This is a mechanised method of path building, which uses an excavator to strip off the top surface, excavate the underlying subsoil and then invert the two layers to produce a path surface. The stony subsoil, suitable for path surfaces, is then at the top, and the unstable peat or topsoil is buried beneath. The aim is to use only the existing natural materials, reorganised by the excavator, to produce a free-draining, hard-wearing surface. Culvert pipes and stones for headwalls will need to be brought in as necessary.
This technique was first used on the Southern Upland Way and West Highland Way by the former Countryside Commission for Scotland, and has since been developed by the Yorkshire Dales National Park and elsewhere.
The success of this technique relies on the quality of the subsoil, and on having a skilled and experienced excavator driver. Subsoils with a high clay content may produce a sticky and unsuitable path surface, but it has been found difficult to predict results even by careful analysis of subsoil samples, and appears to be dependent on the way the subsoil is mixed and dropped by the excavator. Slopes of between 10 and 15 degrees are typical for this type of work, but the technique has been successfully used on slopes as steep as 20degrees, and the only limit is the degree of slope on which the machine can safely work.
The technique is very cheap compared with the alternative of floated aggregate paths on geotextiles, or stone pitching on steeper ground. The fact that no imported materials are used not only saves on material cost, transport and labour, but is a significant aesthetic and ecological consideration for upland landscapes. The actual work can look very drastic, and effective publicity is needed to ensure the work is seen to be in the cause of improvement, and not a form of authorised vandalism!
The details of the technique will depend on the amount of peat or topsoil present, the type of subsoil, the slope, and the type of machine used. Conventional ‘Hymac’ type excavators with a 2.5m wide trackbase have been found more suitable than mini-excavators, even though they are much wider than the finished width of the path. The larger machines have a long reach, and have the power to move greater volumes more efficiently. The ground pressure is no greater for large machines, because of their greater track area, and their longer reach means they make fewer movements on the ground than mini-excavators. Depending on the situation, a skilled driver can complete a length of about 100m in one day. This speed of working makes it easier to take advantage of good weather and dry ground conditions, when the work can be completed with the minimum of damage.
An important factor is the ‘retreat’ route for the machine, as driving it back down the finished path would normally damage the path. There may be a retreat route for the excavator down a different track. In dry conditions, the machine may be driven back down the hillside without causing damage. Some drivers may prefer to work from the top downwards, although in wet conditions the ditch will tend to collect water which can make working conditions awkward. Where paths undulate, with both downhill and uphill sections, this factor is less important. In some locations, helicopters have been used to airlift small excavators onto inaccessible sites.
The basic technique is as follows. Normally the machine would start from the roadside, car-park or other access point and work in an upwards direction (but see above).
To get the reorganisation process started, the machine proceeds forwards up the path for a few metres, so that working backwards, the bucket can reach the start of the path. Vegetation, topsoil or peat and subsoil from the first few metres of the path up to the machine is removed, down to a depth of about two metres, and put temporarily aside. The bucket is swung to the front, and the vegetation is removed and put temporarily aside. From then on, the material can be removed from in front of the machine, and permanently repositioned behind as described on the following page. This minimises handling of material and damage to the surrounding vegetation.
- The peat or topsoil is removed from in front of the machine across the width of the path and ditch, swung to the back of the machine and repositioned along the proposed ditchline to the side of the new path.
- The subsoil is then excavated from across the width of the ditch, swung to the back and repositioned to form the new path. The bucket is used to compact the subsoil to make a firm path.
- The vegetation from the next section forward is then removed and put down behind the machine to cover the ditch and the path edges. Vegetation is removed in thick clods, to include the root systems.
The machine then moves forward, and repeats 1-3 until the path is completed. Local adjustment is needed to remove the excess of excavated material at the beginning of the path, and the lack of it at the end of the path! Some hand- finishing may be needed to landscape the edges of the path and ensure quick recovery of vegetation.
Within this basic technique there will be variations. On deep peat, where a complete ‘swap’ of subsoil for peat would require too big an excavation, the new path can be made by simply laying the subsoil on top, supported by the mat of vegetation. Where the vegetation is sparse, a geotextile layer would be needed to support the subsoil.
Where the ground can support the excavator without damage, the driver may prefer to position the machine to one side of the new path, rather than along the line of the path itself.
Culverts must be included where surface or sub-surface water crosses the path. The inlet and outlet point for each culvert should be marked in advance with easily visible stakes, and the culvert pipe put ready at the side of the path. The excavator then digs out the trench and places the pipe before covering it with inverted subsoil. Headwalls of stone are then built by hand to support each end of the pipe. On some sites this will be the only hand work needed for the path construction. The entire disturbed area can then be seeded as desired.
It may be necessary to import aggregate to use on sections of the path which prove unsuccessful due to high clay content, surface roughness or other problems.
