Revetments

In this book the term ‘revetments’ is used to describe structures which retain slopes below or above paths, and prevent the path either eroding from below, or being covered with material slumping from above. Revetments should have an attractive finish as they will be visible unless covered by vegetation. They usually only require a small amount of backfill material.

The term ‘erosion barrier’ is used to describe structures built to repair bare slopes and gullies caused by trampling- induced erosion. These barriers are usually buried, and therefore their appearance is not important. Erosion barriers require a large amount of backfill material in order to restore the slope to its original profile. Most designs can be used for either purpose, as their function is similar.

Log revetments

These are appropriate on wooded slopes, using either local untreated timber, or for longer life, pressure-treated timbers. Log revetments are also suitable for coastal, lakeside or downland paths where stone is not available. Vertical revetments up to 1.5 metres high can be built, provided that posts can be knocked in securely. Buttresses or stepped revetments may be needed on severe slopes. Careful construction is needed both for strength and good appearance.

The design below uses 1.8m x 125mm horizontals, with 2.75m uprights, spaced 900mm apart. The general procedure for a straight revetment is as follows:

1. Measure the height of the gap to give the number of horizontal layers needed. Allow at least 1.8m extra on either side to prevent slippage at the edges.
2. Mark the position of the revetment with a taut line and pegs. Level the bottom of the gap and excavate narrow slit trenches on either side. It’s important to get the bottom of the trench as level as possible, so that the horizontals fit squarely to the uprights.
   
3. Knock in all the uprights, checking they are exactly in line and vertical. Uprights should be knocked in a third to one half of their length
4. Fit the bottom horizontal timbers. They can be attached by wiring or nailing. Nailing can cause the wood to split, especially if it has not been properly seasoned before use. Three people are needed to do the wiring; one at either side of the post straining the wire with fencing pliers, and one person stapling. Continue fitting to the top. Nailing is easier from the front of the revetment, as there is not usually room to nail from the back. Alternatively, only the top horizontal can be nailed, to hold it securely and discourage vandalism. The top may need to be stepped to fit the gradient of the path. Cut uprights level with the top of the revetment, weathering the tops to shed water.
5. Backfill the revetment with free-draining material, using hard-core at the base, and graded quarry waste or similar for the upper layer. The aim is to allow water to drain freely through the fill and out between the horizontals, rather than it forming a channel at the ends of the revetment. It’s usually necessary to top the backfill up after six months or so as it compacts with use.
6. Seed the lower slope or transplant turves or other plants as required, and put down brashings to discourage people going on it.

The same technique can be used for a revetment above a path. On very steep slopes, two or more revetments can be constructed, wired together with a windlass. As an alternative to wire, Netlon string (available from Netlon Ltd.) can be used, as this does not degrade.

Corners or angles on revetments are always a weak point, so avoid them if you can. If unavoidable, position an upright at the angle, and cut the ends of the horizontals to butt neatly as shown.

Alternatively, ‘log-cabin’ style joints can be made.

Felled tree revetment

This revetment was built in a nature reserve in Gwynedd, North Wales, using sycamore trees felled nearby, of about 450mm base diameter. The trunks were cut into 1.8m and 2.7m lengths, as required for horizontals, and the branches into 1.5m lengths for uprights. A similar method of construction to that described above was used, with the uprights knocked in at 900mm intervals, and the largest section of trunk used for the bottom horizontals. Gaps between the horizontals were turfed to encourage quick recovery of the slope.

Braced timber revetments

This technique uses a ‘skin’ of sawn timber tied by wire to bracing stakes buried in the bank.

The following design has a skin of posts and rails with chain-link fencing, enclosing a rubble wall. It can be used to support an unstable slope above a path, for example where space is limited alongside a river bank, and the path will be lost if the slope collapses. The chain-link fencing is not attractive, but becomes less obtrusive as vegetation grows through. This can be encouraged by filling between the fencing and the wall with turves. This design is suitable for erosion barriers.

