Such anchorages may be rockbolts or steel cables and most commonly serve to stabilize potential rock slides or rock falls. These operate by clamping the unstable rock block back to the stable sound rock behind (Fig. 1). The force exerted by tension on the anchor can be oriented to oppose the forces that tend to push the rock block down the slope. Figure 1b shows a steel cable anchorage used to improve the stability of a rock block resting on a sloping discontinuity. Without the anchorage and assuming c = 0 the factor of safety (Fs) of the block is given by:
If the cable anchorage is tensioned so that the block is pressed against the backing discontinuity with force F the factor of safety becomes:
The angle at which the ground anchor is inclined to the possible sliding plane is critical; anchors must be inclined against the dip of the plane of sliding so that they give no down dip component of force.
Ground anchorages may be grout anchored steel cables or steel bars or mechanically or resin anchored rock bolts. Both are widely used for many engineering purposes but generally cable anchorages are used for deep long anchorages and bolts for shallow work.
For both types of anchorage successful installation and operation requires that the rock is strong enough to withstand the stresses imposed under the surface bearing plate, the rock will not creep under these stresses and thus reduce anchorage load, the rock near surface will not weather away around the bearing plate, and the steel of cable or bolt shaft will not creep or corrode and thereby reduce the tension in the anchorage and thus the force applied. Figure 1a shows examples of potential slope instability and indicates how tensioned ground anchorages might be used to make some slopes safe. Both the long cable anchorages (10 to 40 m+) and rock bolts (3 to 6 m) must be protected against corrosion, especially if they are used as part of a permanent engineering structure.
Anchorages should not be randomly distributed on a slope but used to support key rock blocks. Thus, in Fig. 2, anchorages installed to support the blocks which are free to move (F) will automatically support the intervening blocks.
It is quite easy to identify major rock blocks and to calculate the force necessary to support them to a given factor of safety. However, the major blocks will also be divided by discontinuities which might cause the block to spring apart if subjected to the force from a single massive anchorage. Accordingly the total load required to stabilise a large block may have to be applied by many smaller anchorages located so as to ensure no disruption of the total block (Fig. 1c).
It may be that problems of corrosion, creep or weathering are such that the application of tensioned anchorages seems undesirable. In such a case an alternative treatment would be to install untensioned rock pins which would resist potential rock movements in shear. They may be considered to increase the cohesion across any potential sliding plane. Thus the equation for the factor of safety in the dry condition (Fs) becomes:
where s = the safe shear strength of a steel rock pin and n = number of pins.
Rockbolts and Cables