This is the second in a series of articles about bio-engineering techniques.
Today, we take into account:
- The “Minimum Energy Level” Concept
- Lining systems for slope protection
- Wall systems for slope stabilisation
- The stability of earth slopes and soil reinforcement
- River banks
- Solutions that combine engineering practices and ecological principles
These articles not only give a background into soil bioengineering and ecological systems and the products and techniques which may be adopted, but also goes through the concept of greening traditional gabion structures, and how to account for this in the engineering design.
Key Factors to Consider When Choosing a Possible Solution
1. The “Minimum Energy Level” Concept
In choosing the most appropriate solution the following approach should be adopted:
- Analysis of the problem (causes and effects);
- Evaluation of the stability factors (numerical);
- Use and knowledge of the performance limits, of the materials used;
- Understanding of the performance of the combined use of these materials over time.
The most appropriate solution will be that defined by the “Minimum Energy Level”. This is commonly defined as the minimum amount of intervention on the environment, which is required to solve the problem.
It is illustrated in the figure below and ranges from the lowest level of no intervention through to the highest energy level, which may necessitate the construction of a massive retaining structure, or a similar type of intervention.
Lining / Wall Systems for Slope Protection
2. Types of Intervention for Various Energy Levels
The right solution for the problem is dependent on:
- The ability of the solution to be effective immediately after installation, as well as in the long term;
- Acceptable safety factors;
- The degree and size of the risk associated with the failure of the structure used.
There are three categories of solution that could be appropriate:
- Heavy systems. This category includes systems like gabions or rock mattresses, rip rap, articulated concrete blocks (ACBs)etc.
- Light systems. This category includes erosion control blankets (ECBs), turf reinforcement mats (TRMs), geocells, etc.
- Soil Bioengineering techniques. This category may include many sorts of vegetative species and treatments (without the use of combined inert materials) ranging from herbaceous or woody plants, to a wide variety of treatments and systems. (Di Pietro and Brunet, 2000)
A design index, which will help for choosing the most appropriate solution, is shown in the table below.
Designers Table: Selecting the most appropriate bank erosion control solution
Design Subject | Materials Used |
Greening Gabion Walls for wet and dry banks | · Gabions · Soil Blankets · Organic Revetments · EcoLogs |
Greening Reinforced soil walls for wet and dry banks | · Soil Blankets (in combination with a reinforced soil structure) · Green Terramesh |
Greening stable dry banks at more than 1:1 | · Greened gabion mattresses with soil anchors · Soil blankets · 3D Geomats · Triple twist woven mesh placed on top of these products will provide additional stability. |
Greening stable dry banks between 1:1 and 1:2 | · Soil blankets · 3D Geomats · Greened gabion mattress |
Greening stable dry banks less than 1:2 | · Soil blankets – BioJute |
Greening wet stable banks and water courses | · Soil blankets · 3D geomats · Greened gabion mattress · Organic revetments – Coir rolls, ecoshutter · Plants and seeds – Water margin plants |
Added value | · Floating islands · Fish hides · Fish passes |
Note: Bioengineering is typically suitable for light to moderate loads, which correspond to mean full flow velocities below 2.5 m/s and wave heights less than 0.15m (Escarameia, 2001).
3. Earth slopes and Soil Reinforcement
The stability of slopes is governed by topographic, geologic and climatic variables. Slope movements occur when shear stresses exceed the shear resistance offered by the materials forming the slope.
Soil reinforcement
- Soil reinforcement is the term used to describe a well-established construction technique, which is based on the simple principle that soil stability can be improved by sandwiching a material with a tensile resistance, in layers, within a soil body.
- The mechanism of soil reinforcement is straightforward. The inclusion of a reinforcement in the soil serves to form a composite material whose shear strength is greater than that possessed by the soil alone. The enhanced shear strength enables the soil reinforcement combination to carry loads considerably greater than those that would be carried by the un-reinforced soil. For example, a slope can be constructed at an angle greater than the natural angle of repose of the constituent soil by the inclusion of appropriate horizontal reinforcement elements within the slope.
- A comparison of reinforcement data for inert materials and biomaterials is shown in the table below. Biomaterials refer to both dead and living plants or their by-products. Examples range from biodegradable soil blankets to deep-rooted planting systems.
Soil Reinforcement Data
| Inert Materials | Biomaterials |
Tensile strength (kN/m) | 10-1250 | 1-50 |
Tensile strain (%) | 1-60% | NA |
Long term creep | Exists | NA |
Durability | UV, chemicals, hydrolysis | Drought, fire, rotting, biodegradation |
Facing considerations | Rigidity is more significant Connection strength to reinforcement Durability (chemicals, heat, hydrolysis) | Aspect Slope angle Root structure Establishment period |
Slope stability analysis
- A slope stability analysis using the available geotechnical data will indicate the significance of a change in conditions and its effect on the stability of an earth slope or riverbank.
- Slope instability often requires the evaluation of both hydraulic and soil geotechnical parameters. Experience indicates that there are three main causes of instability (Di Pietro, 2000). The causes, which influence the solutions are summarised in the table below. This is a continuation of the “minimum energy level concept”.
Table - Causes of Slope Instability and Possible Solutions
Cause | Progressive surface erosion (water run-off) | Soil veneer sliding | Deep sliding failures (global instability) |
Initial step | Provide surface protection to prevent the erosion from spreading out along the slope face. | An accurate analysis of the reasons for which instability occurred, and more importantly, the depth of the critical sliding plane is needed. | Detailed geotechnical analysis of slope stability. |
Typical solution | Depending on soil profile, ranges from: Simple seeding treatments to encourage vegetation re-establishment, to A wide variety of erosion-control blankets and geomats, to Heavy-duty linings such as gabions or mattresses, articulated concrete blocks, rip rap etc | Surface vegetative systems combined with deeper drainage treatments distributed at variable distances. | Larger structure (retaining wall or mechanically stabilised earth system) depending on the geomorphology of the area. |
Role of vegetation | Improvement of surface stability through the vegetative rooting system. | Will eventually eliminate / decrease the soil veneer’s dependence on the drainage system. | Unique structural system, which offers a new habitat suitable for the local fauna and flora, thereby enhancing environmental quality. |
4. River banks
The hydraulic force of the flowing water and the rapid change of water level, due to surcharge and draw down, affects riverbank stability.
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REFERENCES
Di Pietro, P. and Brunet, G., 2000. Design Considerations Related to the Performance of Erosion Control Products Combined with Soil Bioengineering Techniques. ASTM Workshop: Testing and Performance of Flexible Erosion Control Materials.
Di Pietro, P., 2000. Soil Bioengineering and Ecological Systems: Erosion problems in natural and altered habitats can be controlled through the use of geosynthetics. In: Geotechnical Fabrics Report, September 2000, Volume 18. Number 7.
Escarameia, M., 2001. The Right Choice for Erosion. In: Soil Bioengineering: Integrating Ecology with Engineering Practice, Ground Engineering, March 2001.
