Retaining walls

Most retaining walls are built to a similar design to free-standing walls. They are made up of horizontal layers of face stones, wedged and packed with smaller stone fillings, and have a batter which tapers from wide at the base to narrow at the top.

The diagram shows the two main variations within this ‘standard’ form. Yorkshire wallers usually build a normal two-faced wall, although the hidden inner face is left rougher and with a steeper batter than the outer face, and large face stones are placed so that they project into the cavity between the wall and the bank. Scottish dykers build a one-sided wall, using various sizes of fillings which bind not only with the outer face stones but with the earth bank behind. A third variation is found in North Wales and other areas where slates, schists or mudstones are common. Here the face stones are generally set with a good length into the wall, and the fill may include soil.

Each of these styles ensures that the wall sits solidly on its own footings. The Yorkshire style may be best where there are plenty of face stones and small fillings, or where the wall projects above the bank so that its top is free standing. The Scottish type is most suitable where many of the fillings are large and irregular and so bind well with the earth bank. The Welsh style is suitable only where there is an abundance of long building stones.

Retaining walls are generally built to protect a vertical or near vertical bank from erosion. Most banks are quite stable, except at the exposed surface, and place little pressure on the wall. Hence the batter of the outside face of a retaining wall is usually the same as for a free-standing wall of the same type of stone. However, where the wall is load bearing and supporting a bank rather than just protecting it from erosion, the batter may need to be more sloped.

The base width depends on the batter and wall height. In general it can be a bit thinner than for an equivalent free-standing wall, because to some extent it is supported by the bank behind it. However, where it is bearing the load of an unstable bank, the base should be wider than for a free-standing wall. The extent to which it should be widened will be entirely dependent on the nature of the load, and will vary from place to place.

For a 3′ (900mm) high wall, a base width of 1’6″ (450mm) is generally adequate. However, where a high wall is subject to vibration from traffic or the bank is unstable scree or wet clay, the wall should be built wider than normal, at 2′ (600mm) for a 3′ (900mm) high wall, or 3-4′ (0.9-1.2m) for a 5-6′ (1.5-1.8m) high wall.

Procedural points

The procedure for building a dry stone retaining wall is essentially the same as for a free-standing wall. The following points show how to adapt normal walling techniques to retaining walls.

1. The wall should be dismantled as for a free-standing wall, grading the stone as you go, and separating throughs, ties, hearting, building stone and any coping. As all the stone will be on one side, more space is required than for a free-standing wall. Set out the coping stones at a distance from the wall equivalent to at least two and a half times its height. Any turf should be piled neatly beyond the coping stone, with roots placed downward to protect them from drying out. Ensure that you leave plenty of space for soil. This should be piled at convenient intervals, as for hearting.
   Often it will be necessary to run the piles of soil continuously through the stripping- out area as shown. Care has to be taken that the soil does not spread out and engulf the building stone. Often it’s possible to pile some of the soil uphill, above the intended top of the wall, ready to shovel down behind the wall.
   
2. Prepare the bank by cutting it back and digging it out to a vertical face the height of the proposed wall. Whilst it is best to disturb the bank as little as possible, you will need to cut it back about 6″ (150mm) deeper than the proposed base of the wall, to give room to place the stones.
   
3. Batter frames are usually impractical to set up, but line bars can be used. It is only necessary to set up bars for the outer face, and the inside can be built ‘by eye’. To help secure the line bar, additional bars or wooden battens can be driven into the bank and tied to the line bar.
   
4. Dig a foundation trench for the footings, down to firm subsoil or bedrock. About 6″ (150mm) is usually deep enough.
5. Lay the footings as for a free standing wall, concentrating on running the stones lengthwise into the bank.
6. If the wall has definite ends or ‘heads’, build these first. Choose large stones, preferably with a flat bed and two good faces.
7. If the wall merges with the bank at either end, you can build it up along the whole section as convenient. Be sure to key the ends into the bank at their sides as well as at the back.
   
