Rockfall, slope and avalanche guide

Recognise the hazard

What is rockfall?

Rockfall is the detachment of rock blocks from a steep slope or road cut and their descent by free fall, bouncing and rolling. The block has usually been separated in advance where joints in the rock intersect; water, frost, roots and earthquakes set it loose.

It belongs to the landslide (mass movement) family, but it moves as individual blocks at high speed. A single block can carry a large kinetic energy even when the total volume is small, which is why protection is sized by the energy of a design block rather than by volume. See rockfall protection.

What are the types of rockfall?

Rockfall is classified by how the block leaves the slope and by how it travels. Leaving the slope, a block falls from an overhang or steep face, topples forward about its base, or slides out along one joint (planar sliding) or along the line where two joints meet (wedge sliding).

On the way down the block falls freely, bounces and rolls; on gentler ground it slides and stops. The failure mode tells which method can hold the block at its source; the trajectory tells where a barrier must stand and how high it must be. A kinematic analysis (in Turkish) of the measured joint orientations shows which modes a slope allows. When a whole slope section fails as a rock avalanche, the hazard is of another order: flexible barriers are designed for individual blocks.

What causes rockfall?

A block falls when the forces holding it drop below those pulling it out of the slope. The preconditions are jointed, weathered rock on a steep face; the usual triggers are water pressure in the joints after rain or snowmelt, repeated freezing and thawing, root growth, earthquakes, and excavation or erosion that undercuts the slope.

Most of these act slowly, over years. The last one, often a storm or a night of frost, only releases a block that was already loose, which is why a slope that has been quiet for a long time can still produce rockfall.

What are the warning signs on a slope?

Fresh stones at the roadside or at the foot of the slope, light-coloured scars on the rock face where blocks have come away, blocks with open cracks behind them or hollows beneath them, new cracks above the crest, and leaning trees or posts. Any one of them is a reason to have the slope measured.

A site survey records the joint sets, the block sizes and the slope profile; these are the inputs of every later decision. See design and engineering and laser scanning and mapping.

Debris flow and landslides

What is a debris flow?

A debris flow is a surging flow of water-saturated soil, stones and wood down a steep channel. It moves in waves with a boulder-rich front and grows on the way by picking up material from the channel bed; the landslide classification of Hungr and colleagues (2014) describes it as very rapid to extremely rapid.

Because it is confined to a channel, a debris flow travels far beyond the slope it started from and reaches the fans, roads and villages at the channel mouth. Protection works in the channel itself: flexible UX and VX ring-net barriers retain a volume of material and slow the surge. See debris flow and landslide protection.

What causes a debris flow?

Three things together: a steep channel, loose material ready in the catchment or the channel bed, and a sudden large supply of water, usually intense or long rainfall, rapid snowmelt or the failure of a natural dam. Slopes stripped by wildfire or clearing give more runoff and sediment and make debris flows more likely.

Because the triggering rain and the flow itself can be measured, monitoring and early warning are often combined with barriers in debris flow channels. See early warning and monitoring.

What is the difference between a debris slide and a debris flow?

A debris slide moves as a more or less intact mass on a sliding surface, often where soil lies on rock, and is not confined to a channel. A debris flow is saturated and behaves like a fluid: it runs in an established channel, moves faster and travels much farther. A slide can turn into a flow when it breaks up and takes on water.

On an open slope, a shallow saturated mass that breaks up and accelerates without a channel is called a debris avalanche. Shallow landslides on open slopes are held by SL barriers; channelled flows by UX and VX barriers.

Choose a protection method

What are the main slope stabilization methods?

They act in three places. At the source they keep material from detaching: scaling of loose rock, drainage, rock bolts and soil nails, anchored high-tensile mesh, shotcrete and, on soil slopes, hydroseeding and erosion mats. On the path they guide or catch it: drapery, catch ditches and flexible rockfall barriers. At the target they shield the road or building, for example with a gallery.

The choice follows from measurements: block size and joint pattern, the depth of the unstable layer, the design energy on the path and the space available. Anchored mesh systems are assessed under EAD 230025-00-0106 and flexible rockfall barriers under EAD 340059-00-0106. See slope stabilization, rock scaling, shotcrete and hydroseeding.

What is rockfall netting?

Rockfall netting is a general term for steel mesh used against falling rock, and it covers three different systems: drapery hung freely over the face, anchored mesh pressed onto the slope with nails and plates, and the net of a flexible barrier that catches blocks below the face. They work differently and are designed and tested differently.

A specification that only says "rockfall netting" leaves the essential choice open. It should name the system, the mesh (double-twist hexagonal mesh, high-tensile mesh or ring net) and the document the product is assessed or declared under.

What is a drapery system?

A drapery is steel mesh hung from anchors along the crest of a rock slope and left free over the face, not pressed onto it. A detaching stone is caught between the mesh and the face and slides down to the toe instead of bouncing onto the road. Drapery guides falling material; it does not hold the slope.

