Brush layering

Category: NBS FOR EROSION CONTROL - Living approach / NBS for erosjonsreduksjon - levende materialer

Description

For a deeper soil stabilization, alternatively to live fascines, the brush layering consists of live cut branches and rooted plants placed in layers into excavated terraces and filled with compacted soil material. Since brush layers are linear structures, they are usually completed with plantation or seeding (Stangl, 2007). As they are placed at the bottom of excavated terraces, the tips of the cuttings, including leaves and buds, exit the face of the slope and can intercept rainfall, reduce the water runoff and entrapt soil sediments (Figure 1). On the other hand, the stem of the cuttings enters into the slope and act like tensile nail inclusion that reinforce the soil (Bischetti et al., 2010).
 

Advantages:

  • Immediate protection from the first vegetative year for slopes with steepness < 40% (Rey, 2009);

  • Suitable for very steep slopes;

  • The deeper the root growth, the higher the stabilizing effect;

 Disadvantages:

  • High demand on material;

  • Not applicable for slopes with limited equipment access (trenches are difficult to realize);

  • Not suitable for rocky slopes or with low water content.

 

Figure 1. Brush layering along a steep slope. (California Department of Transportation: http://www.dot.ca.gov/)

 

Modified brush layers

Alternatively for very steep slopes where rolling rocks or derbis can down the slope, gathering speed and damage the vegetation, modified brush layers can be adopted. These are brush layers supported on a small log or board (2 m in length by at least 5 m in thickness) which create a small terrace to chatch the potential derbis or small rocks. They should be staggered across the slope in order to increase the chances to get the potential masses dowing the slope (Figure 2).

Reinforcing steel bar one m long by 15 mm diameter can be used as strengthen support to hold the modified brush layers in place (Polster, 1999) . 

Figure 2. Top figure: Modified brush layers in Canada (Rey et al., 2019). Bottom figure: (a) Modified brush layers staggered across a slope, (b) detail of a modified brush layer prior to backfilling, (c) detail of the normal backfill which creates a bit of a bench (Polster, 1999).

 


Design methods

Brush layering has been installed widespread around the world by following guidelines given by some handbooks based on past experience (e.g., Schiechtl, 1980; Gray and Sotir, 1996) about bench width, inward inclination and spacing. Usually the trenches have a depth from 0.5 to 1.5 m, with a spacing distance depending on the slope angle and varying from 1 to 2 m (Florineth et al., 2002). The cuttings should be at least 50 cm long with diameter ranging from 2-4 cm with tips extending from 10 to 20 cm from the slope surface (Figure 3). The average number of cuttings is 20 per linear meter (Rey, 2009). Work is carried out from the bottom upwards, so the soil excavated from the previous trench can be used for filling the next shelf. This technique can be used also for construction of new slope of fill material. In this case the live cuttings can be longer (1-5 m) and the slope is built up by compacting the soil layer by layer. The brush layers should be inclined at least 10° so they can be slightly inclined upwards, and their tips can protrude from the slope surface (Morgan & Rickson, 1995). Bischetti et al. (2010) introduced a new model for the design of brush layering based on the calculation of Factor of Safety based on equilibrium limit equations and by considering brush layer design parameters (number of stems per meter, length and diameter of stems, distance between brush layers). From this new model they concluded that by using half of the live materials typically involved in this technique, the same stabilization can be obtained with a great saving of cost and time.   

Period of installation: during plant dormant season

Materials: Branch cuttings and rooted plants capable of aventious root growth (e.g. Salix tetrasperma, Salix purpurea, Salix incana).

Figure 3. Brush layering scheme (Bischetti et al., 2010)

 


Functional suitability criteria

Type of movement

Descriptor Rating Notes
Fall 2 Brush layering is used for both controlling flowing prone slopes and for reinforcing potential slide surfaces.
Topple 0
Slide 8
Spread 2
Flow 7

Material type

Descriptor Rating Notes
Earth 9 Most suitable for earth soil and for entrapping debris sediments along the slope. Not suitable for rock slopes, because the brushes cannot be fixed properly, and vegetation cannot be established.

