Description
A drainage blanket is a very permeable material used to remove water or to control groundwater seepage from cut slopes or beneath fills. Drainage blankets can be used on slopes with different problems: as protection measure for cutting slopes, as road deactivation technique (along the forest roads) or in some cases for natural slopes with a very loose material on the top prone to flooding. In general, this measure is particularly relevant to ensure the stability of any man-made or natural slope located below and near the trench excavation works and when the trench is open during any part of the wet season, where infiltration and water runoff can occur increasing the instability of the slope (REGION SA, 2003).
For cut slopes, the drainage is generally angled across the slope for intercepting the run-off and preventing surface precipitation from percolating into the slope. The drainage aggregate should be brought right up to ground level and not be covered with soil that can destroy the function of the drain (Smart & Herbertson, 1992). The objective of the drainage blankets is to disperse low-velocity flows over the hillslope. To this aim they often require a collection system to outlet the blanket. If large flows are expected, a pipe should be placed at the base for collecting the water (Smart & Herbertson, 1992).
If the drainage is used as road deactivation technique, the material is placed at the bottom of the bench and covered with pullback materials (Forest Road Deactivation Practices, 2002).
When a loose slope needs to be stabilized against flow slide, the compaction of the top 3 layers can be adopted as mitigation measures in combination with drainage blankets. In other cases, rockfill is provided on the existing loose soil layer. For both these two approaches, a drainage blanket must be placed at the interface between the newly compacted fill and the existing material, irrespective of whether seepage is observed from the slope face during the re construction (GEO, 2004).
Advantages
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useful in steep ground to prevent water accumulation from intercepted flows and to maintain the natural drainage of the slope;
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protect the surface of any temporary cutting in loose fill against erosion and direct water ingress.
Disadvantages
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If the drainage blanket is applied on a surficial subgrade material granular and permeable, it may be of little or no benefit to the water drainage;
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High cost if the material to be used is not available near the slope.
Design methods
The drainage blanket consists of a coarse aggregate with a permeability higher than that of the material surrounding the drain. Usually the material is provided near the slope, and the availability of the aggregates should be determined before the measure is taken (CDOT, 2004). The drainage aggregate should be brought right up to ground level and not be covered with soil that can destroy the function of the drain (Smart & Herbertson, 1992). The materials include cobbles or shot rock material with diameter depending on the permeability that the drainage aggregate should have to guarantee a layer more permeable than the surrounding soil.
For cut slope protection along roads the drainage blanket should be used at particular locations to intercept the surface runoff. The drain is generally angled across the slope between the slope toe and the road pavement to reduce the potential erosion. The drainage material should have a gradation and permeability that ensure the drainage without having contamination by fine particles during stockpiling at the site or degradation during transport (CDOT, 2004). To prevent the increasing of infiltration potential and the slope erosion, the drainage blanket can be lined with grouted rip rap and the water discharge dissipated away from the cutting (Figure 1). The suitability of the measure depends on many features: extent of catchment, vegetation, slope, dynamics of the failure zone (Al-Homoud et al., 1999). The addition of vegetation along the slope could significantly reduce the potential erosion.
When a loose slope needs to be stabilized against flow slide, the compaction of the topsoil layer or the rockfill can be adopted as stabilization measures. In these two approaches, the drainage blankets are used as layer between the compacted topsoil and the existing underneath material. An example of these measures is provided by Geotechnical Engineering Office in the Guidelines for Fill Slope Recompaction. Drainage blankets should satisfy the filter requirements and they should be extended up to the excavated surface as far as possible to intercept runoff and subsurface flow (GEO, 2004).
Functional suitability criteria
Type of movement |
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| Descriptor | Rating | Notes |
|---|---|---|
| Fall | 0 | Measure used as runoff protection and flowslide problems |
| Topple | 0 | |
| Slide | 4 | |
| Spread | 3 | |
| Flow | 4 | |
Material type |
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| Descriptor | Rating | Notes |
|---|---|---|
| Earth | 8 | Suitable for earth slopes or embankments, less suitable for slopes with debris layers. Not suitable for rocky slopes. Use of living materials like seeds mixtures are recommended. |
| Debris | 5 | |
| Rock | 0 | |
Depth of movement |
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| Descriptor | Rating | Notes |
|---|---|---|
| Surficial (< 0.5 m) | 9 | Only used as surface drainage system for water runoff or for controlling water seepage at shallow depths |
| Shallow (0.5 to 3 m) | 5 | |
| Medium (3 to 8 m) | 0 | |
| Deep (8 to 15 m) | 0 | |
| Very deep (> 15 m) | 0 | |
Rate of movement |
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| Descriptor | Rating | Notes |
|---|---|---|
| Moderate to fast | 2 | More suitable from extremely slow to very slow rate of movement. |
| Slow | 4 | |
| Very slow | 6 | |
| Extremely slow | 8 | |
Ground water conditions |
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| Descriptor | Rating | Notes |
|---|---|---|
| Artesian | 7 | Generally used for slopes with groundwater seepage issues or surface runoff problems. Useless in absence of a seepage problem |
| High | 9 | |
| Low | 3 | |
| Absent | 1 | |
Surface water |
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| Descriptor | Rating | Notes |
|---|---|---|
| Rain | 9 | It provides drainage from rainwater/snowmelt runoff or for localized springs along the slope. Use of adequate seeds species are recommended. |
| Snowmelt | 9 | |
| Localized | 5 | |
| Stream | 1 | |
| Torrent | 0 | |
| River | 0 | |
Reliability and feasibility criteria
| Criteria | Rating | Notes |
|---|---|---|
| Reliability | 8 | Indirect effects on stability. If not well designed (grading and permeability of the drainage aggregate, flow discharge collection) it can be useless |
| Feasibility and Manageability | 10 | Simple technique. Potential benefits and limits of applicability are well established. |
Urgency and consequence suitability
| Criteria | Rating | Notes |
|---|---|---|
| Timeliness of implementation | 8 | Relatively fast implementation if the material is available in the vicinity |
| Environmental suitability | 6 | Depending on the extension of the measure it can provide relatively negative impact on the surrounding environment |
| Economic suitability (cost) | 6 | It can be costly depending on the extension of the measure and the availability of the material involved |
References
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Al-Homoud, A. S., Prior, G., & Awad, A. (1999). Modelling the effect of rainfall on instabilities of slopes along highways. Environmental Geology, 37(4), 317-325.
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CDOT- Colorado Department of Transportation (2004). Drainage Design Manual.
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Geotechnical Engineering Office (2004). GEO Technical Guidance Note No. 7. Fill Slope Recompaction – Investigation, Design and Construction Considerations.
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Forest Road Deactivation Practice in the Pacific Northwest (2002) https://www.culvertbc.ca/fieldguide2/13.html
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REGION,S.A. The Government of the Hong Kong Special Administrative Region. (2003). GUIDE TO TRENCH EXCAVATIONS (SHORING SUPPORT AND DRAINAGE MEASURES).
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Smart, P., & Herbertson, J. G. (Eds.). (1992). Drainage design. Glasgow, UK: Blackie.