9.1. Falls, rockafall avalanches and Topples prevention and protection

Techniques of prevention

The main implication for a planner is to ensure that there are suitable surveys of areas which are likely to produce falls. These surveys are carried out by engineering geologists and geomorphologists. The purpose of these is the identification of the sectors prone to fall. It is possible with research of surface indices such as distribution of fissures and pattern of rock discontinuities.

As concern rock avalanches, with advanced investigations and development of predictive models it is possible to identify some of the likely areas for failure and to gain some idea of the possible run-out distance for a given volume. The identification of the sectors prone to topple is possible with research of surface indices such as open cracks.

These investigations lead to fall hazard assessment. Then, elements at risk are identified and their vulnerability is assessed in order to include fall hazard in town planning. The result is a zoning map which defines terrains which can be built or not, according to the degree of hazard and vulnerability (see more information on landslide mapping). Monitoring systems should also be installed in high risk areas.

Techniques of protection

These techniques can be only used for reducing controllable and small-scale fall hazards. In high-risk and large-scale cases there is generally no technical solution. These sites require to be continuously monitored. In France, the “Ruines de Séchilienne” rockfall, near Vizille (Isère) is a good example of monitoring strategy in combination with urban planning.

Techniques of protection can be classified in two distinct categories:
– “Active techniques”: they avoid the triggering of the fall. They mainly consist in reinforcement systems which retain the material (rock or soil) on the steep slope or the cliff.
– “Passive techniques”: they seek to control the consequences of the fall. They aim at interposing a “screen” between the steep slope and vulnerable elements.

A. Active techniques:

– Steel reinforcement

Reinforcement systems add strength to the rock mass by increasing the general tensile strength and by improving its resistance to shear along discontinuities. Both active and passive steel reinforcement systems are used for stabilizing rock slopes. The active system involves tensioned or prestressed bars or cables that have been anchored at one end within the rock mass by mechanical means or by cement or chemical grouts or by both. The passive system involves untensioned bars that have been fully grouted throughout their length by cement or chemical grouts. The main advantage of the active system over the passive system is that no movement has to take place before the anchor develops its full capacity; thus, deformation and possible tension cracking of the slope are minimized. Long-term corrosion protection of the anchors must be provided.

Fig. 1: schematic view of an anchored system (from www.prim.net)

Length of bars or cables can vary from few meters to several tens of meters. Rock bolts are commonly used to reinforce the surface and near-surface rock of the slope. Tendon and cable anchors are used for supporting large masses of unstable rock and are accordingly longer than rock bolts. Short reinforcing bars fully grouted into the rock mass are commonly called dowels. Their action is somewhat similar to rock bolts.

The rock bolt exerts a compressive force, which tends to prevent elastic rebound, frost action phenomena, and general relaxation or exfoliation by keeping the rock in its original position. Shear resistance along discontinuities is improved. Rock bolts are often used to minimize the decompression or loosening effects associated with recently excavated rock slopes. Figure 2 shows a group of large diameter bolts concentrated on an area at the base of a slope to maintain the integrity of large fault blocks.

Fig. 2: Concentration of 3,5 cm diameter rock bolts at base of section at Hell’s Gate Bluffs, British Columbia. Rock bolts were tensioned to 72 KN and dropped to 623 kN and then grouted (from TRB, 1978)
Fig. 3: Cliff reinforcement with passive system, Grenoble, Isère, France (from Besson, 2005)

– External support systems

They offer resistance to loads imposed by the slope-forming materials.
Buttresses and bulkheads are designed to take part of the weight of the slope, thus inducing stable conditions and preventing rock falls. They are used for stabilization where failure of overhangs appears to be imminent or where slight cracking or vertical displacement appears to be occurring. Buttresses are usually constructed at highway or railway level.

igure 4 shows a 15 m high reinforced concrete buttress providing stabilization of a large overhanging slope (from TRB, 1978).

