Description
Ditch and embankment are passive control works for rockfall and debris flow risk mitigation used in mountain areas to protect roadways or railways and/or villages. Generally, they are used coupled but not always. It is possible to distinguish three configurations:
- Use of a ditch (generally close to a road, or at the bottom of a slope) (Figures 1)
- Use of embankment (as a terminal barrier)
- Coupled use of the embankments made with the material excavated by the ditch
Ditch and embankment can be involved jointly (Figure 3). They are passive control works placed at the bottom of instable slopes where rockfall, topples or debris flow could occur. In function of slope morphology and other constraints, ditch and embankments can be used jointly or individually. This type of structures can be used for several hundreds of meters.
Figure 1 Excavation of a ditch (source: www.protezionecivile.fvg.it)
ditch and embankment can be used individually. Regarding the ditch, the construction and use of properly designed catchment ditches represent one of the most efficient control measure for rockfall risk reduction. Ditches aim to catch and contain fallen debris and rock avoiding reaching, for example, a roadway and avoiding damaging vehicles(Figure 2).
Figure 2 Sketch of a ditch
If a high depth of the ditch is required, the construction of an embankment along the outside edge can represent an effective solution reducing the excavation of material at the base of the ditch.
Embankments can be used at the bottom of the slope in order to stop falling material, acting as a terminal barrier. It is constructed with a trapezoidal shape with coarse and incoherent material coming from the excavation or from adjacent area. Most common type of embankments are:
- Embankment made with coarse material with a retaining wall on the downslope side
- Embankment made with reinforced earth
Where space is limited, a flexible barrier may be used to extend the height of the embankment so that it can catch the infrequent higher bounce rock or larger volume without substantially increasing the footprint of the structure (Lambert2013).
Design methods
Most of all, the required width and depth of a ditch depends upon the angle and height of the slope. Generally, if the slope is very steep, the ditch dimensions are small because rocks fall vertically avoiding bouncing. However, to avoid rocks bouncing, loose absorbing material, such as gravel or sand, is placed on the base of the ditch with a thickness of 40-100 cm. With regards to the excavation, it is very important to verify that it doesn't increase the slope and cause it to fall.
Where space is limited, a flexible barrier may be used to extend the height of the embankment so that it can catch the infrequent higher bounce rock or larger volume without substantially increasing the footprint of the structure (Lambert 2013). The width usually is between a minimum top crest width of 1.0 meter and a minimum width at impact of more than 2 times penetration depth (Lambert 2013).
Figure 3 Sketch of combined ditch and embankment (APAT, 2002) (Adapted)
Regarding the maintenance, access should be left for equipment to clean fallen rock that has accumulated in the ditch and reduced its effectiveness.
From the impact point of view, this type of structures fits well in the mountain environment, where it is most used. Materials used for the construction allow the growth of green on the structures.
Functional suitability criteria
Type of movement |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Fall | 9 | will be updated |
| Topple | 8 | |
| Slide | 3 | |
| Spread | 1 | |
| Flow | 6 | |
Material type |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Earth | 7 | will be updated |
| Debris | 8 | |
| Rock | 9 | |
Depth of movement |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Surficial (< 0.5 m) | 9 | will be updated |
| Shallow (0.5 to 3 m) | 8 | |
| Medium (3 to 8 m) | 4 | |
| Deep (8 to 15 m) | 1 | |
| Very deep (> 15 m) | 0 | |
Rate of movement |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Moderate to fast | 7 | will be updated |
| Slow | 7 | |
| Very slow | 6 | |
| Extremely slow | 2 | |
Ground water conditions |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Artesian | 8 | will be updated |
| High | 8 | |
| Low | 8 | |
| Absent | 8 | |
Surface water |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Rain | 8 | will be updated |
| Snowmelt | 8 | |
| Localized | 8 | |
| Stream | 3 | |
| Torrent | 2 | |
| River | 0 | |
Reliability and feasibility criteria
| Criteria | Rating | Notes |
|---|---|---|
| Reliability | 7 | will be updated |
| Feasibility and Manageability | 8 | will be updated |
Urgency and consequence suitability
| Criteria | Rating | Notes |
|---|---|---|
| Timeliness of implementation | 8 | will be updated |
| Environmental suitability | 7 | will be updated |
| Economic suitability (cost) | 8 | will be updated |
References
- Eliassen T.D., Thomas E. (2012), "Rock slope catchment ditch effectiveness, an Assessment of Methods used for RHRS Scoring", Vermont Agency of Transportation
- U.S. Department of Transportation (1989), "Rock slopes: design, excavation, stabilization", Publication No. FHWA-TS-89-045
- 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)
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