Description
Geotextiles are permeable blankets used to cover soil surface and reduce erosion from rainfall impact, as a filter to prevent soils from migrating along a slope as well as to drain water from or through soils with low permeability. The presence of geotextile can also improve the surface microclimate, retaining soil moisture to promote seed germination and thus vegetation growth (Bergado & Soralump, 1999). They are also used to protect seeds along slopes in the initial stage of vegetation establishment (Figure 1), when vegetation is the long-term answer to the slope stabilization (Lekha, 2004). These erosion control products are directly applied on the exposed surface and thus they provide an immediate protection against soil erosion by reducing the flow velocity and by cushioning the impact of raindrops (GEO, 2011). This method is appropriate for slopes where the erosion hazard is high, and vegetation is slow to establish, or is in its dormant season (BMP, 2009). Sometimes a mulch layer is placed under the geotextile to prevent soil evaporation and to maintain the ground temperature that minimizes the crust formation, as well as to conserve fertilizers and nutrients that improve the seeds germination (Ahn et al., 2002; Morgan & Rickson 1995).
Materials
Geotextiles, also called Rolled Erosion Control Products (RECPs) (BMP, 2009) are construction fabrics made of fibres or yarns combined into planar textile structures (Holtz et al., 1997). They may be woven by monofilament yarns or slit film, non-woven, heat set and bonded with resin fabrics (Koerner, 1986) and are usually categorised as either temporary biodegradable products, made of natural fibres (jute, biodegradable mats, fibres), or permanent non-biodegradable, made of synthetic fibres (primarily polymers, fiberglass, rubber, steel), or a combination of the two. The difference in using synthetic or natural materials is that the first ones are not biodegradable and so not “eco-friendly” but they can eventually provide a reinforcement for steep slopes (> 35°) in case the vegetation does not provide a sufficient erosion protection, or for slopes where vegetation will be degraded or not well established. On the other hand, natural materials gradually disappear during time and thus are more adequate for slopes where vegetation can be very well established in the long term (Morgan & Rickson 1995). The structure of a geotextile. Usually, when the site conditions are suitable for a quick vegetation establishment, natural fibres blankets (coir, jute, straw, etc) are preferred to synthetic fibres because the material is environmental friendly, easily available and less costly (Lekha, 2004). In general, they will degrade in a period of 3 to 5 years (GEO, 2011).
Advantages:
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Instant rain splash and runoff control. They provide excellent erosion protection also in disturbed areas where vegetation is slow to establish;
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Synthetic mats can be used as reinforcement to add tensile strength to a soil matrix;
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Suitable also along steep slopes (> 3:1 H:V) or slope channels with high water flow
Disadvantages:
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Not suitable for excessively rocky sites;
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Some synthetic materials can produce air/water pollution and, if used for stream bank stabilization this is a threat to aquatic species;
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Some geotextiles are tightly woven, making difficult for grass seed to root into the underlying soil or strangling the plants during their growth (GEO, 2011);
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The slopes must be uniform and relatively smooth before installation to ensure complete contact with the soil. The associated labour cost may be high
Figure 1. Two examples of slopes stabilized with biodegradable (left picture) and synthetic (right picture) geotextiles in Hong Kong (GEO, 2011).
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Design methods
(from Ecan Regional Council http://esccanterbury.co.nz/project/geotextiles-and-erosion-control-blankets/)
Selection of the material
The geotextile is selected in accordance with soil conditions and slope stability (Koerner, 1986) . If the slope conditions are suitable for vegetation growth and the measure is limited to control the runoff erosion, the selection can be oriented to a temporarly biodegradable solution, such as geotextiles or erosion control blankets made of jute, straw or coir fibers. If the potential runoff is high, a woven geotextile is preferred because of its higher strength compared to nonwoven materials. When slope conditions are not suitable for easy vegetation growth or a slope reinforcement is required, synthetics products are preferred. For slope stabilization purposes, geogrids are used when high tensile strength is required, while geotextiles are used for low tensile strength requirement.
Site preparation and seeding
The site should be prepared in order to ensure a perfect contact between the blanket and the soil surface. The surface should be homogeneously smooth and without any rock, clod, vegetation or other obstructions. Sometimes a layer of mulch can be required for improving soil characteristics that enhance vegetation establishment and growth (Morgan & Rickson 2003). The area should be seeded before installing the blanket. If there is an existing turf, first place the blanket and then seed the area.
Installation
Surface erosion control
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The geotextiles are anchored in a trench 150 mm deep and 150 mm wide on the top of the slope and then unrolled along the slope without being stretched;
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A permanent seeding should be applied before placing the blankets (transportation.alberta.ca);
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Two adjacent rolls should have the edges overlapped from a minimum of 150 mm;
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The blankets are anchored to the slope through staples. The staples density depends on slope inclination (Table 1).
Table 1. Staple density (ECan Regional Council, New Zealand. http://esccanterbury.co.nz/project/geotextiles-and-erosion-control-blankets/)
|
Slope |
Minimum staple density |
|
> 50% |
2.0 staples/m² |
|
50% to 33% |
1.5 staples/m² |
|
< 33% |
1.0 staples/m² |
Figure 2. Geotextile design on slopes (Alberta Ministry of Transportation, 2011)
Shallow erosion control and slope stabilization
The surface of the slope is covered by geosynthetic mats while anchor rods are driven through the potential slip zone and embedded in the stable region underneath it. The tensioning of the geosynthetic fabric via the anchors compacts the soil and compresses the potential slipe zone of the slope between the geostynthetic and the underneath stable region. The fabric is unrolled from the top along the slope and a diamond-shaped pattern of uniformly spaced grommets is developed along the slope cover. Anchors are generally metal pipes or rods and their spacing depends on the slope steepness, slope height and general slope stability conditions. The average anchor length depends on the depth of the potential slip zone (Koerner, 1986).
