3.2 Geotextiles: Aggregate Path on Peat
Path construction through peat can require deep excavation to reach a solid base. Where the peat depth exceeds 500mm this may be impractical. Either large quantities of material are required, or, without a firm base, the path structure will be unstable. The use of synthetic geotextiles to provide the foundation, and ‘float’ the path over deep peat has developed from road engineering and construction methods.
Geotextile laid under the path separates the path material from the peat; it prevents aggregate loss and the path subsequently disappearing. Selection of appropriate geotextiles will also provide a strong path base. If laid well this results in a stable and durable path. This technique reduces the amount of excavation and aggregate required compared to excavating to a hard base and infilling with stone.
Bill of Quantities (example)
Construct aggregate path on geotextile to a variable width, between 800 -1200mm. ‘Terram’ 2000 to be used on full length; ‘Tensar’ TS20 to be used on sections where the gradient exceeds 6° and on benched crossfalls. Aggregate base and surface to be a minimum depth of 250mm. No geotextile to be left exposed above the path surface.
The components are as for an aggregate path, with one important addition - the path tray is lined with geotextile. Turf edging may be required to prevent geotextile exposure at the path edge.
These are as for an aggregate path with the exception of the path tray depth. This should be 150mm minimum but need be no more than 250mm. The pressure of use will determine the need for a stronger path base to the maximum depth - for example, if the path will be used by ponies.
A deep path tray should not be excavated where the peat is wet and has minimal vegetative content. Either a shallow tray can be carefully dug, or, preferably, the tray depth formed with good size turfs, to provide a stable path edge, and the path built up over the eroded vegetation surface, rather than dug down into it.
If the peat has no structure or is very wet the formed tray should be increased to 300mm wider than the required path width, on each side. This allows for a greater geotextile width, which will give added strength to the path base, and allow better water drainage from the path base. Good size turfs will be required to place over the excess width of geotextile, and to create the tray edges.
The aggregate material is the same as for an aggregate path.
There is a variety of geotextiles on the market, mainly geared towards road building or engineered landscaping. Two geotextiles are commonly used under aggregate paths - mattings and geogrids.
The matting, of tightly woven synthetic fibres, is the separation material used to ‘float’ the path. The main properties are:
- separates the path material from underlying peat, or wet clay
- semi-permeable allowing water to seep through and drain away from the path structure
- spreads the load across the path width and length and prevents subsidence or sinking into the peat.
Mattings come in several grades, the highest provide greater load bearing strength, which will be required over very deep or wet peat. Lower grades are suitable where the peat layer is thin or has a higher mineral content. The one most widely used in Scotland is "Terram" 2000.
The geogrid is a thick plastic mesh, which provides a strong path foundation, and also ‘holds’ the aggregate.
The main properties are:
- provides a strong path foundation
- spreads the weight of path use over the full path length and width
- grid structure prevents path material from moving along, or across the matting and migrating from the path sides into the peat
Geogrids may therefore be used with lower grade matting for additional strength over deep, wet peat.
They are particularly useful to prevent movement of the base aggregate where there is a cross-slope or a downhill gradient. The one predominantly used in Scotland is ‘Tensar’ TS20.
Geotextiles can be obtained from most industrial suppliers. They are normally supplied in rolls, of variable width and length. Whole rolls of matting may be cut, off-site, to a suitable width using a chain saw or hack saw. The lengths required can be cut on-site using a sharp knife or heavy duty scissors.
Method of Construction
Form the path tray
- excavate the tray as for an aggregate path, with the exception that the depth does not need to reach a solid base
- form a base that is level and even for laying the geotextile
- remove any protruding bog wood, stone or boulders to prevent distortion or puncturing of the matting
- if the peat is very wet, or has no vegetative content, form the tray depth and sides with good size turfs, after laying matting
Lay the geotextile matting
- line the path tray with the geotextile matting, cutting it to the required width
- to take up curves and bends in the path either fold the matting or cut it to suitable lengths, allowing an overlap of at least 300mm
- secure folds or overlaps with larger aggregate stone to prevent them protruding up through the path material
- if the matting is laid wider than the path place large turves, or boulders, over it to secure and form the required path tray width
- to prevent aggregate migration into the peat, or peat into the path structure, the matting may be folded halfway up the tray sides
- to prevent subsequent exposure at the path edges tuck folded up matting back from the path tray and dig under the surrounding vegetation, or place turfs over the top
Lay the geogrid
- where required, lay the geogrid over the matting, cut to the required path width, and for bends in the path alignment; as with the matting joins should overlap by 300mm
- where there is an excess on either side, due to the variable path width, it should be dug into the tray edges, or, if the matting is folded up, cut to the exact size
- the geogrid should not curve up the tray sides; it is important that no geogrid edges are left exposed after the surface has been laid and compacted
Incorporate drainage features
- construct drainage features as for an aggregate path, with the exception that geotextile should be laid to continue into construction trenches
- for ease of laying, and to provide additional strength, cut the geotextile to allow a full overlap across the drainage trench width
Construct the aggregate path.
- take care to prevent any puncturing of the matting when laying and compacting the lower layer of base, or sub-base material
- care is also required if moving aggregate along the prepared path tray with a powered track barrow; minimise the number of movements as far as possible
- make sure that any turfs already laid are effective in covering the geotextile and containing the aggregate
- the path edges may require further turfing and landscaping, to define the line and ‘soften’ the appearance
- use excess turf and spoil from the tray excavation to re-instate any eroded or damaged ground, and for in-filling any borrow pits (see Introduction to Restoration Techniques)
Key points to watch out for:
- avoid the possibility of geotextile becoming exposed - keep path construction deep enough to cover it and to avoid erosion
- ensure matting, or geogrid, is not sticking up after surface compaction - use good size turf effectively at path edges
- incorporate drainage features into the path
Other methods have traditionally been used to float paths over deep peat, particularly heather or tree brashings, or wooden stakes. These rely on the preservative nature of the peat and, depending on its acidic balance, may break down quickly.
Where it is not feasible to excavate a path tray a ‘causeway’ may be constructed with a geotextile base. The path sides are contained by large boulders with spoil and turfs, which must be placed to provide a solid edge. If the path is traversing a slope the uphill slope may provide an edge. A double course of boulders or very large turves will be required to take a causeway path through a particularly wet area.
With appropriate and sensitive geotextile use, a well constructed path should withstand the pressure of use, and drainage features protect it from erosion. It will need some maintenance over the years but not as frequently as the drainage features:
- if geotextile is exposed, dig it securely back under the path surface, or path edges, and re-surface over the top
- top up surfacing where it has settled, compacted or eroded, incorporating a camber or cross-fall as required
- repair path edges with turf where they have collapsed or been trampled and eroded
- make sure all laid geotextile is well covered and will not be exposed by path surface or edge erosion
- clean up and remove any off-cuts of geotextile that may be left on site
HEALTH AND SAFETY HAZARDS
- take care when cutting geotextiles, use sharp tools - blunt ones lead to excessive force being used and potential accidents
- if using chain saws, or similar, to cut rolls of matting, make sure the correct PPE is being worn and the operator is trained
- do not under estimate the amount, and type of geotextile required
- ensure that the path gradient, or crossfall is not too steep, increasing the risk of erosion and exposure of geotextile