Florin Florineth1 and Christoph Gerstgraser1
1 Institute of Soil Bioengineering and Landscape Construction, University of Agriculture, Forestry and Renewable Natural Resources, Asenauerstraße 42, A-1190 Vienna, Austria.
Abstract
For centuries wood and living plants were the only material for hill and slope stabilization works. Today, some of these old techniques have been modified and applied again. Furthermore, new methods have been created which mainly use living material such as willow branches, willow cuttings and rooted deciduous trees.
The time factor for implementing vegetation for slope stabilization works depends on the technique of the soil bioengineering measure and the type of plant. Cuttings and rooted plants should only be used during the dormant season and sodding techniques should be used during the vegetation season.
Various methods are available for hillside and slope stabilizing. Often used methods of seeding are dry-seeding and hydroseeding. On exposed areas the seed will be protected with straw in combination with bitumen or meshes of jute and wire.
After revegetation with seeding, the stabilization can be increased through transplanting stump sprout deciduous trees.
With different types of brush layering, loose rock slopes can be stabilized. If the plants are rooted, they are called hedge layer. If they are unrooted, they are called brush layer. A combination of both is a hedge brush layer.
A useful method for dewatering and stabilization of wet slopes is the use of drain and slope fascines. The fascines consist of live branches of willows that are tied together with wire.
There are numerous different hillside and slope stabilization methods which utilize plants in combination with constructions of wood, stone and wire such as planted pole walls, live slope grids, live wooden cribwalls, vegetated stone walls and vegetated gabions. Choosing the right method depends on various factors such as the position of slope, ground and available material.
All the soil bioengineering measures described below are being used as hill and slope stabilization in South Tyrol (Italy) and Austria.
Methods of seeding
The most common method of hill and slope stabilization is the seeding of a grass and herb mixture.
Dry-seeding is an easy method where seed (10-25 g/m2) and organic fertilizer (100 g/m2) are scattered by hand or machine. It can be applied on flat slopes with rough surfaces. It is also possible to use hayflowers instead of common seed and then we call it hayseeding.
On steeper slopes where it is necessary to cover the soil quickly, a cover crop seeding is used. Special types of rye (in fall) and barley (in spring) are spread in a mixture of 10 g/m2 and covered with soil. On this surface normal seed (10 g/m2) and organic fertilizer (100 g/m2) were spread.
On less steep slopes with rough surfaces and no erosion problems tree and shrub seeding can be spread. Seeds of trees and shrubs are mixed with sand in a ratio of 1:3 and spread as broadcast, pit or row seeding.
On steep slopes which have a smooth surface and mild climate and are also mainly in forests mostly hydroseeding is used. Seed (25 g/m2), organic fertilizer (100 g/m2), mulch (e.g. cellulose, straw 80 g/m2) and an alga product as glue (100 g/m2) are mixed in a special barrel with water and pumped out onto the slopes (2 l/m2). On very steep slopes it is advisable to fasten a jute mesh on the slope because it fixes the hydroseed.
Bitumen straw seeding from Schiechtl is the best method on exposed areas and areas mainly above the forest line. In a 3-4 cm thick layer of straw (700 g/m2), seed (25 g/m2) and fertilizer (100 g/m2) are spread and covered with an unstable bitumen emulsion. The straw effects a mechanical protection and the bitumen emulsion absorbs the warmth, which is necessary above the forest line, and functions like a greenhouse.
On very steep slopes and on failure edges a stable jute mesh instead of the bitumen emulsion covers the straw. We call this method jute mesh straw seeding. The life span of the jute mesh depends on the climate and the weather but is normally 1 to 2 years. For areas where this lifetime is too short, it is better to use a coconut mesh, which lasts about four years.
On areas where rockfall could happen or rocks could break out of the soil it is useful to nail an iron mesh into the ground covering the straw layer. This method is called wire mesh straw seeding.
Another quick method for slope stabilization is the use of sods. They can be obtained from the original slope, for example, as a result of road construction. After the earthwork is completed, they can be put back to cover the new slopes.
Slope plantings
Deeper, rooted woody vegetation helps to prevent shallow mass movements. Therefore, after a first slope stabilization with seeding, some stump sprout deciduous trees are transplanted as naked rooted, container or root ball plants. It is advisable to use stump sprout deciduous trees, because if they are cut down by hand or are damaged as a result of an avalanche, or by windthrows or snowfall, they have sprouting capacity and can still grow. Transplanting must be carried out carefully, otherwise root regeneration is slow. As a general rule, the younger a plant is transplanted, the more successful is its root system.
On less steep slopes with rough surfaces and no erosion problems, it is also possible to create a deciduous tree stand only by tree and shrub seeding.
Brush layering
The use of rooted and unrooted layers is one of the best methods to stabilize loose rock slopes. Fresh, green willow cuttings and rooted plants are layered on 1-1.5 m wide berms on the face of the slope. The berms should have an angle of inclination (sloping up) of 10-20 percent to the outside and are dug out by hand or machine. Wherever there is a risk of slope failure, ditches should only be dug in short segments. The plants are placed close together and the tips or the leafy ends should be allowed to protrude slightly beyond the face of the slope. Rooted and unrooted plants should be used as vegetation and these should be resistant to rock fall and rubble and have the ability to produce adventitious root systems. The plants are buried with the material from the next, above berm, so that 15-20 cm of the tips protrude. Vertical spacing between layers is dictated by the erosion potential of the slope and may be between 1 and 2 m.
This method is also very useful in protecting dumped slopes and dams where the plants are layered in a successive terrace system during the dumping. On very steep dams a coconut mesh should be used to stabilize the earth between the brush layers.
