall, foundation, etc.) to the ground in which they are anchored. They are generally prestressed.
There are two operating principles:
1. Anchoring structures
The purpose of the ground anchor is to generate a force across a structure, either to compensate for an uplift force or compress the foundation on the ground. It must mobilise a volume of ground with a sufficient weight to offset the required force. The bond length is designed to transmit the forces to the ground, and the free length is defined according to the required volume of ground. The prestressing force plays a vitally important role in reducing or preventing vertical movement. In case of repeated forces, it eliminates the risks of fatigue on the bonding.
The ground anchor can be used to stabilise a retaining wall by transferring the forces caused by the natural thrust of the ground and the working loads beyond the slip circle. Forces are transmitted to the ground via the bond length. It is generally prestressed to control the movement of the wall during the various construction phases.
Soil Nails & Rock Bolts
Most of the time, these anchors are created using bars inserted in a bore hole and held in place using grouting or a mechanical anchor. Their purpose is to improve the resistance of the ground.
Soft Ground: Soil Nails
Soil nails are 20 to 50 mm diameter bars, inserted in 70 to 150 mm bore holes. They are generally over 6.00 m long and may be as much as 20 m. They are bonded along their entire length by cement grouting. They are said to be “passive” and are subjected to tensile, bending and shear stresses by the movement of the ground.
Rock: Rock Bolts
Rock bolts are 15 to 32 mm diameter bars, inserted in 30 to 60mm bore holes. They are generally between 3.00 and 6.00m long. They may be bonded along their entire length by cement grouting or anchored at various points at the base of the hole using resin or a mechanical anchorage. Anchors with continuous grouting are said to be “passive” and are subjected to tensile and shear stresses by the movement of the ground. Bolts with grouting at various points (resin or plug) are often prestressed by tightening with a wrench or a jack.