Retaining walls are used to prevent large blocks in the slope from failing and to control or correct failures by increasing the resistance to slope movements. The space along railways and highways is often too narrow for normal gravity walls, but tied-back walls may be used to overcome this problem. They need only have the strength required for bending and shear resistance between rock bolts. Figure 5 shows an application of a tieback retaining wall formed of galvanized steel members for protection of a high sedimentary road cut.

Fig. 5 : Galvanized sheet steel retaining wall that is anchored by shallow rock bolts to prevent failures of high cut slope in sedimentary rock above highway in Hamilton, Ontario (from TRB, 1978)

The box gabion catch wall is a rectangular basket divided by diaphragms into smaller rectangles that are filled with stones. The basket is formed of hexagonal steel galvanized wire mesh. The wire mesh tends to reinforce the stone in tension. The gabion is a flexible structure that tends to deflect and deform instead of breaking by rock impact. Gabion catch walls are recognized as a feasible alternative to more rigid concrete walls for protecting the roads from rolling stones as large as 0,6 to 1 meter.

Fig. 6: Schematic view of a gabion catch wall (from Slosson and al., 1992)
Fig. 7: Gabion catch wall along edge of high slope in unconsolidated material on main highway (from TRB, 1978)
Fig. 8: Cliff reinforcement with a combination of concrete buttress and gabion catch wall, Peyrolles, Bouches-du-Rhône, France (from Besson, 2005)

– Shotcrete

It is a concrete that consists of mortar with aggregate and that is projected by air jet directly onto the surface to be treated. It is used to prevent weathering and spalling of rock surfaces and to provide surface reinforcement between blocks. The shotcrete acts as a protective membrane on the surface of the rock and helps to maintain the adjacent rock blocks in place by mean of its initial shear and tensile strength. Shotcrete can be used in combination with steel wire mesh and rock bolts to give structural support and also to form buttresses for small loads (figure 9).

The most important advantage of shotcrete in treating rock slopes is that offers a rapid, mechanized and often uncomplicated solution to rock fall problems. Deterioration of shotcrete can result from frost action, groundwater seepage, or rock spalling due to lack of shotcrete bond.

Fig. 9: Shotcrete in combination with steel wire net, Vinay, Isère, France (from Besson, 2005)

– Rock face purge

Instable rock or fall is artificially caused. The aim is to obtain a bare rock face and thus to eliminate fall risks. Different techniques are used to cause the fall, such as mining, chemical grout into boreholes, hydraulic breaking out, etc. Purge is a temporary protection technique because the bare rock will be submitted to fragmentation phenomena, frost action, and will be more liable to fall again. Therefore, rock face purge requires regular monitoring.

B. Passive techniques:

– Intercepting slop ditches and shaped berms

Slope ditches are used to intercept rock falls partway up the slope. Shaped berms are used at the top of the slope or can be installed in combination with a ditch. These methods ensure that rock falls get caught while they are on their way down the slope. The shaped berm is generally 4-6 m high, material (soil) is compressed and upstream face is often reinforced with gabions or tyres (figure 10). Depending of the nature of the terrain, ditches and berms are generally simple to construct and easy to maintain. An intercepting slope ditch should only be installed on a slope that can accept the introduction of a ditch without the stability of the entire slope being impaired.

Fig. 10: Schematic view of a slope ditch in combination with a shaped berm (from www.prim.net)
Fig. 11: This scheme shows a successful example of rock fall hazard mitigation with ditch and berm in Vail, Colorado. The ditch and berm were designed with the aid of a rock fall simulation software developed by university researchers (from NRC, 2004)
Fig. 12: Housing protection with shaped berm, Lumbin, Isère, France (from www.irma-grenoble.com)
Fig. 13: Aerial view of a rockfall protection system with shaped berm (red colour), Crolles, Isère, France (from www.prim.net)

– Anchored wire mesh

It is a versatile and economical material for use in protecting the right-of-way from small rocks. Layers of mesh are often pinned onto the rock surface and installed as a drapery to prevent small loose rocks from becoming dislodged. They also can be used to guide falling rocks into a ditch at the base of the slope. This practice is commonly used on talus slopes in steep mountainous terrain. Mesh can be combined with long rock bolts to provide a generally deeper reinforcement. Conditions for the use of mesh are particularly suitable if no individual rocks are larger than 0,6 to 1 meter and if the slope is uniform enough for the mesh to be in almost continuous contact with the slope.

