The restoration of vegetation in areas, in combination with the protection and/or rehabilitation of remnant vegetation, can reverse the negative impacts and effects of clearing and disturbances to the land (through earthworks, mining etc.). Vegetation plays a huge role in the health of the environment; It controls erosion, reduces land degradation and salinity, and provides a habitat for biodiversity and animal species. Furthermore, it provides stability to landform surfaces through root reinforcement.
The Art of Evaluation
A successful vegetation program requires time spent on understanding what is happening in and around the slope. Observations must be made regarding the behaviour of the slope and note any deviations from that, especially during periods of heavy rain, strong wings and freeze-thaw events. In order to achieve appropriate batter surface stabilisation, it is important that all factors are considered, such as: slope gradient and soil, drainage and vegetation type.
1. Slope gradient and soil
When talking about a batter, the most significant aspects are the slopes gradient and the length. The selection of a stabilisation technique should be based on the steepest gradient and the longest slope; it is possible to use different stabilisation techniques concurrently, such as the use of GeoSpray and vegetation together. The slope gradient influences the ability to apply and hold topsoil or solutions, the ability to incorporate ameliorants and the type of erosion risk, while the length influences the potential for erosion. Slope interruption devices such as Terra-Tubes are used to minimise soil erosion by reducing the slope length. They are designed to absorb and dissipate the energy caused by surface runoff.
Soil testing is recommended to ensure that soil is properly managed and ameliorated for the best chance of success and stabilisation. Methods such as Hydromulching and Hydroseeding are able to be tailored through the addition of growth mediums, tackifiers, fibres and mulches to assist in preparing the soil for vegetation.
Water is the most common cause of erosion and slope instability. It is important that water does not pool either on or above the slope, especially when there are dispersive soils present. There may be times where concentrated or overland flows are not diverted away. These conditions require a more intensive stabilisation technique to withstand the flow velocities and avoid erosion and slumping. Upstream surface flows that divert away from batters mean that the batter and vegetation have only to withstand raindrop splash impact and overflow that is generated on the batter itself.
The type of existing vegetation and its state is a good clue to the health and condition of the slopes soil. As you identify what vegetation is growing on the batter, you are able to determine the environmental conditions that the plants tolerate and decide as to whether that is the best choice of species.
Vegetation influences slope stability in two ways: through hydrological effects and mechanical effects. Hydrological effects refer to the removal of water by evapotranspiration through vegetation which leads to a reduction of pore-water pressure and an increase of soil suction, resulting in an overall increase of the soils strength. The mechanical effect refers to the increase of strength through the plant root matrix system. The density and tensile strength of the root within the soil mass contributes to the ability of the soil to resist stress.
Grass shields the soils surface form raindrop impacts and slows the movement of water across the surface soil, reducing the loss of topsoil, and encourages rainfall to soak into the soil for root absorption. By absorbing water through the roots, it increases the stability of the slope by removing the water from the soil. The roots of plants physically reinforce soils, resist erosion and increase the infiltration of water into the soil by forming physical pathways. Plants have extensive root systems that lock the layers of soil together in an interlocking grid, reducing displacement and stabilising embankments and batters.
Vegetation selected must be suitable for the climatic conditions of the batter and the ongoing maintenance. Temporary vegetation such as the use of cover crop seed species, assist with managing soil and restarting the biological process until permanent vegetation is placed. Permanent vegetation is grown as part of the landscape and achieves both visual and ecological outcomes. Both temporary and permanent vegetation should provide reliable growth and establishment, provide erosion control and adaptability to climatic conditions.
There are multiple techniques that are effective for the stabilisation of batters which include; hydromulching, hydroseeding, straw mulching, compost blankets, bitumen emulsion, soil binders, erosion control blankets and GeoSpray. Hydromulching is a one step process that involves spraying a slurry of water, seed, fertiliser, cellulosic mulch, tracking dye and a binder/tackifier onto the soils surface to provide a temporary layer of erosion control from wind and water until vegetation germinates. It is important to note that hydromulching is not hydroseeding; hydroseeding is simply a method of applying seed and does not provide extra protection against erosion control until vegetation is established. Hydroseeding and hydromulching have proven effective on revegetating and stabilising batters as part of mine site rehabilitation projects that include coal mines, copper mines, ash dams, tailings dams and power stations.
Straw mulching refers to the application of cereal or cane straw mulch to the surface of the soil to prevent the effects of raindrop splash, usually applied after hydroseeding. Straw mulch is susceptible to wind movement so the addition of a binder, such as a tackifier or bitumen emulsion, is added. It is important to note however that the binder only bonds the straw together, not to the soils surface so it is critical that upstream flows are diverted away from the batter.
Bitumen emulsion is a specific type of soil binder that can be used for a variety of applications to glue the soil particles together and form an erosion resistant crust. Bitumen emulsion works to aid is vegetation establishment, conserve moisture with the soil and increase infiltration. Tackifiers refer to a soil binders that are not bitumen-based but work in the same way. They are generally mixed with water and applied to the batter or are included into the hydromulching slurry. The effectiveness is often reliant on the soil type, heavy and well compacted soils may limit the ability of the tackifier to penetrate the surface and bond particles.
GeoSpray is a synthetic erosion control blanket that has a flexible membrane that works to protect the soil surface from erosion and can be altered to support the growth of vegetation or prevent it. GeoSpray is a highly effective technique for the stabilisation of batters and is quicker and safer to apply than conventional erosion control blankets such as jute mesh, Geofabrics, concrete canvas and shotcrete.