Bio-Engineering Techniques for Soil Erosion

This article is for anyone who is concerned and interested in the protection and mitigation of ecosystems, especially engineers, landscape architects, biologists and soil conservationists. It illustrates the compatibility of incorporating environmentally sound concepts into the design of engineering solutions.

Important:

It must be stressed from the onset that any soil bioengineered technique adopted, must:

• primarily be technically sound from an engineering aspect, and
• secondarily satisfy environmental requirements.

We will delve into:

• Soil bioengineering and ecological systems
• How do we combine soft and hard engineering?
• Products and techniques which may be adopted, including the concept of greening traditional gabion structures, and how to account for this in the engineering design.
• Design considerations for bio-engineered structures

Read on to find solutions that combine engineering practices and ecological principles.

What is Soil Bioengineering?

Definitions

The method of construction using living vegetation and non-living organic matter, often in combination with structural elements and manufactured products, is referred to by a host of terminology as shown below. 

·        Bioengineering

Bioengineering is the use of biological, mechanical and ecological concepts to control erosion while preserving ecological value.  It relies on living and non-living plants, typically in combination with traditional construction material, to stabilise soil and to provide good wildlife and fisheries habitat in riparian systems (University of Minnesota, 1999).  In its strictest definition, it refers to a plant-only solution.

·        Soil bioengineering

Soil bioengineering is the combined application of engineering practices and ecological principles to design and build systems of living plant material, frequently with inert material such as rock, wood, geosynthetics, geocomposites and other manufactured products to repair past and / or control soil erosion and shallow slope failures. (Sotir, 2001).

·        Eco-engineering

Ecological engineering (Eco-engineering) entails the use of mechanical elements (or structures) in combination with biological elements (or plants) to arrest and prevent slope failures and erosion.  Both biological and mechanical elements must function together in an integrated and complementary manner.

·        Other definitions

Biotechnical engineering has also been used to define this method of construction.

Irrespective of the terminology chosen, each technique refers to the integration of sound engineering practices with ecological principles.  It is a method of construction using living vegetation and non-living organic matter, often in combination with structural elements and manufactured products.

For the purposes of these articles, this technique will be referred to as soil bioengineering.
 

How Do We Combine Hard and Soft Engineering Techniques?

Essentially there is incompatibility between engineering requirements and creating a good ecological environment.  However with care, botanical understanding and an innovative approach to the detailing of the face, it is possible to create conditions in a structure favourable to the greening process.

Soil bioengineering is often used in combination with conventional engineering, offering an enduring alternative that increases permanence, effectiveness and aesthetic appeal.

The Purpose of Soil Bioengineering

Vegetation is an excellent defence mechanism which nature has produced to protect soil against erosion.  Sometimes, however, erosive forces are too large or vegetation needs to be developed under difficult conditions and nature needs a helping hand at erosion control.  In this case, inert materials need to be brought into the solution.

Soil bioengineering brings together biological, ecological, and engineering concepts to produce living, functioning systems.  The structural components initially protect the site mechanically and develop a stable, healthy environment for the plants to establish.

Vegetation will have a protective function in waterside applications:  The stems and leaves reduce the hydraulic loads (active role of the vegetation) while the roots improve the stability of the subsoil against erosion (passive role of the vegetation).  In some cases the vegetation plays only an aesthetic role. (Pilarczky, 1997).

Where is Soil Bioengineering used?

1. Erosion and flood control;
2. Wave protection in channels and coastal zones;
3. Slope stabilisation;
4. Habitat, and aesthetic enhancement; and,
5. Water quality improvement.
   
   

The operating concepts of Soil Bioengineering are:

• Mechanical
  Plant roots function as fibrous inclusions reinforcing the soil and increasing the resistance to sliding or shear displacement. Stems and trunks can act as buttressing agents to help prevent shallow slope failure.  Slope instability and erosion are reduced by transpiration of moisture and interception of rainfall.
• Hydrological
  Improved internal drainage and reduced seepage, thereby increasing the safety factor on slopes. Biomass increases surface roughness, which retards flow.

• Biological and ecological
  At one with nature. Pioneer plants provide immediate habitat improvements.  Biodiversity and habitat value are increased as vegetative invasion and natural succession occur, creating self-sustaining plant communities.
  
  

Benefits and Features of Soil Bioengineering

The benefits and features of soil bioengineering practices are shown in the table below.

* Environment Agency, undated.

Design Considerations for Bioengineered Structures:

1. Stability: The bioengineered structure must be capable of supporting the loads, stabilising the underlying soil and preventing erosion.
2. Flexibility: The ability to absorb settlement deformations without impairment of its other functions.
3. Durability: The structure should remain effective for the duration of the required design life at least.
4. Maintenance: The design should allow for maintenance, including the repair of local damage and the replacement of deteriorated materials.
5. Safety: Consideration must be given to eliminating potential risks to the labour force and the public.  All factors relating to safety should be incorporated, including consideration of all possible activities that may be taking place on and around the site, whether authorised or not.
6. Cost: The project will need to fulfil all the functional requirements while staying within budget for both construction and maintenance.

Also consider:

1. The usefulness of soil bioengineering techniques may be limited by the following conditions:
   a.  Lack of fertile soil or moisture to support the required plant growth;
   b.  Soil-restrictive layers, such as igneous intrusions, may prevent required root growth;
   c.  Banks exposed to high velocity water flow or constant inundation; and,
   d.  Climatic constraints.
2. Particularly in urban stream environments, vegetative techniques alone are often insufficient for reversing channel instability due to constrained space and modifications in the hydrological and sediment transport regime. Consequently a combination of hard and soft engineering is required to restore the natural channel geometry.

3. Soil bioengineering practices are most successful where the medium has sufficient fines, nutrients, sunlight, and moisture to support plant growth.

4. It is highly recommended to consult specific practitioners for specialised areas such as biological, geotechnical and hydraulic assessment. A multidisciplinary approach allows the engineer to conduct static and hydraulic checks, the landscape architect to take care of the environmental impact of the river works, the botanist and the zoologist to choose grasses, trees and shrubs suitable for the region and to indicate the need for maintaining / creating areas different in water level in order to promote the settlement of species typical of that region.

5. The design engineer must always recognise the possibility of complete failure of the vegetation and consequent increased risk of slope instability. For this reason, vegetation would not normally be allowed to be the prime factor governing slope stability where the consequences of failure threaten life or property. (Greenwood, 2001).

The next article in this series will focus on choosing the right soil bio-engineering solution for your project.

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