Bio-engineering Techniques for
Slope Stabilization and Control
of Sediment Generation
Detailing; Hand & Mechanical Site
Preparation and Planting methods.
Kay & Associates, International Training Consultants
Bio-engineering is the successful use of vegetation in concert with engineering structures to increase slope stability against shallow mass wasting.
Plant material increases soil strength through the transfer of root tensile strength to soil shear strength, buttressing and arching. Bio-engineering systems provide additional support beyond that which can be provided by single plants. As the plants mature they increase in strength and provide increased resistance to natural forces.
The following bio-engineering solutions uses native plant species to;
Stabilization of the surface layers of the slopes from wind, gravitational, and hydraulic forces is achieved with the benefit of energy dissipation of water and detritus moving down-slope. Infiltration of water increases the amount of water for plant establishment while vigorous plant growth dissipates excess slope water to the atmosphere. Entrapment of sediment is accomplished through filtering action of established vegetation and a developing duff layer.
Natural vegetation establishment is enhanced by providing suitable microsites for plants by;
Seeds from natural dispersal tend to remain in place and seedlings have a chance to develop in the stabilized surface areas. The temporary vegetation from the Bio-engineering treatments acts as pioneering species, allowing time for natural succession to form a more permanent vegetation cover.
2. Willow Whips and other plants that sprout from cuttings.
Willow whips are expected to be cut (make clean cuts with unsplit ends) locally by local labour in the dormant season. (from when the leaves start to turn yellow in the autumn until growth starts in the spring) These willow whips will be approximately 2 meters long and selectively harvested from convenient sites without damage or harm to the natural environment. The whips will be placed in bundles and wrapped in medium weight polyethelyne (leave ends open) and tied with cord. (the poly is to protect the stems during handling and to retain moisture, do not leave out in the sun. Place in the shade) These willow whips will be kept in cold storage until time for use. It may be advantageous to take the time at the harvesting stage to prepare the required lineal meters of wattle bundles. (see: contour-wattling for instructions in making the wattle bundles)
It is important to remember that in each cutting that you are handling a live plant. Handle gently, and don't damage or bruise the stems.
In the harvesting and planting phases, do not let the cuttings dry out. Do not leave them out in the sun or spread out on a windy slope. (keep covered or in the shade)
Keep all planting stock moist, cool and covered when in storage, transport and on the jobsite when planting.
b) Pre-planting preparation
Soaking of the cuttings prior to planting has shown to be beneficial to survival rates. A period of 24 hrs to 72 hrs (one to three days) is recommended.
3. Planting of Cuttings
Different methods for planting of the willow whips may be utilized.
a) Live Staking (Upright Planting)
The Live Staking (upright planting) method is commonly employed on slopes requiring remediation work for slope stability and in conjunction with the methods described below.
The willow whips are usually cut into 500mm lengths for planting vertically in the ground.
Planting spacing on most slopes will be from 200mm to One meter apart. The worker will be instructed by the Bio-engineering supervisor in what to look for in ideal planting sites to get the best plant survival and growth.
The willow whip (if strong enough and the soil is loose) may be pushed or driven into the ground. Otherwise a suitably deep hole may be made in the ground using a metal bar or other suitable method (see tools). Approx. 2/3rd's of the whip should be below ground and there should be two buds visible on the exposed 1/3rd above the ground. The cutting should make contact with the bottom of the hole (no air space). Compact the dirt around the stem.
Contour-wattling for slope stabilization and also to provide sediment control in fine textured and sandy soils adjacent to watercourses
Contour-wattling is the placing of bundles of twigs in a prepared trench and burying them across the slope at regular contour intervals resulting a lightly terraced slope. All work starts at the base of the slope.
Wattles are constructed by the laying of willow whips in alternate directions to form a bundle 200 to 300mm in diameter.
The bundles are compressed tightly and firmly tied with binder twine every 300 to 400mm. (do not allow bundles to dry out)
An excavated trench (approx. ½ the depth of the bundle is excavated along the contour of the slope. Note: trenches must kept horizontal to prevent the mis-directing of surface water. (use an inclinometer) Trenching should not precede the placing of the wattles by more than 1hr to minimize the drying out of the soil.