For brush layering we have three different techniques with different names:
· brushlayer consists of only willow cuttings, which have the ability to regrow;· hedge brush layer or rooted and unrooted layer is a combination of willow cuttings and rooted plants;
· hedge layer or rooted brush layer is a method where only rooted plants are used.
Drain and slope fascines
Drain and slope fascines are useful for dewatering and stabilization of wet slopes. They consist of live branches of willows and are tied together with at least 2 mm thick wire. The lowest third should also contain dead branch material which channels the water unhindered. The tip end of the branches always points to the flow direction. Usually the fascines are 30 to 60 cm thick and have no length limit. They are put into ditches and then must be covered with soil so that all the branches are embedded and can take root and grow. To prevent the fascines from sliding or washing away, they are fixed every 2 m with wooden poles.
The effect of the fascines is that they channel water immediately after placement and, after the plants have formed roots, they will desiccate the slope further by transpiration.
Planted pole wall
We use the so-called planted pole walls instead of wattle fences on slopes, since wattle fences are covered with little soil with consequent short lifetime and no protection against soil movement. Iron or wood poles are hammered into the ground and a larch log is attached to them. The deciduous trees are placed on the logs and covered with soil. On sites where there is light rock fall and shallow soil movement and erosion has not been stopped completely, this method can correct these problems gradually. In addition, due to the stabilizing function of the plant, the logs prop up the soil material. It is necessary to use stump sprout deciduous trees since they are so durable that mechanical damage through rock or rubble does not matter much.
Live slope grid
Very steep eroded slopes with compact soil can be secured with a live slope grid. It is essential in repairing a steep slope where the soil must not be dug up and the slope angle cannot be reduced. On vertical larch logs, which are driven into the soil and propped up on the base of the slope, logs are nailed horizontally on to it. The space between the logs is filled up completely with live branches and soil, so that very dense vegetation is quickly obtained. The whole slope grid is secured with iron poles and covered with soil.
Live wooden cribwall
Live wooden cribwalls are one of the best methods to securing immediately endangered parts of slopes and toes of slopes. They can be erected as single or double cribwalls and are built from logs and anchor logs held together with nails or bolts. The anchor logs should not be above each other but should be placed alternately. For higher stability against sliding, cribwalls should not be placed horizontally on the slope but at an angle of 10-15 percent towards the slope. Additionally, the whole construction is secured with 2-2.5 m long iron poles, which are hammered into the ground. The space between the logs is filled with soil material and plants, which should not stick out of the wall more than a quarter of their length. To reach vegetation quickly it is advantageous to use green willow branches and strong rooted pioneer plants as mentioned for the construction of the hedge brush layer. In order to achieve good plant development the face wall should be 50 percent. If this is not the case the plants will not receive enough light.
Vegetated stone wall
Vegetated stone walls are useful for the stabilization of slope toes or steep slope cuts. They are flexible, permeable, durable and can be adapted to every slope angle. During the building of the stone wall, live plants are placed into the joints between the stones so that they reach into the soil behind the stones. The joints must be filled with soil material to ensure plant growth. Green willow branches and rooted plants which have the ability to produce adventitious root systems should be used as vegetation. The branches should not protrude from the wall more than 10 cm to prevent desiccation. Additionally, a greening with hydroseeding is possible. The function of the plants is the stabilization of the construction with their root system and the absorption and transpiration of water by the vegetation, which drains the slope.
Vegetated gabion
Vegetated gabions are used to stabilize slopes and slope toes where a lot of smaller stones are available. They are constructed with a fine wire mesh that is filled with coarse gravel or smaller stones, earth, live cuttings and container plants. To ensure that the plants are covered with material, it is necessary to lift the mesh and shake it to settle the gravel. The plants should protrude slightly beyond the face of the gabion but not more than 10 cm to prevent desiccation. Finally, the wire mesh is pulled together and sewn shut with wire. Moreover, vegetated gabions are a fast and simple construction to secure wet slopes because they are elastic and improve drainage through plant transpiration.
Technical terms
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Adventitious roots |
Roots that develop from shoots |
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Berm |
Flat section of a terrace of a slope |
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Container plant |
Plants where the roots are secured with a small container of soil or peat |
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Cuttings |
Portion of a branch (usually willows) from the parent plant for the production of a new independent plant by inducing it to form shoots and roots |
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Pioneer plant |
Plants that settle in raw mineral soils and ameliorate them for succeeding plants |
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Root ball plant |
Plants where the roots are covered with a ball of soil |
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Shrubs |
A woody plant smaller than a tree and generally producing several basal shoots instead of a single stem. |
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Slope toe |
The basis of a slope |
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Stump sprouting |
Shoots developing between the stem base and the root crown, e.g. on the root neck of cut trees. |
HECKENBUSCHLAGE HEDGE BRUSH LAYER
Draufsicht/Top view
HANGFASCHINE SLOPE FASCINE Draufsicht/Top view
HANGFASCHINE SLOPE FASCINE Querschnitt/Cross section
DRAINFASCHINE DRAIN FASCINE Draufsicht/Top view
DRAINFASCHINE DRAIN FASCINE Querschnitt/Cross section
LEBENDER HANGROST LIVE SLOPE GRID Ansicht/Frontal view
LEBENDER HANGROST LIVE SLOPE GRID Querschnitt/Cross section
BEPFLANZTE HOLZKRAINERWAND LIVE WOODEN CRIBWALL
Einwandig Single cribwall - Doppelwandig Double cribwall
BEPFLANZTE TROCKENSTEINMAUER VEGETATED STONE WALL
BEPFLANZTE DRAHTSCHOTTERKÖRPER VEGETATED GABION