Table 2- Typical anchor spacing requirements (Koerner, 1986 modified)
|
General slope stability |
Typical slope angle |
Typical slope height |
Typical anchor spacing |
|
Questionable |
30°-45° |
0-3 m |
approx 6 m |
|
Marginal |
40°-55° |
2.5 m – 5 m |
approx 5 m |
|
Poor |
45°-60° |
3.5 m – 6 m |
approx 3 m |
|
Very poor |
> 60° |
> 5.5 m |
apporx 1.5 m |
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Functional suitability criteria
Type of movement | ||
| Descriptor | Rating | Notes |
|---|---|---|
| Fall | 0 | Protect slope surfaces from loss of soil due to water runoff. The choice of vegetal species to put under the geotextiles is very relevant. Better are of straw. |
| Topple | 0 | |
| Slide | 7 | |
| Spread | 5 | |
| Flow | 5 |
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Material type | ||
| Descriptor | Rating | Notes |
|---|---|---|
| Earth | 9 | Suitable for covering slopes also with high erosion hazard. Not suitable for excessively rocky sites |
| Debris | 8 | |
| Rock | 1 |
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Depth of movement | ||
| Descriptor | Rating | Notes |
|---|---|---|
| Surficial (< 0.5 m) | 9 | Suitable for surficial erosion or for shallow slope stabilization |
| Shallow (0.5 to 3 m) | 5 | |
| Medium (3 to 8 m) | 2 | |
| Deep (8 to 15 m) | 1 | |
| Very deep (> 15 m) | 0 |
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Rate of movement | ||
| Descriptor | Rating | Notes |
|---|---|---|
| Moderate to fast | 4 | Depending on the material used it can contrast form extremely slow to moderate shallow soil movements. |
| Slow | 6 | |
| Very slow | 8 | |
| Extremely slow | 8 |
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Ground water conditions | ||
| Descriptor | Rating | Notes |
|---|---|---|
| Artesian | 3 | Applicable irrespective of groundwater conditions. Relevant the vegetal species choice. |
| High | 5 | |
| Low | 3 | |
| Absent | 3 |
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Surface water | ||
| Descriptor | Rating | Notes |
|---|---|---|
| Rain | 6 | The geosynthetic layer acts like a filter for many types of surface water. Space between soil and geotextiles must be avoided, water runoff under the geotextiles is very frequent error in the execution of the project. |
| Snowmelt | 6 | |
| Localized | 4 | |
| Stream | 3 | |
| Torrent | 2 | |
| River | 2 |
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Reliability and feasibility criteria
| Criteria | Rating | Notes |
|---|---|---|
| Reliability | 8 | The reliability of the measure depends on the installation effectiveness (e.g. the geotextile layer is well in contact with the soil surface; the staples spacing is well distributed; two adjacent layers are well overlapped on the edges) |
| Feasibility and Manageability | 10 | Feasible for different slope types (stream banks, slopes, channels etc.) and inclinations (also steep slopes) |
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Urgency and consequence suitability
| Criteria | Rating | Notes |
|---|---|---|
| Timeliness of implementation | 8 | Fast implementation and immediate effectiveness |
| Environmental suitability | 6 | Some geosynthetic materials can negatively impact the air/water quality |
| Economic suitability (cost) | 6 | Labour costs are usually higher than other erosion control measures. In addition, sometimes a preliminary site preparation is needed. |
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References
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Ahn, T. B., Cho, S. D., & Yang, S. C. (2002). Stabilization of soil slope using geosynthetic mulching mat. Geotextiles and geomembranes, 20(2), 135-146.
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Alberta Ministry of Transportation (2011). Field guide for erosion and sediment control. http://www.transportation.alberta.ca/Content/docType372/Production/FieldGuideforErosionandSedimentControl-June2011.pdf
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Bergado, D. T., & Soralump, S. (1999). Geosynthetics for Erosion Control and Preservation of Environment. In Proc. Construction and Environment Forum, Cebu, Philippines.
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California Stormwater Quality Association. Best Management Practices (BMP) (2009). Hydroseeding http://www.stancounty.com/publicworks/pdf/Development/NPDES/EC-04.pdf
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Environment Canterbury Regional Council. “Erosion Control. Soil and surface stabilisation practices: Geotextiles and erosion control blankets”. http://esccanterbury.co.nz/project/geotextiles-and-erosion-control-blankets/
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GEO (Geotechnical Engineering Office). (2011). Technical guidelines on landscape treatment for slopes.
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Holtz, R. D., Berg, R. R., & Christopher, B. R. (1997). Geosynthetic engineering. BiTech.
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Koerner, R. M. (1986). U.S. Patent No. 4,610,568. Washington, DC: U.S. Patent and Trademark Office.
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Lekha, K. R. (2004). Field instrumentation and monitoring of soil erosion in coir geotextile stabilised slopes—a case study. Geotextiles and Geomembranes, 22(5), 399-413.
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