The very strong construction described below should only be necessary for revetting banks of streams or rivers along which paths pass. The procedure for building is as follows:

1. Clear debris and loose rock from the line of the revetment.
   
2. Construct coffer dam to keep as much water as possible out of the work site. It is difficult to make it watertight, but is effective as long as water is stopped from actually flowing through the site.
3. Excavate holes and carefully align the posts in position, using temporary braces.
4. Fill with 3:1 concrete, and leave for at least two days to set.
5. Remove temporary braces. Drill posts and boards, and bolt in position. Attach bracing wire.
6. Build stone wall and backfill with rubble. Surface path as necessary.

Gabions

A gabion is a box-shaped basket of square or hexagonal mesh, made of galvanised steel or polypropylene. Gabions are supplied flat for assembly on site, positioned and then filled with stone to form strong, durable revetments. They are available in various sizes from several suppliers. Because of their appearance, they are not suitable for use in the uplands, or on sites where vegetation is unlikely to disguise them. Gabions are useful for some lakeshore, riverside and coastal locations, where space is limited and secure revetments are needed. Gabions can be hidden by vegetation growth in many of these locations. Gabion-like baskets or rolls can be made in-situ from steel or polypropylene mesh, and are suitable for erosion barriers or other structures which will be buried, where appearance is unimportant.

For most footpath sites, a single layer of gabions will be sufficient, set in the ground with sides vertical. For two layer revetments, the face of the wall should be angled to 10 degrees to increase stability. The face can be stepped or flush, the latter discouraging access. Structures higher than twice the width of the gabion should be designed with engineering advice. Where scour action is likely on river banks at the base of the revetment, secure foundations will be needed either by sinking a gabion below the base of the bank, or by using mattress-shaped gabions to create an apron.

The minimum size for stone to fill the basket is 10mm larger than the mesh size. Stones larger than 200mm are not suitable for rough filling, as they will pack unevenly into the basket. For good appearance, the exposed walls of the gabion should be carefully constructed like a dry stone wall. Where there is sufficient suitable stone, construction of the fill in a similar way gives a very strong structure that will not settle. Normally it is sufficient to roughly fill the centre with smaller stones behind the constructed walls. Fill the gabion in layers, one third at a time, fitting the bracing wires to keep the gabion in shape.

Gabions are assembled with ring clips or by threading with wire, according to manufacturer ’s instructions. Dig out a trench for the foundations, either level, or to create a 10 degree raked wall. For a row of gabions, wire or clip them together in position, and then anchor one end with posts driven in to secure. Thread four crowbars through the other end and tension with a Tirfor winch. Keep the gabions under tension while they are filled. Fill the gabions in layers, one third at a time, fitting horizontal wires tightened with a windlass. Slightly overfill each gabion to allow for settlement, stretching the lid to close using a crowbar, but being careful not to damage the galvanising.

Angles can be made by cutting a gabion and fitting it to the required shape.

Where the gabions are to form the path, the top can be surfaced with an aggregate surfacing. Otherwise, grade soil over the top of the gabions and either seed or transplant turves or clumps of vegetation.

For shrubby growth, insert willow or alder cuttings during construction. Choose willow or alder according to which is typical of the habitat. Put them randomly across the gabion, protruding about 150mm from either side. When the structure is complete, cut to the first bud, from where shrubby growth will develop.

An alternative method is to use larger willow stakes placed to make the front face and top of the gabion, thus hiding the stone. These will sprout along their length. At water margins, reeds and other marginal plants can be planted into the top of the gabion.

Various geotextiles are available which are designed to stabilise water margins with vegetative growth, and give a ‘softer’ finish than gabions or other constructed revetments. For details contact suppliers.

Dry stone revetments

The technique of building a dry stone revetment is similar to that of building a dry stone wall. The basic procedure is as follows:

1. Cut back the bank as necessary, to the sharpest angle possible without it slumping.
   Cut back enough to allow for the width of the stone plus some room to work. In order to keep the work site tidy, and prevent slumping of slopes, only prepare as much as you can revet that day.
   
