A flexible spiral rope net that holds the snowpack on the slope in the avalanche starting zone: swivel post, MF/RF/WF net panels, pressure plate without anchor. Type E Dk 4.0 N 3.2 is certified with ETA 20/0515; the design follows the WSL 04/2007 guideline.
What it does
Unlike a rockfall barrier, an avalanche prevention barrier does not catch a moving mass; it holds the snowpack on the slope where the avalanche would release, in the starting zone. The snowpack leans on the net with a pressure that depends on the slope angle, the snow height and the glide factor. SPIDER® AVALANCHE takes this pressure with a spiral rope net woven from high-tensile steel wire and transfers it from the posts to the ground through a pressure plate, and from the ropes to the slope through spiral rope anchors.
The system is a Geobrugg AG product. The technical data on this page are taken from the manufacturer's SPIDER AVALANCHE E Dk 4.0 N 3.2 installation manual (9 December 2025, 1st edition).
Components
| Component | Function |
|---|---|
| Swivel post | Carries the net surface; its hinged base sits on the anchor head of the pressure plate and takes an angle to suit the slope |
| Pressure plate (PP) | Spreads the compressive force of the post into the ground without anchor drilling; three sizes |
| MF net panel | Rectangular spiral rope net for the middle fields of the line |
| RF / WF net panel | Trapezoidal net for boundary (RF) and end (WF) fields; the net surface stays closed down to the ground |
| Top support rope and triangle rope | Connect the net edges to the post head and the anchors |
| Upslope and downslope spiral rope anchors | Transfer the support and tensioning rope forces into the slope |
| Tensioning ropes, 5/8'' chain shackles, rope clips | Panel-to-panel and panel-to-rope connections |
The shortest line consists of two fields (RF/WF); for two sections, two swivel posts and two MF panels, or one MF and one RF/WF panel, are used. Depending on the terrain, as many sections as needed are added in series.
Pressure plate sizes
| Plate | Dimensions | Weight | Permitted compressive force |
|---|---|---|---|
| PP small | 600 × 650 mm | 46.5 kg | F < 195 kN |
| PP medium | 750 × 800 mm | 72 kg | F < 300 kN |
| PP large | 900 × 900 mm | 101 kg | F < 405 kN |
The plate size is chosen from the manufacturer's table according to the Dk/N type and the kind of field (MF, RF, WF); there are separate application details for loose rock and for sound rock. Because the pressure plate needs no anchor, drilling work is reduced on soft and sliding ground; the ground under the plate is levelled if necessary.
Design parameters
| Parameter | Value (type E Dk 4.0 N 3.2) |
|---|---|
| Dk – effective net height | 4.0 m |
| N – glide factor | 3.2 |
| fc – altitude factor | 1.1 |
| ψ – slope angle | 45° |
The system family is produced for effective heights Dk 2.5 – 4.5 m and glide factors N 2.5 / 3.2; which type is needed is determined by the measured snow height in the starting zone, the slope angle and the ground roughness. The design basis is the WSL (Swiss Federal Institute for Forest, Snow and Landscape Research) Guideline for Defence Structures in Avalanche Starting Zones, 04/2007 edition. Type E Dk 4.0 N 3.2 is certified with European Technical Assessment ETA 20/0515 according to EAD 340109-00-0106.
Layout rules
- In a discontinuous layout the lateral gap A between neighbouring structures is at most 2 m (except for parts of the terrain not affected by avalanches). For neighbouring structures offset from each other along the slope, the gap is closed in proportion to the offset.
- Where these rules are met, the fields next to a lateral gap are built as boundary fields (RF); where they are not, as end fields (WF).
- There are separate details for downslope (hollow) and upslope (crest) changes of direction; at a change of direction the net surface must stay closed.
- Post and anchor inclinations for slope angles of 30°–55° are taken from the manufacturer's table; the setting-out accuracy is given separately in the manual.
Installation
The line is installed by helicopter or from the ground, depending on access. For helicopter installation, post and net panels are assembled at the staging area into a "flight unit"; one unit is carried per flight, the helicopter approaches from downslope and the net corners are left in line with the upslope anchors. For this work the manufacturer foresees at least six installation engineers (five on site, one at the staging area) and one flight assistant at each of the two points. When the post sits on its ground plate, the tensioning ropes are tensioned by hand and locked with clips; only then does anyone climb onto the net and the helicopter releases its rope. At this stage the structure is in a temporary state with limited lateral stability; work proceeds accordingly until the line is complete and adjusted with the alignment rope.
Final check
When installation is complete, the items on the manufacturer's list are checked one by one and recorded:
- that the rope and net guides at the post head are fitted correctly,
- that the top support rope is connected at the right points with the right number of clips, and the clip torque is applied,
- that the net is connected correctly to the support ropes and the nets to each other,
- that the mountain-side and valley-side connections are built correctly,
- that boundary reinforcement ropes are present where required,
- that the change-of-direction details are applied and the net surface stays closed.
On every line Artusa installs, this check is entered on the line's barrier tag as an "installation" record; the checks before winter and after snowmelt are written in the same log. For the service levels, see the inspection, maintenance and service life page.
Snow-free season
The manufacturer states that the system has also passed field tests for rockfall of up to 500 kJ in the snow-free season. The structure is all steel; corrosion protection follows the manufacturer's coating concept and, unlike timber systems, it is not affected by pasture and scrub fires.
What Artusa does
During the survey we determine the boundary of the starting zone, the slope angle, the measured snow height, the ground type (for choosing the pressure plate) and the access (helicopter or ground). Dimensioning is done together with the manufacturer; supply, installation and acceptance inspection are ours. A tag plate is left on the installed line. For a quotation use the contact page; at sites without snow height measurements we first propose a measurement programme.
Sources: Geobrugg AG, SPIDER AVALANCHE E Dk 4.0 N 3.2 Manual, 9 December 2025, Ed. 01. Images: Geobrugg AG.
