Geotextiles

The term geotextiles covers a wide range of materials used in road building and other civil engineering schemes for separation of soil layers, filtration, reinforcement and erosion control. About 60 different products from 24 manufacturers are available in the UK, including synthetic and natural materials in a range of different mesh sizes, thicknesses and designs (Landscape Design Extra, No.61, Nov. 95). In footpath work, geotextiles have been used for many years for supporting aggregate paths on soft ground. They are also used for erosion control and vegetation restoration

An aggregate path laid directly on soft ground will soon fail, as the stone gets ‘punched’ into the sub-grade, and fine material from the sub-grade is ‘pumped’ upwards by the action of walkers. The geotextile keeps the layers separate, and by keeping the aggregate together, gives structural strength to the path. Geotextiles have the advantage over other types of sub-base of being light to handle and easy to install, and their efficiency compared with a stone sub-base may also allow a reduction in the thickness of the base layer.

Many different types of geotextile have been tried in many different situations, with varying success. The most problematical sites are deep peat, where the stronger geogrids have been found more successful than the thinner, flexible materials (see below). The geogrid and aggregate in effect float over the structurally weak peat, creating a ‘raft path’. However, in the uplands generally there is a move away from using any synthetic materials, and instead relying on traditional methods of stone pitched or slab paths, or subsoil paths using no imported material. The susceptibility of upland paths to erosion means that the geotextile layer can easily become exposed by erosion of the aggregate, leaving an unsightly and dangerous path.

For path work, geotextiles have been most successfully used on sites with gentle gradients over damp mineral soils or shallow peat. They are particularly useful for paths which receive heavy use, either by people, horses or occasional vehicles. Geotextiles are essential for the successful construction of cycle paths, mostly along old railway tracks and canal paths (Sustrans, 1994). The products described below are those that have been found satisfactory, but other products may perform equally well. Manufacturers will provide technical advice for particular sites and applications as required.

Geotextiles used for footpath construction divide into two main types. The thinner and cheaper textiles such as Terram, Lotrak or Typar mainly act as separators. They are best used on flat or gently sloping paths on damp mineral soils and shallow peat. These fabrics are of varying permeability, according to their grade. Higher permeability is needed on wetter ground to allow soil water to move upwards, preventing the path from moving under pressure from water below.

The stronger, costlier materials such as Tensar Geogrids and Wyretex provide structural reinforcement for paths over very soft ground, and on slopes. They have a large mesh size, and are thus permeable. They do not act as separators for fine particles, which can be washed down through or pumped up from beneath. For this reason, they can be used together with a separator, for example Tensar laid over Lotrak. However, this may be an over- specification, as geogrids have been successfully used on their own on deep peat, with no problem of sediment being pumped upwards, nor loss of fine materials downwards. Most mineral soils, even when wet, should be strong enough to support the path with only the use of a separator, on flat or gently sloping ground.

To further complicate the picture, the thinner geotextiles such as Terram, Lotrak and Typar do add some structural strength to the path, by providing tensile reinforcement to support the aggregate, keeping it together. However, for footpath use, the width of the fabric and the depth of aggregate used are at a minimum, compared with their more normal use for roads and large constructions, and the whole structure may fail to bind. A common fault has been for the geotextile to stretch and sag over peat and other soft ground, and on slopes, causing the aggregate layer to slump. The thinner geotextiles are not suitable on their own for sloping ground, as the aggregate tends to slip off them. All geotextiles are unsightly if they become exposed due to erosion or path failure, but the woven materials such as Lotrak have the further disadvantage that if exposed the material shreds into long, hazardous strands.

The geogrid type textiles, with their large mesh size, allow the aggregate to lock down into the mesh, making a strong structure. Paths built in the Yorkshire Dales using only geogrids with aggregate have been successful on deep peat. Paths on sloping ground have also been constructed successfully by this technique.

Choice of geotextile also depends on the load bearing capability of the ground, and the maximum loading the path will have to take. In nearly all cases the maximum loading will be exerted by vehicles used during construction, and not by its end use. Even a loaded wheelbarrow exerts six times the pressure of the average walker. For this reason, in some areas helicopter airlifts only are being used for constructing geotextile and aggregate paths, to lower the materials directly into position with a minimum of handling on the ground. This allows the building of lightweight paths fit for their end use, and not determined by the needs of the construction vehicle.

For efficient use of the geotextile, the width of the fabric needs to be simply divisible into the required path width. Where helicopter airlifts are not being used, the width may also be dictated by the width of any machine being used to construct the path. However, an effort should be made to vary the width of geotextile paths, to make them look less artificial. The tendency has been to produce long straight lengths of even-width path, often surfaced with uniform material.

