Description
Terminal walls or barriers are commonly used as a passive defence measures to protect buildings and infrastructures and they are constructed across the path of a a landslide to encourage deposition by presenting a physical obstruction to flow. The structures must be sufficiently stiff to withstand the kinetic energy imparted by the falling material. There exist a variety of barrier types with a wide range of energy capacities in function of their materials and geometry. Once materials and debris have been deposited upstream of a terminal structure, the coarse-grained debris must be removed from the area.
Rigid barriers are usually located close to the base of the instable slopes. This type of structure is bit deformable, heavy and characterized by big dimensions. They are able to withstand very strong impacts.
Generally, they are built in reinforced concrete, anchored in the ground with micropiles or bolts. Often, on the top of the barrier, a flexible barrier can be installed in order to avoid the overpassing of material (Figures 1 and 2).
Rigid barriers have a significant environmental impact (Figure 3) due to the type of construction and materials involved. The use of vegetation on the barrier can mitigate the environmental impact (Figure 4).
Figure 1 Rigid barrier in reinforced concrete to protect the road.
On the top of the barrier, a flexible barrier is placed. (APAT, 2002)
Figure 2 Rigid barrier in reinforced concrete.
On the top, a flexible barrier made with metallic pre-cast panels. (APAT, 2002)
Figure 3 Reinforced concrete barrier (Lam et al, 2017)
Figure 4 Rigid rockfall barrier with vegetation (Rockfall: Design considerations for passive protection structures – 2016)
Disadvantages
- Used to stop rockfall, so periodical removal of falling material is needed
- Need of a quite large space
Design methods
The design phase need the evaluation of the magnitude or volume of the landslide, the likely flow paths, potential runout distance, impact forces, run-up, and probable storage angle. In case of debris flow, these structures are usually located as far as possible downstream from the apex of the fan to maximize the runout distance and deposition area, and to minimize the impact forces and run-up (VanDine 1996).
With regards to the size of the wall, for the height a freeboard above the design bounce height must be included. Where space is limited, a net fence may be used to extend the height of the barrier.
To ensure the effectiveness of the structure, the facing element of the control works must be durable to resist environmental exposure. Drainage systems or channel should be taken into account to reduce the chance of ponding, erosion or instability processes.
A service road should be included for regular maintenance and removal of debris fallen into the embankment.
The maintenance access route should preferably have a minimum width of 2 m to allow access by small plant for debris clearance, as necessary. The periodical removal of debris accumulated behind the barrier has to be done to ensure the adequate storage capacity.
The retention capacity of the barrier is affected by the size of the buffer zone, the height of the barrier, and the storage angle which is the angle between the upper surface of the debris piled up behind the barrier and the horizon. The location of the barrier should be chosen appropriately to ensure wide space for all the volume of the design debris event. It is also necessary to ensure that the barrier is high enough to account for the possibility of debris run up" (Lo D.O.K., 2000).
Functional suitability criteria
Type of movement |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Fall | 8 | Will be updated soon |
| Topple | 8 | |
| Slide | 4 | |
| Spread | 2 | |
| Flow | 6 | |
Material type |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Earth | 7 | Will be updated soon |
| Debris | 7 | |
| Rock | 9 | |
Depth of movement |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Surficial (< 0.5 m) | 8 | Will be updated soon |
| Shallow (0.5 to 3 m) | 8 | |
| Medium (3 to 8 m) | 6 | |
| Deep (8 to 15 m) | 3 | |
| Very deep (> 15 m) | 1 | |
Rate of movement |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Moderate to fast | 9 | Will be updated soon |
| Slow | 7 | |
| Very slow | 6 | |
| Extremely slow | 5 | |
Ground water conditions |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Artesian | 9 | Will be updated soon |
| High | 9 | |
| Low | 9 | |
| Absent | 9 | |
Surface water |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Rain | 9 | Will be updated soon |
| Snowmelt | 9 | |
| Localized | 9 | |
| Stream | 4 | |
| Torrent | 3 | |
| River | 1 | |
Reliability and feasibility criteria
| Criteria | Rating | Notes |
|---|---|---|
| Reliability | 9 | Will be updated soon |
| Feasibility and Manageability | 6 | Will be updated soon |
Urgency and consequence suitability
| Criteria | Rating | Notes |
|---|---|---|
| Timeliness of implementation | 5 | Will be updated soon |
| Environmental suitability | 4 | will be updated |
| Economic suitability (cost) | 4 | Will be updated soon |
References
- Rockfall: characterization and control - Transportation Research Board – A. K. Turner, R. L. Schuster. 2012.
- Overturning stability of L-shaped rigid barriers subjected to rockfall impacts – C. Lam, A.C.Y. Yong, J.S.H. Kwan, N.T.K. Lam – 2017
- 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)
- Debris Flow Control Structures for Forest Engineering - D.F. VanDine 1- Res. Br., BC Min. For., Victoria, BC, Work. Pap, 8, 1996.
- Design of rockfall protection embankments: a review – S. Lambert, F. Bourrier - Engineering Geology 154. pp 77–88. 2013
- Supplementary Technical Guidance on Design of Rigid Debris-resisting Barriers - GEO Report No. 270 J.S.H. Kwan (2012)
- Technical Guidelines on Landscape Treatment for Slopes - GEO Publication No. 1/2011 - The Government of the Hong Kong Special Administrative Region
- Use of standardised debris-resisting barriers for mitigation of natural terrain landslide hazard - GEO REPORT No. 182 H.W. Sun & T.T.M. Lam. 2004
- DESIGN BASIS FOR STANDARDISED MODULES OF LANDSLIDE DEBRIS-RESISTING BARRIERS - GEO REPORT No. 174 H.W. Sun, T.T.M. Lam & H.M. Tsui. 2003
- Lo D.O.K. (2000), "Review of natural terrain landslide debris-resisting barrier design", GEO REPORT No. 104
- The Landslide Handbook— A Guide to Understanding Landslides - Appendix C, Introduction to Landslide Stabilization and Mitigation, Lynn M. Highland, Peter Bobrowsky
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