4.1 Materials and Use
The most important factor when choosing materials for site restoration is to achieve a natural finished appearance. Sensitive use of materials appropriate to the site, and to the purpose of the restoration, is essential.
Careful attention must be paid to:
- matching vegetation type
- following topographical and geological characteristics
- sensitive collection of materials
The natural, on-site, materials used are:
- Stone
- Turf
- Spoil
supplemented, as necessary, by:
- Seed
- Fertiliser
- Biodegradable geotextile
STONE
Stone that will remain visible should be in its natural form and weathered, preferably lichen or moss covered, to blend with the surroundings. Stone for site restoration can be sourced from:
- surface stone and boulders, within proximity of the path
- excavation, or movement, during path and drainage work
- borrow pits, excavated for path surface material
When moving stone to the restoration area, with winches or power barrows, care is required to avoid further ground damage.
General Principles of Use
The main use for stone is to stabilise slopes and prevent off-path movement. Specific use depends on size:
- large boulder stone - to stabilise and stop use of path edges; to block use of alternative path lines
- medium size stone - to create use channelling mounds, with spoil and turf, at path sides and corners; to revet banks and steep slopes
- smaller stone - scattered, with spoil and seed, to help revegetate and prevent use of eroded ground
Boulders for blocking use should be large enough to dig partially into the ground whilst leaving the majority exposed as an obstacle. For stability, dig in to at least one third of the depth. It may be necessary to use stone wedges to ensure that no movement occurs.
Blocking stone should appear as natural as possible, with weathered and lichen covered surfaces exposed. Transplanted turf can be used to cover unweathered sides, or to landscape around, particularly on slopes or the path edge.
Unweathered or broken stone can be used to create blocking or channelling mounds, if it will be turf covered. Stone exposed at the surface should be weathered and placed to look natural. Small stone spread over eroded areas should be scattered to achieve a naturally appearing, random cover. The stone scatter should deter use and break up the visual impact of large bare areas.
TURF
Turf used for restoration should closely match the vegetation of the site, wherever possible making use of turf cut during ditching or path excavation. Turfs may also be gathered randomly from the surrounding ground, but out of sight of the path.
Grass turfs are usually best for blanket coverage; taller vegetation, such as bushy heather or rushes, is better for blocking off-path use. Leggy and woody heather should be avoided as it will not survive transplanting.
General Principles of Use
Unless in short supply, turf is used in all site restoration, including:
- defining and establishing a path edge
- blocking use, with boulders
- revegetating eroded ground, braids and short-cuts
- covering stone and spoil mounds and banks
- landscaping backfilled borrow pits
Transplanting
To improve the chance of success turf should be dug large and deep with a good, soil covered, root system. Soil around a turf will normally hold a seed source, and when transplanted the turf will provide shelter for the germination of this, or introduced seed. Turfs should be carefully handled and transplanted into prepared holes, or onto loosened ground or spoil.
Ideally turf should not be transplanted during periods of dry weather as success is dependant on rain to help root systems establish. Periods when frost is expected should also be avoided as it can damage establishing roots.
Turf should be left for as short a time as possible before transplanting. When not used immediately cover the root system with soil, to protect it from drying out. Stack turfs excavated during path construction, with root systems facing inward, until it is required. To help retain moisture lay the turf on, and cover with, synthetic material or matting.
Spot turfing
Turfing, with one or two large turfs, is appropriate at path edges, around large boulders, or to block path braids. Over larger, eroded areas spot turfing can create "islands" of vegetation, when not enough turf is available for total coverage.
To appear natural turfs should be randomly positioned. Spacing will depend on availability, but over large areas aim for at least 30% cover.
Blanket turfing
In this method turfs are laid for total cover over bare ground, or mounds. Grass turf can be cut to the shape required to achieve good coverage. This may include shaping the under-side to the ground being turfed over. Adjoining turf edges and surfaces should be flush, but with undulations incorporated for a natural appearance.
Turfs will shrink slightly, especially during periods of dry weather. To prevent the root system drying out, and vegetation dying, turfs should fit tightly together, with overlapping joins. Fill gaps between or beneath with spare spoil or turf off-cuts.
The join with existing vegetation should match in type, form and surface contours. When turfing over mounds, surface matching is achieved by shaping either the underside of the turf edge or an excavating small tray and slotting the turf in. Alternatively carefully lift existing vegetation and fill underneath with spoil to raise the level. Finally match the surfaces by tamping down firmly.
SPOIL
Depending on the geology of the site, spoil will have a mineral or organic content, and be acid or alkaline - usually acidic. Some sites may allow a choice of spoil for specific use. The more organic the spoil, the better for establishing vegetation. Mineral spoil, with a higher content of stone particles, is more suitable for stabilising slopes, forming mounds and backfilling borrow pits.
Spoil is generally won from path tray and ditch excavations, or from borrow pits if alternative material is available for backfilling. To prevent damage to vegetation, store spoil on bare ground, or on a tarpaulin or geotextile.
General Principles of Use
Spoil is the bulk material used to:
- create mounds and banks, with stone and turf, to channel use onto the path
- spread over eroded areas, as the base for revegetation with turf, seed and fertiliser
- backfill revetments to stabilise slopes and banks
- backfill redundant borrow pits
When used for mounds and banks the spoil should be shaped to imitate natural landforms. Any larger, unweathered or broken stone in the spoil can be used to create the bulk of the mound. Smaller stone and spoil content is then compacted over and around to form the shape, ensuring that it is contoured to naturally match and join the surrounding ground level and slope.
