Living vegetation, that is purposely arranged and embedded in the ground in selected patterns and arrays has been used for many decades as a natural soil bioengineering tool to reclaim land, control erosion, and stabilize slopes. The acceptance of this 'soft' engineering approach in recent years has increased significantly partly because more information about vegetation is now available to planners and design and construction engineers. 

Vetiver grass is not just another grass, it is a special grass like bamboo. Although technically a grass, vetiver plants are used in land and slope stabilization applications behave more like fast-growing trees or shrubs. Its extensive, thick root system binds the soil, making it very difficult to dislodge and extremely tolerant to drought. 

 

Characteristics of Vetiver Grass 

Structured Root System 

Vetiver's extremely deep and massive finely structured root system can extend to two to three meters in the first year. Its roots have a high tensile strength that has been proven positive for root reinforcement in steep slopes, thus preventing surficial erosion and shallow mass movement.

Vetiver roots can penetrate a compacted soil profile such as hardpan and blocky clay pan common in tropical soils, providing a good anchor for fill and topsoil. This creates a structurally stable composite earth mass.

When planted closely together, vetiver plants form dense hedges that reduce flow velocity, spread and divert runoff water, and create a very effective filter that controls erosion. The hedges slow the flow and spread it out, allowing more time for water to percolate into the ground.

Tolerates Environmental Variation 

Vetiver tolerates extreme climatic and environmental variation, including prolonged drought, flooding and submergence, and temperature extremes. It displays a high tolerance level to soil acidity, salinity, and acid sulfate conditions.

Power of Soil Penetration

Another attribute that sets vetiver apart from other tree roots is its power of penetration. Its strength and vigor enable it to penetrate difficult soils and rocky layers with weak spots. Combined with its remarkable reinforcement and ability to become quickly established in difficult conditions, these characteristics make vetiver more suitable for slope stabilization than other plants. 

 

This article was contributed by our expert Roberto Orozco
 

Frequently Asked Questions Answered by Roberto Orozco

Q1. What are the main methods of slope stabilization?

Grading and Drainage

This method involves reshaping the slope and improving the drainage to reduce the water pressure on the slope. The slope is graded to a shallower angle, and drainage systems such as surface drainage, subsurface drainage, or retention ponds are installed to control the water flow.

Retaining Walls

Retaining walls are structures built at the base of the slope to hold back the soil and prevent it from sliding or eroding. These walls can be made of concrete, stone, timber, or other materials and can be designed to match the aesthetics of the surrounding landscape.

Soil Nailing 

Soil nailing involves drilling holes into the slope and reinforcing them with steel bars or other materials to hold the soil in place. This technique helps stabilize slopes with loose or unstable soil.

Rock Bolting

This technique is similar to soil nailing but involves reinforcing the slope with steel bolts anchored into the rock. This method stabilizes slopes made of rock or a combination of rock and soil.

Slope Grouting 

This method involves injecting a grout mixture into the soil or rock to improve its strength and stability. The grout mixture can be made of cement, bentonite, or other materials.

Terracing

Terracing involves constructing a series of flat platforms or steps on the slope to reduce the steepness of the slope and provide a stable surface for vegetation to grow. This method is useful for slopes with moderate gradients and can be used in conjunction with other stabilization techniques.

Vegetation 

Planting vegetation on the slope can help stabilize the soil and reduce erosion. The roots of the plants hold the soil in place and absorb water, reducing the amount of runoff and erosion. 

Q2. What other plants are used for Slope Stabilization and how are they determined in various locations? 

Many different types of plants can be used for slope stabilization, and the specific plants that are used depend on several factors, such as the climate, soil conditions, and the slope gradient. Here are some examples of plants that are commonly used for slope stabilization: 

Grasses 

Grasses are a popular choice for stabilizing slopes because they have deep root systems that help hold the soil in place. They are also fast-growing and can quickly establish themselves on bare soil. 

Shrubs 

Shrubs such as willows, dogwoods, and sumacs are useful for stabilizing slopes because they have deep root systems and can tolerate various soil conditions.

Trees

Trees such as conifers, oaks, and maples can be used for slope stabilization because they have deep roots and can absorb large amounts of water. 

Ground Covers 

Ground covers such as creeping thyme, sedum, and creeping juniper can be used for slope stabilization because they spread quickly and cover the soil, reducing the amount of runoff and erosion. 

Several factors, including the local climate, soil type, slope gradient, and drainage conditions, determine the specific plants used for slope stabilization in a particular location. A landscape professional or ecologist can help determine the best plants for a particular slope based on these factors. They will consider factors such as the plant's ability to establish and grow in specific conditions, its ability to tolerate drought, flooding, and other extreme conditions, and its compatibility with other vegetation in the area. They will also consider the slope's specific needs, such as the need for erosion control, stabilization against landslides, and aesthetic appeal. 

Q3. What are the advantages of bioengineering techniques in construction? 

Bioengineering techniques use living plants and other natural materials to provide stability, erosion control, and other environmental benefits. Here are some advantages of using bioengineering techniques in construction:

Environmentally Friendly 

Bioengineering techniques are environmentally friendly because they use natural materials and do not require the use of synthetic chemicals or other harmful substances. 

Cost-Effective 

Bioengineering techniques are often cost-effective because they use readily available materials and can be easily installed with basic equipment and labor. 

Aesthetically Pleasing 

Bioengineering techniques can enhance the aesthetic appeal of a project by creating a more natural and visually pleasing landscape. 

Sustainable 

Bioengineering techniques promote sustainability by using natural processes to improve soil stability, water quality, and biodiversity. 

Effective 

Bioengineering techniques have proven effective in controlling erosion, stabilizing slopes, and mitigating other environmental risks. 

Increased Habitat 

Bioengineering techniques can create habitats for wildlife and enhance biodiversity. 

Low Maintenance 

Once established, bioengineering structures such as vegetation and soil bioengineering structures require little maintenance, reducing long-term costs. 

Flexibility 

Bioengineering techniques can be adapted to various environmental conditions, making them suitable for a wide range of construction projects. 

Overall, bioengineering techniques provide a sustainable, cost-effective, and environmentally friendly approach to construction that promotes long-term stability and biodiversity while enhancing the aesthetic appeal of a project.

Q4. How effective is vetiver grass on stabilizing hill slopes a numerical approach? 

Vetiver grass effectively stabilizes hill slopes through its extensive root system and ability to reduce erosion. Studies have reported varying degrees of success with vetiver grass, depending on the slope gradient, soil type, and rainfall intensity. 

One study conducted in the Philippines found that vetiver grass reduced soil erosion by 71% on slopes with a gradient of 18% compared to an untreated slope. Another study in India reported that vetiver grass reduced soil erosion by 85% on a slope with a gradient of 35%. 

A study in Thailand also reported that vetiver grass was effective in stabilizing slopes, reducing soil erosion by 92% on a slope with a gradient of 25%. In this study, vetiver grass was compared to other vegetation types and was found to be the most effective in stabilizing the slope. 

Overall, the effectiveness of vetiver grass in stabilizing hill slopes varies depending on the specific site conditions and slope characteristics. However, numerous studies have demonstrated its effectiveness in reducing erosion and stabilizing slopes, making it a valuable tool for bioengineering applications.