9.4.2 Rock anchor

Rock anchors are tendons which are placed in competent rock or soil to control displacements and provide vertical and lateral support for engineered structures and natural slopes. The primary function of rock anchors is to modify the normal and shear forces acting on the sliding planes.  These anchors may be fully grouted and untensioned, or anchored at the end and tensioned.

A rock anchor generally consists of a bar or cable of high-strength steel tensioned inside a borehole to about 60 to 70% of its yield strength. Tension in the member is transmitted to the surrounding rock mass by anchorage points at the ends. The length of the rock anchor can be from 3 m to over 100 m. Holes for installation of the anchors are normally drilled that cross the potential failure plane.

Normally, the bottom one-third of the hole is line-loaded with quick-set resin cartridges, and the top two-thirds of the hole is loaded with slow-setting resin cartridges. The threadbar is then inserted into the hole and rotated, usually via the rock drill, to break the plastic containers and mix the resin and the catalyst. After the resin has set, the anchor plate, bevel or flat washers, and the end nut are added. Wedge washer may also be used where the end plate is not perpendicular to the threadbar. The threadbar can be tensioned by tightening the end nut to load the anchor.


Figure 17: Polyester Resin Rock Anchor System (http://www.williamsform.com)


Figure 18: Rock or Earth Anchors (http://www.williamsform.com)