Fig. 14: Anchored double-twist, hexagonal wire mesh being draped f on a high rock face to prevent rock falls onto highway (from TRB, 1978)
Fig. 15: Wire mesh blanket used to control rock falls near Kelso, Washington. Falling rocks roll down surface of slope under mesh and drop into ditch (from TRB, 1978)
Fig. 16: Wire mesh being draped on the “Bastille” rock face to prevent rock falls onto buildings, Grenoble, Isère, France, 2007 (www.irma-grenoble.com)

Anchored wire mesh can be installed at toe of rock face or on the slope and not only as a drapery onto the rock surface. They aimed at dissipating the kinetic energy of the rolling rocks and stopping their trajectory.

Fig. 17: Anchored wire mesh to protect buildings against rock falls, Pont-en-Royans, Isère, France, 2004 (from www.irma-grenoble.com)
Fig. 18: Anchored wire mesh, France (from www.prim.net)
Fig. 19: Anchored wire mesh, Venosc, Isère, France, 2004 (from www.irma-grenoble.com)
Figure 20 shows high-impact rock catchment fencing, engineered to catch large blocks travelling at high speeds (USA). This “cable wall” consists of steel posts set in concrete, cables strung horizontally between, and smaller cables between the cables to form a net (from www.prometheusconstruction.com).

– Rigid fences

Rail walls consist of vertical posts and horizontal members that are extended between the vertical posts. The posts are usually scrap steel rails set in holes backfilled with concrete. The horizontal members are either ties or scrap rails. These rigid protection systems are generally less efficient than flexible screen such as wire mesh.

Fig. 21: Rail wall with vertical posts seated in cast-in-place footings and horizontal scrap rails cut into 3 to 4 meters lengths. The top of the wall is anchored by cables to bedrock for support (from from TRB, 1978)

– Role of forest

Vegetation has positive and negative effects on slope stability (More information on landslide causes). Within the framework of fall risk prevention, forest can act as a natural barrier to intercept rolling rocks.

A team of researchers of Grenoble, in France, is currently working about the protection function of forest against rock fall hazard. Different in situ experiments are led to reconstruct natural rock fall and rocks trajectory. Trajectory modelling will be developed, in order to be used by local practitioners.

Fig. 22: In situ rock fall experiment, Haute-Savoie, France (from www.grenoble.cemagref.fr)

First studies showed that for a slope between 25 and 35°, forest plays an important protection role, stopping 80 % of rocks. Protection effect of forest acts on stop distance and bounce height of rocks. An experiment has been led with a 3 tons boulder rolling at 80 km/h with 2,5 meters high bounces. Forest stopped the boulder 80 meters further. Researches are also led with different tree species. Such studies contribute to develop long-term strategies for natural hazards prevention.

– BESSON L., 2005. Les risques naturels: de la connaissance pratique à la gestion administrative. Editions Techni. Cités, Voiron, 60 p.
– BRUNSDEN D., PRIOR D., 1984. Slope instability. John Wiley & Sons Ltd, Chichester
– DIKAU R., BRUNSDEN D., SCHROTT L. & IBSEN M.-L. (eds.), 1996. Landslide Recognition: Identification, Movement and Causes. John Wiley & Sons Ltd, Chichester
– MATE, METL, 1997. Plans de prévention des risques naturels prévisibles (PPR). Guide général. La Documentation française, Paris, 76 p.
– NATIONAL RESEARCH COUNCIL, 2004. Partnerships for reducing landslide risk. Assessment of the landlide hazards mitigation strategy. The National academies press, Washington DC
– SLOSSON E., KEENE A.G., JOHNSON J.A., 1992. Landslides/Landslide mitigation. In: Reviews of Engineering Geology, Volume IX, Colorado
– TRANSPORTATION RESEARCH BOARD, 1978. Landslides: Analysis and Control. National Academy of Sciences, Special Report 176, Washington DC