The general method for all geotextiles is to excavate the path to the required depth, removing any large stones which may distort the geotextile, and making as level a bed as possible. This excavation is called the path tray. The geotextile is then laid either only across the base of the tray, or with the edges part way up the sides and covered with turf, as shown below. For upland areas where vegetation growth is slow, it is not recommended to bring the edges up to the surface to fully contain the aggregate, as even if then covered with turf, the edge of the fabric is likely to become exposed. In most situations there is little problem with aggregate moving ‘sideways’ into the soil, and geotextile laid only on the base of the path tray is sufficient.

For paths over wet mineral soils, and where grass growth is lush, the geotextile can be laid directly on the ground, with the edges or ‘shoulders’ carefully constructed with turf. This method is used by Sustrans for constructing cycle tracks along canal paths and other places where the water table is high.

An overlap of 300mm is the normal recommendation for joining lengths. Geotextiles can be cut with sharp shears, snips, or heavy-duty scissors. Terram in a roll can be cut with a hacksaw or chainsaw. Geotextiles should be stored out of sunlight, and unprotected geotextile should never be driven over by a vehicle. Aggregate is tipped by dumper, barrow or from a helicopter hopper, and spread by grader or rake, and compacted with a vibrating roller or plate.

Terram

Terram Ltd. produce many types of geotextile, including woven and non-woven fabrics. Non-woven polyethylene/ polypropylene fabric is available in many grades, and is widely used for road building and other construction work. To select the optimum grade of fabric, a choice needs to be made between the strength of the fabric and its filtration characteristics. Lower strength fabrics are more permeable, but may be too elastic. The higher strength fabrics are less elastic, but may not be permeable enough in wet conditions, and swelling can occur underneath the fabric due to excessive ground water pressure. Cost increases for the stronger fabrics. The grades used for footpath work are given below. Terram 1000 is recommended by the manufacturers for most situations, and is easily obtainable from suppliers, lists of whom are available from Terram Ltd.

Lotrak

Lotrak is a woven polypropylene geotextile, available in many different grades. The grades suitable for path construction over soft ground and shallow peat are given below. Further information is available from the manufacturers, Don & Low Ltd., from whom details of local suppliers can be obtained.

Lotrak 10/7 is suitable for most sites, where it will act as a separator and filter. Lotrak 16/15 should be used for paths which have occasional vehicle use, or for paths over shallow peat. Trials on shallow peat in the Yorkshire Dales showed that woven textiles were more stable and less elastic than non-woven geotextiles, and Lotrak 16/15 was successfully used for paths on peat, providing peat depth did not exceed 450mm.

A path width of 1.75m was excavated to a depth of 150mm, the Lotrak 16/15 unrolled and covered with a 200mm layer of Type 1 aggregate. The 300mm layer of unexcavated peat was compressed below the geotextile and provided effective support.

Typar

Typar, manufactured by Du Pont, is a thermally bonded polypropylene geotextile, with high tensile strength and high resistance to puncture.

Tensar geogrids

Tensar geogrids are made of sheet polypropylene, punched and stretched under heat, to form a grid of high tensile strength. They have many uses in civil engineering. Tensar SS35 (now superseded by SS40) has been used for path construction over deep peat in the Yorkshire Dales. Pathcraft Ltd. prefer to use SS2, a lighter geogrid, over Terram 1000, for aggregate paths on slopes. The geogrid prevents the aggregate slipping, and the Terram prevents loss of fines from the surfacing and ‘pumping up’ of peat from below.

The method used in the Yorkshire Dales is to excavate a path tray to a depth of 150mm, and lay down the geogrid. Type 1 aggregate is then spread to a depth of 200mm, and compacted by twin-drum vibrating roller. Experiments were tried with simply laying the grid on the surface and spreading the aggregate on top, with careful construction of the aggregate edge to a maximum 1 in 1.5 angle. However, aggregate spread sideways with use, widening the path and making the aggregate layer too thin to protect the geotextile. The excavated method is recommended. Paths have been successfully constructed over peat deeper than 1.5m.

The ability of the aggregate to lock into the sub-grade through the geogrid means that paths can be constructed up slopes as steep as 20 degrees. These must be combined with drainage works to ensure water does not flow down the path, and revegetation measures to promote the long term stability of the path.

Wyretex

Wyretex is a woven material made from polypropylene and galvanised steel wire, available in various mesh sizes. It is used for sub-base material for aggregate paths, as reinforcement for grass paths, as well as bank stabilisation, windbreaks and many other uses. The grades given below are recommended for sub-base use, and for grass paths.

The following method of path construction can be used. The thickness of the base layer, which is thinner than in the examples given above, is according to manufacturer ’s recommendations.

Many other man-made and natural materials have been tried for sub-base use, especially on deep peat. These include chestnut paling fences, brushwood, sheep’s wool and polystyrene blocks. None of these have been found to be successful (see Countryside Commission, 1991).