Description
Flexible barriers represent a protection against landslide, mostly debris flow or rockfall, because they intercept and contain detached material during its movement and dissipate the energy of the latter. Flexible barriers consist of a sequence of functional modules made up of an interception structure, a support structure and connection components. They are linked to the foundations which in turn are anchored in the ground (Peila et al, 2009). The interception structure is represented by the principal net panel made up of metallic cables, wires and bars of different types and materials. Cables and bars, anchored into the ground, can transfer the dissipated energy into it.
This type of structure is deformable, lightweight with strong elements, and characterized by high quality and lifetime.
They are very easy and rapid to install, and easy to maintain. For all these reasons, they can be used also high on very steep slopes. The structure is design to catch and stop falling material, boulders or debris. Often, this type of structure is used coupled with other type of passive control measures such as rigid reinforced concrete barriers, ditches and/or embankments.
Flexible barrier can have two types of configuration:
- Barriers with flexible nets installed on rigid support structures such as concrete walls;
- Barriers with flexible nets with supports structures and anchors directly into the ground or into embankments
The environmental impact of flexible barriers is very limited, and they can be painted with colours similar to vegetation, ground or rock in situ, in order to ensure a better visual impact.
Figure 1 Flexible steel barrier to protect the road
Figure 2 Debris flow flexible barrier installed in a gully ( from: geobrugg.com)
Figure 3 Debris flow flexible barrier installed in a creek channel (geobrugg.com)
Advantages
- Able to block also very large boulders
- Cheaper with respect structural works
- Easy installation
Disadvantages
- Periodical removal of debris accumulated behind the structure
- Installed high on the slope, so not easy access
Design methods
The recent endorsement, by the European Organization for Technical Approvals (EOTA), of a European Technical Approval Guideline (ETAG 027, 2008), which defines how to test and assess the performance of a net fence, is therefore a great innovation that will change both the market and the design procedures of these devices (Peila et al, 2009).
Below design prescription by ETAG 027 are summarized:
- " Energy can be dissipated by the net fence must be greater than the computed energy of the block;
- the interception height (hi
) of the net fence is greater than the design interception height (hp
), which is determined from the trajectory forecast, taking into account the computed block passage height relative to the slope (95% percentile) in the design position plus a clearance (f
) that is not less than half the average size of the block: hi ≥ hp + f
; - the maximum barrier elongation towards the valley (da
), multiplied by a safety factor (γE
), must be smaller than the design distance between the net fence alignment and - the area that has to be protected (dp
): da γE≤ dp
.
This means that the barrier should always be installed at a sufficiently safe distance from the protected area " (Peila et al, 2009).
Figure 5 Scheme of rockfall barrier (after EOTA, 2013)
(NZGS, 2016)
Functional suitability criteria
Type of movement |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Fall | 9 | Will be updated soon |
| Topple | 8 | |
| Slide | 3 | |
| Spread | 1 | |
| Flow | 7 | |
Material type |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Earth | 3 | Will be updated soon |
| Debris | 9 | |
| Rock | 9 | |
Depth of movement |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Surficial (< 0.5 m) | 9 | Will be updated soon |
| Shallow (0.5 to 3 m) | 8 | |
| Medium (3 to 8 m) | 4 | |
| Deep (8 to 15 m) | 3 | |
| Very deep (> 15 m) | 1 | |
Rate of movement |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Moderate to fast | 8 | Will be updated soon |
| Slow | 7 | |
| Very slow | 3 | |
| Extremely slow | 2 | |
Ground water conditions |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Artesian | 8 | Will be updated soon |
| High | 8 | |
| Low | 8 | |
| Absent | 8 | |
Surface water |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Rain | 9 | Will be updated soon |
| Snowmelt | 8 | |
| Localized | 9 | |
| Stream | 5 | |
| Torrent | 5 | |
| River | 0 | |
Reliability and feasibility criteria
| Criteria | Rating | Notes |
|---|---|---|
| Reliability | 7 | Will be updated soon |
| Feasibility and Manageability | 6 | Will be updated soon |
Urgency and consequence suitability
| Criteria | Rating | Notes |
|---|---|---|
| Timeliness of implementation | 7 | Will be updated soon |
| Environmental suitability | 7 | will be updated |
| Economic suitability (cost) | 6 | Will be updated soon |
References
- Technical Note: Design of rockfall net fences and the new ETAG 027 European guideline - D. Peila and C. Ronco. 2009
- NZGS (2016), "Rockfall: Design considerations for passive protection structure" (https://www.building.govt.nz/assets/Uploads/building-code-compliance/b-stability/b1-structure/rockfall-design-consideration/rockfall-design-passive-protection-structures.pdf)
- Ispra, "Atlante delle opere di sistemazione dei versanti – Difesa massi"
back to top