It suits faces where the blocks are relatively small and the source cannot be treated block by block. The toe needs room for the material that collects, and emptying it belongs in the maintenance plan. The mesh may be double-twist hexagonal mesh, a high-tensile mesh such as DELTAX, or ring net for larger blocks. Where the surface itself must be held, anchored mesh is used instead.

What is high-tensile steel mesh?

High-tensile mesh is woven from steel wire with a tensile strength of at least 1,770 N/mm², roughly 3.5 to 5 times that of the wire in double-twist hexagonal mesh (350 to 500 N/mm² under EN 10223-3). A given tensile resistance is therefore reached with a much lighter mesh.

It is used as anchored mesh, pressed onto the slope with nails and spike plates (TECCO), and as drapery (DELTAX). A mesh of ordinary wire reinforced with ropes is not a high-tensile mesh, even when it is sold as one.

Size the barrier

What are the parts of a rockfall barrier?

A flexible rockfall barrier consists of the net (ring net or high-tensile mesh, often with a finer secondary mesh), steel posts on base plates, upper and lower support ropes, uphill retaining ropes, lateral anchor ropes, brake elements in the ropes, and the anchors. The brake elements deform plastically and absorb energy, so the net can stop the block without breaking.

The kit is tested and certified as a whole, so its parts cannot be exchanged with those of another system; after an impact, damaged parts are replaced with identical ones in the manufacturer's order. See rockfall barriers, GBE and RXE.

How are rockfall barriers tested and classified?

In Europe a flexible barrier kit is tested at full scale under EAD 340059-00-0106, which replaced ETAG 027 in 2018, and receives a European Technical Assessment (ETA) and CE marking. A concrete block strikes the middle module of a three-module test barrier; in the maximum energy test it must travel at least 25 m/s on average over the last metre before impact.

The results place the kit in an energy class from 0 to 8. MEL is the maximum energy stopped in a single impact, from 100 kJ in class 0 to more than 4,500 kJ in class 8. SEL is the service energy the barrier must stop twice without repair in between; MEL must be at least three times SEL. The ETA also gives the maximum elongation and the residual height after impact: category A at least 50 %, B between 30 and 50 %, C 30 % or less of the nominal height.

How are the energy class and the position of a barrier chosen?

From a trajectory analysis of the slope. The mass and velocity of the design block give its kinetic energy (E = ½ m v²) and the bounce height along the slope. Under ONR 24810 the design energy is multiplied by a partial factor of 1.00, 1.05 or 1.15 by consequence class, the MEL is divided by the same factor, and the smallest class that passes is chosen.

Design values are taken as high fractiles of the simulated trajectories: 99 % for energy and 95 % for bounce height under ONR 24810. The barrier goes where the energy and bounce height are manageable and anchors can be built, not necessarily next to the road; moving it uphill lowers the energy but makes access and installation harder. Our energy class and trajectory pre-analysis tools (in Turkish) give a first estimate.

Avalanches

How are avalanches prevented?

In three places. In the starting zone, rows of snow nets or snow bridges and reforestation hold the snowpack so that an avalanche does not release. In the avalanche path, dams deflect or stop it. At the road or building, a gallery protects the target. Artificial release, road closure and early warning are temporary measures.

What are avalanche snow nets?

Snow nets are flexible steel structures set out in rows across a steep starting zone. They hold the snowpack in place so that it cannot start sliding and grow into an avalanche. They are designed under the Swiss FOEN/WSL technical guideline (2007) using the extreme snow depth with a 100-year return period.

Flexible nets are light enough to be flown in by helicopter and installed where access is difficult. See SPIDER Avalanche and avalanche prevention.

After installation

How often should rockfall protection be inspected?

At least one visual inspection a year and an extra check after every significant event, such as an impact, a storm or an earthquake. The frequency is set in the project from the event rate and the corrosivity of the site, and the checklist and reporting belong in the contract.

After an impact the line is made safe and the event recorded; the net is emptied; brakes, ropes and anchors are checked; damaged parts are replaced with identical ones and the system is re-tensioned in the manufacturer's order. See inspection and maintenance, post-earthquake barrier inspection and the barrier ID tag.

Sources

• EAD 340059-00-0106 (2018), Falling rock protection kits. Replaces ETAG 027 (2013).
• EAD 230025-00-0106 (2016), Flexible facing systems for slope stabilization and rock protection.
• EN 10223-3:2013, Hexagonal steel wire mesh products for civil engineering purposes.
• ONR 24810:2021, Technical rockfall protection.
• FOEN/WSL (2007), Defence structures in avalanche starting zones, technical guideline.
• Hungr, O., Leroueil, S. and Picarelli, L. (2014), The Varnes classification of landslide types, an update. Landslides 11, 167–194.

Artusa measures, designs, manufactures and installs these systems and has been Geobrugg AG's representative in Türkiye since 2009. Request a site survey or a quote.