Description
Live crib walls are a particular form of gravity-retaining structures made of on-site fill material, timbers and layers of live branch cuttings aimed to provide linear and/or spatial slope stabilization (Schiechtl and Stern, 1992, Morgan and Rickson, 1995, Stangl, 2007) (Figure 1). Their main function is to stabilize steep banks and protect them from undercutting, but they are also considered a flexible solution with regards to stabilize the toe of a slope from minor movements and settlements (Gray and Sotir, 1996). The main structure is made by using timber forming a like-wall framework, then live cuttings or branches are planted into the structure as it is built, similarly to brush layering, to have an additional reinforcement when roots gradually take over (Morgan and Rickson, 1995, Gray and Sotir, 1996). Recently, this technique is increasingly adopted in Nepal by using bamboos as surrogate of timber of other trees, such as willow or chestnut, since local people can easily find them on site (Lammeranner et al., 2005; Florineth et al., 2002).
Advantages
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Immediate protection for mass overturning and long-term benefit for stabilization by vegetation when established (Morgan and Rickson, 1995);
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Suitable to stabilize loose materials at the toe of the slopes or to stabilize the base of the slopes which have failed;
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If robust timber species are used it can be used to stabilize the long profile of stream beds and to avoid regressive erosion along riverbank.
Disadvantages
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Timber species with durable structures cannot be available on site;
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Not designed for or intended to resist large, lateral earth stresses;
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Not adapted when big soil volumes need to be stabilized.
Figure 1. A detail of a vegetated cribwall after 6 months (left)–1994 and after 4 years developing (right)-Suldenbach-1998 (Florineth et al., 2002).
Design methods
Vegetated crib walls should be built by using timber and the logs and anchor logs held together with nails and placed horizontally to form an angle of 10-15° towards the slope to increase stability. Once timbers are laid horizontally at the base of the slope in longitudinal direction, these are fixed in position by nailing sharpened interlocking elements running back into the slope. How closely these are placed depends on the slope inclination to be retained (Morgan and Rickson, 1995). Longitudinal elements can have a length depending on the width of the slope to be stabilized (e.g. 10 m Lammeranner et al., 2005). Transversal elements (headers) should be at least 1-2 meters long to well intersect into the slope. The interlocking between longitudinal and transversal elements can be obtained through cutting notches and fixing with wire (Lammeranner et al., 2005). The next horizontal course is then placed on top of the headers, and back-filled with earth and a layer of live wood. Installation notes for a live crib wall along a river bank are given by ICIMOD. However, the inherent ability of wood to resist biological deterioration should be taken into account during the design by calculating the bending strength of the wood as function of the diameter, the design strength and de decay depth at a time t (Wang et al., 2007). Examples of design of live crib walls and calculations of decay effects on the wood in the long-term period are reported in Tardío and Mickovski (2016).
Period of installation: not in the rainy season. The water can wash out the soil during construction of the layers.
Materials: robust and durable timber species such as European Larch, silver fir, oak, European chestnut, black locust, bamboo; branch cuttings of indigenous plants; wire.
