Natural or constructed rock slopes which are, on the scale of the total slope, inherently stable may, through a process of natural decay or as a consequence of poor construction, give rise to rock falls. While any rock slope anywhere may pose such problems most of them that have required attention are, in the author’s experience, road and railway cuttings, coastal cliffs overlooking or supporting structures e.g. light houses, hotels and cliffs underlying historical monuments, castles, cathedrals, earth mounds and the like. While the rock falls may not be particularly large, perhaps no more than a few tonnes weight, they pose a hazard to traffic and structures below them and to the stability of structures built near the edge of the cliffs. The remedial works that may be undertaken are briefly described in Table 1.
The purpose of these is to prevent rock falls occurring by preserving and supporting the rock face. Preservation always involves cleaning down loose debris, plants, trees etc. the sealing of joints and cracks by grouting and surface coatings and the installation of drainage works, usually sub-horizontal boreholes inclined to the rock face thus acting as drains. It is essential, if surface coatings are applied, such as cement based shotcrete or gunite, that drainholes are installed to drain off any groundwater that might build up behind the surface coating.
The support works for unstable rock slopes involve the installation of tensioned rock bolts and untensioned reinforcing pins, underpinning of overhangs and the installation of buttresses. Both underpins and buttresses are usually anchored into place.
The first task is to choose the location and type of works to be undertaken. Usually it follows the procedure given below:
- A sketch topographic map of the slope is prepared using simple survey methods (Fig. 1). This may serve to delineate the limits of the slope on which work has to be done. A map of the slope area chosen is then prepared showing contours to a vertical datum plane.
- Geological features (major joints, faults etc.) are plotted on the map. Orientations of discontinuities are measured.
- Potentially unstable blocks are recognised by the geology they present. Weights of blocks are calculated, using the surface outcrops and orientations of the joints bounding them, to determine their shape in three dimensions.
- The amount of force or reinforcement required to raise the factor of safety of the slope to a given value is calculated.
- The total force or reinforcement required is divided by that which can be applied by the chosen design of bolt or pin.
- The number of bolts and pins needed for stabilisation is distributed within the block area with regard to the internal jointing in the block, working space limitations etc..
In the design of the works careful consideration must be given to safety. Bolt, pin, drain and grout holes are usually drilled by rotary percussive methods using compressed air as a flushing medium. If any rock block is truly and dangerously unstable then drilling a borehole through it could, for the driller, become nothing more than an elaborate method of self destruction. If any large block should become dislodged on to the scaffolding it could well cause the scaffolding to collapse, endangering those
working on it or below it. Accordingly the works should be planned to either allow dangerous blocks to be supported by cables and nets anchored to more stable areas or to fall through prepared gaps in the scaffolding. The scaffolding should also be anchored to safe parts of the rock face and, if the works are being undertaken above trafficked areas, fences or barriers should be erected to prevent any small rock falls reaching them.
Grouting should be undertaken in small stages, each stage, once grouted, thence being supported by bolts or pins before proceeding with a higher stage. In Fig. 2, which shows part of a rock slope, grouting is undertaken in stages from the scaffolding platforms so that stage 4 grouting is limited to that stage (Fig. 2b) and is undertaken after stage 4 bolting has secured the face below. Thence stage 5 bolting precedes stage 5 grouting. Grouting, usually cement grout, should not be undertaken with high pressures and any open surface cracks should be sealed before it begins. The grouting is intended to prevent the uncontrolled passage of rainfall or groundwater through the face and fill voids that could enable blocks to move and possibly fall or slide. However drainage holes must be provided, otherwise water flowing around the grouted area could bring damage to adjacent faces.
Stabilisation work is expensive and it may be preferable and cheaper to protect sensitive areas against damage from rock falls by the installation of ditches and fences (Fig. 2d). The fence is to catch any rocks that may bounce from either the bottom of the ditch, or other rocks already in it. Filling the ditch with an absorbent bed of sand helps reduce bounce. Nets may be draped over the face. In Fig. 2e the nets are held away from the face by spacer bolts so that falls are guided to be bottom of the face. Nets in close contact with the rock or fastened to the face are not desirable for while they will, for a time, stop debris reaching sensitive areas the debris will build up so that, as the net deteriorates, eventually the net and debris may come down in one major event.
Forward protection works seek to deflect or catch rocks falling from rock faces high above target zones from rolling or bouncing down the underlying slope.
Fences or walls may prevent this movement if correctly sited. Correct siting depends upon an appreciation of the shape and frictional properties of the slope below the rock face (Hongey Chen et al. 1994). Such fences or walls may be severely damaged when struck and it may be better to utilise flexible net catch-traps (Fig. 1f) in which the weight of net slows down boulder velocity without imposing too much strain on the structure. Whatever the character of the protection work, they require maintenance.
Apart from replacing any damaged or deteriorated nets, fences, posts etc. any rock collected in ditches, traps or nets must be cleaned out and removed. Some thought should be given to how this may be done at the time of construction of the works.