2. Pile the material neatly along the outer edge of the path, leaving a clear path for walkers so they do not have to spread down the slope or trample the spoil material.
3. Excavate a trench about 150mm deep for the foundations.
4. Starting with the largest stone available, build up the revetment in horizontal courses, placing the stones level or sloping down towards the bank, with their long axis going into the bank. If wedges are necessary, place them from the inside. Pack behind each course with stone filling.
5. Some wallers like to pack turf and soil between the stones to encourage growth of vegetation which binds and camouflages the revetment.
6.  There are various ways of finishing the top of the revetment, depending on the situation, the stone available, and the local walling style. If suitable stones are available, they can be laid flat on the top of the revetment, with the ends embedded in the bank. They must be large enough that they cannot easily be dislodged. Alternatively, the top can simply be covered with turf.

Low revetments are very likely to be used as seats, which may or may not be desirable. This use can be discouraged, for example on narrow paths, by either building the revetment too high to be comfortable, or by placing a rubble coping on the top. The ‘buck and doe’ style of coping should only be used where it is part of the local walling style, as it is too decorative for many rural situations.

The method of building revetments described above is suitable for revetments up to about one metre high, either above or below the path. If built below the path, top with turf or flat coping stones. Do not use rough coping stones in an effort to keep people away from the edge, as the stones will only get dislodged. The revetment must anyway be strong enough to withstand the weight of people standing near the edge. The revetment can be extended up into a low wall, if this is thought necessary for safety.

If necessary, build two or more revetments, each up to a maximum of one metre, rather than one very high revetment. Although massive slumps cannot be prevented, these revetments reduce most types of erosion on unstable banks.

Steep revetments over one metre tall should be built as retaining walls. Two different designs are shown here. For further information on walling see Dry Stone Walling.

In a deeply gullied path, a revetment can be built with a stone drainage channel at the base as shown above. The path itself will need surfacing with stone pitching, flagstones or compacted aggregate, to prevent further erosion.

Mortared revetments

Mortared stone revetments can be built along river banks, to prevent erosion of the bankside path. Avoid using mortar in other situations, especially in the uplands. A well-built dry stone revetment has the advantage that it will flex rather than break up under severe frost conditions. If the mortar is relied upon for strength, there is always the possibility of the structure collapsing if the mortar fails.

This design is suitable for the outer bends of rivers where erosion is greatest. The stepped design looks less obtrusive than a straight wall, and allows the water to flow more smoothly as levels rise.

Tyres

Tyre revetments are not generally recommended for countryside use, but may be suitable for recreation areas or other intensively used sites. Old tyres are available, usually for free, from tyre depots.

1. Clean back the slope, and pile up the spoil for re-use.
2. Lay the bottom layer of tyres and fill with soil. Fill the gap behind with any available spoil. Compact with a roller or punner.
3. Continue placing and compacting the layers, and cap with a layer of concrete to prevent vandals dislodging the tyres.

Depending on the situation, the tyres should be completely disguised with vegetation within a couple of years. This can be encouraged by including turves or roots of plants in the tyres, or by seeding. In shady situations, plant with ivy (Hedera helix) or periwinkle (Vinca minor).

Other methods of revetment include the use of concrete filled sandbags. These are best for bankside revetments, or for erosion barriers which will be mainly buried, as they are not attractive.

Geotextiles

A huge range of products are available for many uses in civil engineering (Landscape Design Extra, No. 61, Nov. 95). Their use for stabilising slopes and encouraging revegetation is discussed here. A particular application for reinforcing steep sided revetments (over 45 degrees) is briefly described here. It may be useful as an alternative to timber or stone revetments on sites where material is not available to hand, and access is difficult. This technique can accommodate corners and curves. It also has the advantage that most of the geotextile is buried within the revetment. On many sites the exposed face will be hidden by vegetation.

The following method was used by the National Trust for Scotland at Grey Mare’s Tail, near Moffat, to support a path across a slope. The base of the path was roughly levelled, and the geogrid unrolled along it. Infill was then compacted into place, and the geogrid lifted up and over to enclose the infill, with turves laid under the exposed face of the revetment. An aggregate path was then laid along the top.

For larger revetments, the geotextile can be laid in two or more layers, as shown. The face can be turfed or seeded as work proceeds. Tensar SR geogrids (Netlon Ltd.), Terram ParaGrid (Terram Ltd.) and Lotrak (Don and Low Ltd.) are all recommended for this type of slope reinforcement. Further advice for particular sites should be obtained from the manufacturers.