Placing, Staking and Covering of the Wattles
The bundles of wattles are laid in the trench allowing the fringe ends to overlap.
Stakes (minimum 600mm length) are driven through the bundles (beside the tie string) approximately every 500mm. The purpose to the stakes is to retain the bundle in place (particularly from frost heaving) until the roots can take over.
The bundles are partially backfilled using native upslope material. The soil should be worked around and into the bundle itself. Workers should walk on the backfilled material and also on the bundles as much as possible to help work the soil into the bundles and to provide compaction of the backfill material. The excavated material from the next trench provides this backfill material. The backfilled portion must be outsloped as not to retain or pond water. A side portion of the bundle forming the terrace "wall" will always be left exposed for propagation. The finished slope will have a series of out-sloped terraces with the wattle bundles just poking out at the face of each terrace.
Alternate Wattling Method
In moist soils, the driving in of long (minimum 1meter) stakes (Steel or Wood) along the desired terrace line (150-200mm apart, depending on the size of the longitudinal whips used) with 150-200mm left exposed. Then long whips of Willow are placed against this "fence" and then backfilled with native material, creating a bench face with the exposed wattling. This procedure is continued up the slope.
c) Brush Layering (Horizontal Planting), Full Continuos Bench
Brush Layering (horizontal planting), Full bench is also used to reduce the amount of slope angle and provide continuos reinforced bench support.
Useful for slopes that are over-steepened that will benefit from reduced slope angle.
It is also beneficial if there is an opportunity for mechanical assistance of slope preparation. You may wish to consider the use of a small mini-excavator or a "spider" excavator. These smaller, lighter machines can be flown in by Helicopter to sites not accessible by road. The "Spider" excavator, waking on it's four feet, also has the ability to traverse slopes of up to 100% to gain site access with surprisingly negligible ground disturbance.
A flat bench will be excavated with mechanical equipment or by hand work using mattock and shovel, at the base of the slope. This bench will be the full length of the slope and usually 1 to 2 meters in depth. The willow whips will be placed side by side (about 100mm apart) on this bench with the tops facing outward and with a 150mm overhang.
Slope material (native material, dirt, stones etc) will be excavated down from the slope on to the top of the previously prepared bench with its side by side placed willow whips. The depth (ranging from 200mm to 2meter) of this covering material (and the next bench) will vary with the slope angle and slope materials.
Length of Whips.
Whips will be 1 to 2 meters long and laid horizontally on the constructed bench and backfilled (covered over to a depth of 200mm to 2meters) in the terracing process. (There must be a minimum of 150mm of the willow whip freely protruding to establish new growth)
Size and spacing of Benches.
These benches, running the full length of the slope, consisting of one single layer of Willow whips are constructed to the top of the slope. The bench height will vary due to site conditions but is usually in a range of 200mm to 2 meters.
d) Brush Layering (Horizontal Planting), Random Bench
The Brush Layering (horizontal planting), Random Bench method is selectively employed in dry raveling sites to establish "islands" of vegetation and to ensure the planted stems maintain moisture in the dry period of the year and to provide protection from raveling material until the plant is established.
For slopes subject to raveling, a random spacing of short benches may be employed. The object is to create "islands" of Vegetation on the slope.
Length of Whips.
Whips will be 1 to 2 meters long and laid horizontally on a constructed bench and backfilled (covered over) in the terracing process. On some slopes, due to the nature of the ground material it will not be possible to use the full length of the willow whips.
Size and Spacing of Benches.
These benches, 1 to 2 meters in width, consisting of one single layer of Willow whips will be interspaced, in a staggered fashion, on the slope approximately every 3 to 4 meters in suitable random locations.
A flat bench will be excavated by hand with a mattock at the desired location on the slope. (The "Spider" excavator may be a useful tool on some slopes) This bench will be approximately 1 to 2 meters in width and 1 to 2 meters in depth. The willow whips will be placed side by side (about 100mm apart) on this bench with the tops facing outward.
Native slope material (dirt, raveled stones etc) will be excavated down from the slope on top of the previously prepared bench and the side by side placed willow whips. The depth (approximately 200 to 500+mm) of this covering material will vary with the slope angle and slope materials. There must be a minimum of 150mm of the willow whip freely protruding to establish new growth.