Functional suitability criteria
Type of movement |
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| Descriptor | Rating | Notes |
|---|---|---|
| Fall | 3 | Most suited to rotational or pseudo-rotational slides. May be useful to reduce toppling hazard in certain conditions. Assume walls are constructed in contact with slope, not outboard as containment structure. |
| Topple | 3 | |
| Slide | 8 | |
| Spread | 3 | |
| Flow | 3 | |
Material type |
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| Descriptor | Rating | Notes |
|---|---|---|
| Earth | 7 | Mainly applicable to landslides involving earth and debris. Applicability in rock limited by typical slope geometry and failure mode |
| Debris | 6 | |
| Rock | 3 | |
Depth of movement |
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| Descriptor | Rating | Notes |
|---|---|---|
| Surficial (< 0.5 m) | 4 | Typically, applicable to shallow to intermediate depth landslides. Minimum size of elements makes this approach impractical for superficial landslides. Terracing with live crib walls 2 m high it is possible manage very high slopes. |
| Shallow (0.5 to 3 m) | 9 | |
| Medium (3 to 8 m) | 6 | |
| Deep (8 to 15 m) | 3 | |
| Very deep (> 15 m) | 2 | |
Rate of movement |
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| Descriptor | Rating | Notes |
|---|---|---|
| Moderate to fast | 0 | Should be carried out preferably on very or extremely slow landslides; with due care it can be carried out in slow landslides |
| Slow | 2 | |
| Very slow | 6 | |
| Extremely slow | 7 | |
Ground water conditions |
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| Descriptor | Rating | Notes |
|---|---|---|
| Artesian | 5 | Applicable in all groundwater conditions. Stone filled crib walls are intrinsically free draining Adequate drainage must be provided at the interface between low permeability backfills, if any, and natural soil. Adequate drainage must be provided at the interface between low permeability backfills. Use of adequate vegetal species is recommended. In arid sites it is possible use successfully rooted plants instead of living stakes. |
| High | 6 | |
| Low | 5 | |
| Absent | 3 | |
Surface water |
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| Descriptor | Rating | Notes |
|---|---|---|
| Rain | 6 | Not applicable in contact with watercourses. This technique is effective when placed in riverbanks. Use of adequate vegetal species is recommended. |
| Snowmelt | 6 | |
| Localized | 5 | |
| Stream | 5 | |
| Torrent | 4 | |
| River | 6 | |
Reliability and feasibility criteria
| Criteria | Rating | Notes |
|---|---|---|
| Reliability | 8 | The reliability of the technique depends on the reliability of the evaluation of the stability of the treated slope and of the foundations. |
| Feasibility and Manageability | 8 | Relatively simple technique. Potential benefits and limits of applicability are well established. |
Urgency and consequence suitability
| Criteria | Rating | Notes |
|---|---|---|
| Timeliness of implementation | 8 | Downgrade to 6 where elements need to be lifted using cranes in confined workplaces or on steep slopes |
| Environmental suitability | 10 | It only involves natural materials (both for the structure and for the planting) and native species are usually preferred. |
| Economic suitability (cost) | 8 | If local timber is provided the costs are not excessively high |
References
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Gray, D.H., Sotir, R., 1996. Biotechnical and Soil Bioengineering Slope Stabilization. A Practical Guide for Erosion Control. John Wiley & Sons, New York.
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Florineth, F., Rauch, H. P., & Staffler, H. (2002). Stabilization of landslides with bio-engineering measures in South Tyrol/Italy and Thankot/Nepal. In International Congress INTERPRAEVENT 2002 in the Pacific Rim-Matsumoto/Japan Congress Publication (Vol. 2, pp. 827-837).
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International Centre for Integrated Mountain Development (ICIMOD). Chapter 4: Bioengineering Measures. http://lib.icimod.org/record/27708/files/Chapter%204%20Bioengineering.pdf
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Lammeranner, W., Rauch, H. P., & Laaha, G. (2005). Implementation and monitoring of soil bioengineering measures at a landslide in the Middle Mountains of Nepal. Plant and soil, 278(1-2), 159-170.
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Morgan R.P.C., Rickson R.J. (1995). “Slope Stabilization and Erosion Control: A Bioengineering Approach”. E & F Spon, London, England
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Schiechtl H M and Stern R 1992 Handbuch f¨ur naturnahen Erdbau. Osterreichischer Agrarverlag Wien. 90, 108 pp. 153.
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Stangl, R. (2007). Hedge brush layers and live crib walls—stand development and benefits. In Eco-and Ground Bio-Engineering: The Use of Vegetation to Improve Slope Stability (pp. 287-296). Springer, Dordrecht.
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Tardío, G., & Mickovski, S. B. (2016). Implementation of eco-engineering design into existing slope stability design practices. Ecological Engineering, 92, 138-147.