In this database are not referenced living materials like plants stakes and seeds. With this technique of brush layering it is possible to use rooted plants installed with buried stems subhorizontally in the same manner of the living stakes with roooted plants installed as the stakes. With buried stems it is possible work with success also in Mediterranean sites with long summer aridity or in sites with water scarcity.
Debris 8
Rock 2

Depth of movement

Descriptor Rating Notes
Surficial (< 0.5 m) 10 Measure used for both controlling erosion of surficial layers and stabilizing shallow covers. The trenches can reach up to 1 m of depth and roots can grow deeper.

Living materials as plants or living stakes choice is very relevant to the success of the project.
Shallow (0.5 to 3 m) 6
Medium (3 to 8 m) 1
Deep (8 to 15 m) 0
Very deep (> 15 m) 0

Rate of movement

Descriptor Rating Notes
Moderate to fast 2 Adequate for contrasting small volumes of slow moving soil. Less suitable for contrasting higher volumes of fast moving soil.
Slow 6
Very slow 8
Extremely slow 10

Ground water conditions

Descriptor Rating Notes
Artesian 7 Applicable irrespective of groundwater conditions. Indirect effects on groundwater levels due to root-water uptake from plants during evapotranspiration.
However, it can require a good availability of soil moisture into the soil for all the seasons of the year. Additional irrigation can be applied in dry season for vegetation establishment.

In arid conditions use rooted plants with buried stems instead of living stakes.
High 8
Low 7
Absent 5

Surface water

Descriptor Rating Notes
Rain 9 Typically used for reducing the rainsplash erosion.

This technique is effective when placed in riverbanks.
Snowmelt 7
Localized 7
Stream 8
Torrent 2
River 8

Reliability and feasibility criteria

Criteria Rating Notes
Reliability 8 A well implemented measure can be reliable permanently.
Feasibility and Manageability 10 Suitable also for very steep slopes, it does not require any maintenance after the installation. Not applicable to slopes with limited equipment access.

Urgency and consequence suitability

Criteria Rating Notes
Timeliness of implementation 8 The measure is implemented during the dormant season and is immediately effective against erosion. It does not require long time for the realization.
Environmental suitability 10 It involves only live or woody materials (indigenous plants are preferred), suitable with the surrounding environment.
Economic suitability (cost) 8 High demand of material. The cost can be low if the material is provided directly on site. Equipment is needed for the bench excavation.

References

  • Bischetti, G. B., Chiaradia, E. A., D’agostino, V., & Simonato, T. (2010). Quantifying the effect of brush layering on slope stability. Ecological Engineering, 36(3), 258-264.

  • 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).

  • Gray, D.H., Sotir, R., 1996. Biotechnical and Soil Bioengineering Slope Stabilization. A Practical Guide for Erosion Control. John Wiley & Sons, New York.

  • Polster, D. F. (1999, November). Soil Bioengineering for Steep/Unstable Slopes and Riparian Restoration. In Fourth Annual Roads, Rails and Environment Workshop, November(Vol. 2, No. 3, p. 1999).

  • Rey, F. (2009). A strategy for fine sediment retention with bioengineering works in eroded marly catchments in a mountainous Mediterranean climate (Southern Alps, France). Land Degradation & Development, 20(2), 210-216.

  • Rey, F., Bifulco, C., Bischetti, G. B., Bourrier, F., De Cesare, G., Florineth, F., ... & Peklo, K. (2019). Soil and water bioengineering: Practice and research needs for reconciling natural hazard control and ecological restoration. Science of The Total Environment, 648, 1210-1218.

  • Schiechtl, H.M., 1980. Bioengineering for Land Reclamation and Conservation. University of Alberta Press, Edmonton, Canada

  • 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.

back to top