(If fairly large boulders are readily available and can be moved by hand they can be placed on the bench created to assist in providing physical protection)
( 4 ) Live Pole Drains
Water piping or seeping out of a bank creates problems that can lead to slope instability. The use of a "Live Pole Drain" system can control and direct this potentially problem water.
Starting at the water source location, excavate a small (2-300mm) trench to the desired discharge location. Take a pre-prepared bundle of Willow whips (3-400mm in Diameter) and place this bundle in the trench and then place others continuously end to end to the drain point. Backfill the trench, leaving just the top surface of the bundle of whips exposed. Ensure the discharge location is well armoured to receive the anticipated water flow.
This drainage system will convey a considerable amount of water with the added benefit of being a "living drain" with live willows growing out of it and dispersing water to the atmosphere. These drains can be used in many different ways. Eg. In a "Y" formation.
( 5 ) Sediment Control Tools
a) Wattlling for sediment control If regular silt fence is left in place after a project work period it will require regular monitoring and eventual removal of the slit fence and also the removal of the collected silt.
As a practical alternative to regular silt fencing a unique sediment control barrier, is to construct a high wattle fence lined with silt fence or filter cloth. The area for trapped sediment would be planted with site-suitable species as Willows, Cottonwood, Cedar, and other wet-site shrubs and plants. It may be desirable to plant the sediment catchment area after the initial flush of sediment. It is recommended that the area be hand grass seeded (and fertilized) each growing season.
This sediment control device is now a permanent living structure that will trap sediment and not require the removal of that sediment.
b) Brush layering for sediment control A sediment entrapment area can be created by slightly modifying the previously mentioned brush layering technique.
By constructing a shallow depression behind the brush layer, a sediment catchment area can be created. This area should also be planted etc. as in the above "Wattling for sediment control"
c) Monitoring Regularly scheduled (and after peak storm events) monitoring a necessity just as it would be required for any control structure or moderate to high risk site.
( 6 ) Conifers and Hardwoods
Where they are native and suit the site, species such as Conifers and/or Hardwoods that can be planted as rooted stock may be a desirable addition to a bio-engineering stabilization project.
Larger species have the great ability to use water to dry the slope and their root systems will enhance the slope integrity and assist to achieve long term stability.
Consideration should be given to slope steepness and wind conditions and the mature size and height of the larger species.
Species selected should be native to the area, suited to the site conditions and suited to the site altitude.
( 7 ) Other Slope Stabilization Species
There may be other suitable shrub and tree species that are available near the remediation site that propagate from cuttings or that can be grown from seed and planted as rotted stock. Each region of the globe has its own native species that can be used as remediation tools. Observing similar sites nearby to the treatment area will give an indication of all the species of Trees, Shrubs, Plants and Grasses that you should be considering for your remediation work.
For example, in the American Pacific Northwest, Willow and Black Cottonwood will grow profusely from cuttings. In the same area, species that have performed well that are available in rooted stock are: Red Ozier Dogwood, Alder, Sumac among others.
Plants (ferns, wildflowers, vines, vetch and others) are also a viable tools for soil stabilization and sediment control. They can be transplanted directly from adjacent sites (may available from specialist nurseries) or be seeded.
It is essential to use care in non-native species selection. The introduction of a species that may be considered a nuisance(such as damaging to other native species), noxious(poisonous) or a weed must be avoided.
( 8 ) Grass seed and fertilizer.
Dry grass seed and fertilizer should be applied, using a broadcast spreader, the same day that the ground is disturbed, while the ground is friable (loose and open) to be most effective. This method can be used effectively even on quite steep slopes. However, after the ground has been armoured and compacted by sun and rain the seed and fertilizer has a tendency to roll down the slope and for the wind to blow the seed away.
The use of dry seeding in friable ground can be very cost effective and with good results. By the seed being retained (trapped) in the ground the seeding can be effectively done even when dry conditions can be expected after seeding as the seed remains in place until moisture conditions are right for seed propagation.