8. Build up in horizontal layers, with the biggest stones at the bottom. Keep to the principles outlined here, paying particular attention to running each stone lengthwise into the wall.
   In the Yorkshire style, build up both the inner and outer faces together, with fillings between, as in a free-standing wall. Don’t worry if the inner face is irregular but be sure that the stones sit securely. Pack more fillings between the inner face and the bank. These tie the wall to the bank and ensure good drainage. Keep the level of the fillings as high as the face stones.
   In the Scottish style, place and wedge as necessary each face stone, and pack behind it with fillings to give a solid, single-faced structure.
9. A certain amount of earth always slips down from the bank as you work. In some Scottish and North Welsh walls, soil may anyway form part of the integral structure. Any earth must be packed down well as you proceed. Place the hearting as normal, with shallow layers of soil, about 2-3″ (50-75mm) before compaction, filling all voids. Ensure that the soil is very well compacted by walking on the lower layers, and compacting the upper layers with a tamper or lump hammer. The soil and stone must be evenly distributed in any layer, so that pockets of soil do not develop. Where there is a lot of soil for filling, it is best to use up the hearting lower down, using a soil only fill higher up the bank.
10. Place throughs at the same level and spacing as for a free-standing wall of equal height, with their length projecting into the ‘cavity’ between the wall and the bank. Their outer face can be flush with the wall, or protruding, depending on the local style. Wallers disagree as to whether the backs should project into the bank. Some maintain that digging throughs into the bank helps tie the wall to the bank. Others say that any digging destabilises the bank, and that the backs or ‘tails’ of the throughs are then held by the bank and cannot settle with the wall. As the wall settles, the throughs begin to slope, acting to shed the stones sitting on top of them.
11. If you have plenty of big stones, use some of the most awkwardly-shaped ones as fillings, rather than breaking them up. In the Scottish style, a proportion of big fillings is important to strengthen the wall, while small fillings are necessary to wedge up face stones. If you try a face stone and it doesn’t fit, use it to pack the wall cavity.
12. As with free-standing walls, the type of coping used on retaining walls varies from area to area, and often from wall to wall. Try to determine the original form of coping on repairs and rebuilds, and check the local coping style when building a new wall. Flat, slab coping is far more common for retaining walls than for free-standing walls. Construct as described here.
    Many retaining walls do not have a formal cope, but are finished with soil and turf instead. In this case it is important to ensure that the final layer of stones is very solid, as until the turf becomes established the stones are fairly easily displaced. Make sure all the stones are set with their lengths into the wall, and avoid using very thin, light stones. It doesn’t matter if the top layer is slightly thicker than the preceding layer, nor if it has an irregular top, as unlike a stone coping, the turves will sit on any irregularities. If there is a limited supply of turves, complete the outer edge of the revetment top first, before starting on the next row in. You may be able to cut additional turves from a nearby field. See here for details of turf tops.
13. If the top of the wall projects above the bank top, build the projecting part as a two-faced, free-standing wall. If this ‘parapet’ is no more than 2′ (600mm) high, there should be few problems. Taller ‘parapets’ should only be built on top of retaining walls built to the Yorkshire pattern, like a free-standing wall, or otherwise the weight of the parapet may cause uneven settlement and eventual collapse of the retaining wall.
14. Any loose stones or earth on the bank should be shovelled down behind the top of the wall, to help bind the wall and bank together.

Problem situations

1. Where the wall runs up a slope, start the wall at the downhill end, as with a free-standing wall. Make sure this end is provided with a really solid head. On a steep slope, the bottom end stone must be massive and absolutely stable to support the weight of material above. When in doubt, concrete the end to keep it from shifting.
2. Where the wall is built to retain scree or other loose material on a steep slope, several adaptations are necessary. The diagram illustrates a method which has been used successfully by volunteers at Cwm Idwal in Snowdonia, and at other sites where there may not be a level base from which to start, and where the aim is to blend the stonework with the existing hillside as much as possible.
   
   It is important to base the wall on bedrock. This may mean starting it considerably farther down the slope than planned, at least in places, and building up a level platform until the real footings can be placed. In order to try and promote a quicker growth of vegetation over the wall to help stabilise it, any small gaps which cannot be filled with stone can be plugged with turf. However, turf must not be used where a stone could be used instead, as this will create a weak point in the wall.
   This technique and the following two points outline some of the work necessary to repair badly eroded slopes. Successful restoration may require retaining walls of the type described above, as well as stone pitched paths, stone-lined drains and revegetation by turfing and seeding. These techniques are described in Footpaths – Erosion control and vegetation restoration.
3. When winning stones from a hillside for use in a wall, take them from uphill to make it easier to shift them, and to prevent destabilising the slope below the wall. To avoid initiating erosion, fill holes left by the removal of large stones with any nearby loose material such as stones, gravel or earth. Choose material which is itself liable to be eroded downslope or which has slipped down onto paths, drains or areas of turf.
4. Where runoff is likely to affect the wall, a cut-off drain can be dug above the wall to intercept the flow of water and drain it away from the wall. Preferably line the base and sides of the drain with flat stones to prevent it slumping, or fill it with stones.
   
5. Where the base of the wall is a path, or is in soft soil or peat, dig down to a firm base to provide good foundations.
6. Where the bank is of inherently unstable material such as sand or wet clay, ordinary retaining walls may not be sufficiently strong. An alternative technique is to use gabions, which are large rectangular wire baskets that are filled with stones and built up in rows like giant blocks. They are frequently used in civil engineering for roadside, riverside and coastal embankments, and may be suitable for some conservation projects where the gabions can be hidden with vegetation or with properly constructed dry stone walling. They are not generally suitable for use on mountain sides or for areas where dry stone walling is traditional, as the random nature of the stone structure is obtrusive in such settings. For riverside revetments, ‘soft’ materials such as vegetation or geotextiles are increasingly being used.
7. Where the wall supports a path or track, a substantial wall should be built, finished with large flat top-stones that cannot easily be dislodged. With any revetment or other structure adjoining a public right of way, including footpaths, bridleways and roads, the safety of the user must be considered. The relevant transport authority should be consulted over any such work.