Rockfall protection, Geosynthetics to the rescue

What are rockfalls?

This post explains what causes rockfalls and how rockfall protection systems, from netting to barriers, control them.

A rockfall is the free fall, bouncing or rolling of rocks and debris down a slope under gravity. Rockfalls are common on mountains and cliffs. They are usually set off by weather changes such as freeze-thaw, by heavy rain, or by natural events such as landslides and earthquakes. Strong winds and storms can uproot trees whose roots have wedged into cracks, loosening rock. Urban growth and road cuts through hillsides add to the hazard.

Once a rock breaks loose, it gathers speed and energy as it moves down a steep slope, and it can knock smaller rocks and debris loose along its path.

Rockfalls threaten lives and property. Along roads and railways they damage infrastructure and block traffic, and serious falls can shut a route, causing financial losses and disruption.

Protection from the Hazard:

Most rockfalls happen on rock-cut slopes. The timing of an individual fall cannot be predicted, but risky slopes can be identified and ranked with the Rockfall Hazard Rating System (RHRS), so protection can be put in before an accident rather than after it.

Mitigation is either active or passive. Active measures stop rock from coming loose in the first place, for example mesh pinned tightly to the slope with anchors, or rock bolts. Passive measures let rock fall but control where it goes, for example draped mesh that guides falling rock to the toe of the slope, or barrier fences that catch it.

Rockfall mitigation systems are made mainly of high-tensile steel wire and are usually grouped with geosynthetics in geotechnical engineering. They come as drapery, mesh and barriers. A well-designed system dissipates energy in a controlled way, absorbs high impact energy, can take repeated impacts, resists corrosion and is tested and certified to a recognised standard.

The systems absorb the impact of falling rocks and hold the rocks and debris, reducing the danger to people and property below.

Rockfall Netting/Drapery:

Rockfall netting or drapery is made of hexagonal double-twisted wire mesh. On its own it is generally suited to controlling rocks up to about 2 feet (0.6 m) in diameter; larger blocks need cable nets or high-energy panels.

Mesh drapery is often used to protect railway lines and other transport corridors. It is strong enough to hold falling rock against the slope, while letting debris move down beneath it to a collection ditch at the toe. Because of the double twist, the mesh does not unravel if one wire is cut.

HEA Panels:

High Energy Absorption (HEA) panels are made from a single continuous high-tensile steel wire rope, woven into a mesh and knotted or clipped at each crossing. They are used where rocks are too large for standard mesh.

Barriers:

Rockfall barriers are flexible fences designed to stop rocks with high impact energy. They consist of steel posts, support and anchor ropes, nets and concrete foundations, and are often supplied preassembled. Barriers designed for low anchor forces need shorter anchors and less drilling, which lowers installation cost, and preassembled nets shorten installation time.

Barrier kits are tested in full-scale vertical free-fall impact tests and certified for a stated energy rating. The nets are made of high-tensile steel wire. They need little maintenance, but should be inspected regularly and cleared of caught rock after an impact. They have a low visual impact on the landscape.

Rockfall protection from Ocean Geosynthetics

We work with clients to understand each slope and its requirements and recommend a suitable system. For examples, see our rockfall barrier projects for landslide prevention in Uttarakhand and in Wayanad, Kerala.