Hydro-seeding - Is the application of seed using a water slurry(containing the seed and fertilizer) combined with a mulch agent(to retain moisture and also physical protection from rain damage) and a tackifier(to make the mixture stick). There are many combinations of mix and a variety of specific materials and agents to use in site-specific locations.
Bonded fibre matrix Is a product (containing seed, fertilizer, mulch and a specialized bonding agent) that is applied (using hydro-seeding equipment) as a solid blanket over the soil. This can give immediate weather protection from soil erosion for up to two years when it is expected that emergent vegetation will have matured.
It is recommended that you consult an expert in hydro-seeding to get;
Timing of the application will need to be considered (Eg. weather seasons)
Always, a custom grass seed mixture, specific for the site and the season of application is strongly recommended, along with a recommended fertilizer mix, specific to the site soils. (you may need soil tests) There will also need to be specified application rates for both the seed and fertilizer. (The grass seed supplier may be able to assist and may have local knowledge of the area)
The addition of native species to the seed mix is gaining strong acceptance. There is an increasing availability of native species from the seed suppliers. Also shrub and tree seed may be considered to add to the mixture. Eg. In the American Pacific Northwest, Alder should be considered if the work commences after natural Alder seeding for the year has taken place.
In any seed application it is essential to ensure that specified application rates are being actually applied and specified products are being used.
The only sure way to confirm is to place random test cards(150mm square and for dry seeding, coated with a sticky substance) on the site prior to seeding and then sent to a laboratory for a painstaking seed count.
For realistic on-site supervision you may;
Caution: The specifications for the area to be treated if taken directly from a map or aerial photo will not equal the actual ground distances on a slope and will lead to a shortage of materials for the actual ground area required to be treated.
( 9 ) Project Timing
Harvesting of cuttings (Willow Whips) will ideally take place in the dormant season. (no leaves on the stem) However successes have been obtained with cuttings taken at all times of the year.
Soaking of the cuttings a minimum of 24 hrs prior to planting has shown to have effectively increased survival rate.
Bio-engineering field work will take place after any work of buttressing for slope stability in the immediate area is completed. (The machine construction of reinforcing walls at the base of the slopes. Eg. Such as the placing of large boulders to a wall height of 2meters or engineered structures)
Ideally, the bio-engineering field work should commence in the spring. Consideration will want to be given to such limiting factors as;
Planting (live staking and grass hydro-seeding) should be limited to seasons in which adequate moisture is expected to be available for propagation.
( 10 ) Existing Vegetation
The existing natural vegetation should not to be destroyed or damaged in the processes of completing the prescription. Where there is existing vegetation, it is expected that (where practical) it will be incorporated into the remedial work plan.
( 11 ) Naturally re-vegetating species
It is expected that wind born and animal transported seeds from adjacent vegetation (including fruit bearing shrubs) will establish themselves in the stabilized soil with the assistance of the emerging vegetation.
( 12 ) Maintenance
Scheduled on-going Inspection and Monitoring is required to;
( 13 ) Biodiversity
A well rounded prescription addresses the key elements to maintain plant bio-diversity and wildlife concerns.
( 14 ) Hand Tools and Materials:
( 14 ) Worker Safety
A short orientation course "Safety on Steep Slopes" should be presented to all workers and supervisors prior to Bio-engineering work commencement. This will encompass;
All new workers to the site are to have taken this course before being allowed to work on the slopes. No person is to work alone. Use the "buddy system".
Also a written;
(a) Safety plan
(b) Emergency evacuation plan
are to be in place prior to work commencing.
For additional training also see other courses offered by Kay & Associates.
Biotechnical Slope Protection and Erosion Control - Gray/Lessier - 1989
Caminos Rurales Con Impactos Minimos - Keller/Bauer/Aldana - 1995
Forest Road Engineering guidebook - BC Ministry of forests - 1995
A Guide for Management of Landslide Prone Terrain in the Pacific Northwest Land Management Handbook # 18 BC Ministry of Forests - Chatwin/Howe/Schwab/Swanson - Revised 1991
Eric L Kay, Forest Road Consultant
Telephone 250 337-5096 Fax 250 337-5096
Kay and Associates, International Training Consultants.
8712 Island Highway, Black Creek, BC. Canada V9J 1K5
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