It is always preferable that shaped mounds are blanket turfed, incorporating weathered stone, if appropriate to the surrounding area. If turf is limited, careful spot turfing can be used, with stone; if no turf is available the mound should be seeded, and fertilised to help stabilise the surface.
Where mounds or banks are created on wide areas of erosion it is important to use similar turf and stone around them to help avoid an unnatural appearance
SEED
Imported seed must be specifically selected for the site, and the nature of the restoration required. Specially prepared seed mixes will depend on:
- soil and natural vegetation
- altitude and climate
- whether a nurse crop for natural vegetation is required
- whether the seeded area will be walked on
- type of use - for instance, whether deep rooting for slope stabilising or fast growing cover for path sides
Consult a seed specialist on the suitability of particular cultivars in the seed mix. On designated sites advice should be sought from the local SNH office for suppliers of special mixes, or local sources where it is necessary to use native seed.
General Principles of Use
Depending on site designation, and ground conditions, seeding may be used in all types of restoration work:
- revegetating mounds or banks and eroded ground
- recovering braids and short-cuts
- slope stabilisation
- landscaping borrowpits
It is particularly useful where turf is in short supply, but is not necessary where turf is available for blanket cover. The best time of year for seeding is either late spring or early autumn. With the climatic variability of upland sites, sowing between these times can be successful, particularly if temperature and moisture levels are suitable.
Ground preparation
Damaged ground requires preparation before seeding. This involves either spreading spoil, or scarifying the ground to break up the surface layer. A texture that will hold and protect the seed from blowing away is required. Scarifying also encourages water to soak into the ground, which aids germination, and reduces the risk of rapid water run-off, taking seed with it.
Site conditions will dictate the extent to which ground preparation is feasible. On exposed sites, particles of loosened soil will blow or wash away, particularly on sloping ground.
If fertiliser is to be used it can be applied during ground preparation, preferably a couple of weeks prior to seeding.
Seed sowing
The best method on upland sites, is broadcasting by hand, avoiding windy days. Even with prepared ground wind will inevitably reduce the revegetation success on the required area. Long periods of dry weather should also be avoided.
The rate of seeding depends on the seed mix, the vegetation cover required, the ground conditions and the expected success of germination. Advice on this can be provided by the seed specialist. A rough guide is approximately 25gm, or one handful, per square metre of ground.
FERTILISER
Fertilisers add essential mineral elements to nutrient poor, eroded ground or spoil. Different compositions and forms release nutrients over varying periods of time, some as slow as two years. This is beneficial where the soil condition is particularly poor, or germination and vegetation recovery will be slow due to extremes of climate. For sites requiring rapid recovery, appropriate fertiliser use will encourage root and shoot development.
Depending on the mineral balance, fertiliser may affect the natural balance of surrounding vegetation, particularly in wet areas, where it may leach out. Specialist advice should be sought, on the appropriate type and quantity for the soil, natural vegetation, and revegetation required.
Some sites may have restrictions which will prevent the introduction of nutrients, and the local SNH office should be consulted. Fertilisers are generally obtained from agricultural or seed suppliers.
General Principles of Use
As with seeding, fertiliser can be used as part of any site restoration work. It can be mixed with the soil prior to seeding to promote quick and strong germination and growth; or applied to encourage natural re-vegetation. On an upland site, fertiliser is best broadcast by hand, and gloves should always be worn. The rate of spread will depend on the nature of the site and the vegetation cover required. Specialist advice should be sought for individual sites, and specific fertilisers.
Following seeding, especially if the required vegetation recovery is slow, a follow up application may be necessary, at the start of each growing season.
GEOTEXTILES
An extensive range of geotextiles has been developed for formal, or engineered landscaping work. Most are synthetic mats and meshes which stabilise the soil structure. These are only covered as vegetation grows through. In an upland environment they will potentially remain exposed, and are therefore not suitable
For upland site restoration mattings of coir, jute or straw, are preferable, as the natural fibres retain moisture, and biodegrade over time, leaving no sign of use. If required, special seed mixes can be incorporated within the matting.
Geotextiles are obtained from specialist suppliers.
General Principles of Use
Geotextiles are useful where soil, seed or fertiliser cannot be retained on eroded slopes and banks. With their woven mesh they provide a "bound" structure to the surface, and encourage quick vegetation establishment over exposed slopes. Natural fibre mattings also provide a water retaining medium for seeding of large eroded areas where there is no organic soil.
Natural fibre mattings are normally supplied in rolls, with instructions for laying. They can be cut for the area and shape required, or to go around large boulders. Holes can also be cut for spot turfing.
Fibre geotextiles are most successful where they maintain contact with the ground. Level the ground prior to laying, and remove large loose stones. Apply approximately 50% of the seed and fertiliser under the matting and 50% over the top.
It is essential that the matting is firmly attached to the ground, particularly if the site is exposed to high winds. Slot the cut edges into the ground, or tuck under, to prevent fraying. Secure all edges firmly, at regular intervals, with long metal pins or pegs. The matting should also be secured with pins across its width.