By Paul Wheeler
Geosynthetics can provide a cost-effective solution to many of the workaday problems landscape architects encounter; and with a little investigation can significantly unlock the designer’s imagination.
Geosynthetics is the generic term for a range of products used primarily in civil engineering that have become widespread and increasingly sophisticated over the last 40 years. Their main functions are separation, filtration, drainage, containment and reinforcement. Common engineering applications include highways, railways, coastal protection, landfill, retaining structures and foundations and underground utility construction. They are also commonly used in landscape applications.
Landscape professionals will be familiar with geotextiles, but these are, in fact, just one class of geosynthetic. Wikipedia lists eight, namely: geotextiles, geomembranes, geocells, geonets, geogrids, geosynthetic clay liners, geofoams and geocomposites.
You won’t come across all of these products in landscape applications; but a better understanding of geotextiles and other geosynthetic materials could greatly enhance the toolkit of possibilities for the landscape professional.
Common landscape applications include weed suppression, root protection and control, separation layers below paths, drainage and perhaps living roofs. Mostly these involve standard geotextiles, geomembranes and geocells.
But there are mixed views on their benefits, and a number of landscape professionals who are cautious about their use, primarily because they are aware of projects where they have not worked.
Tim O’Hare, founder of soil and landscape science consultancy Tim O’Hare Associates, believes that geosynthetics are too often specified incorrectly. For example, he cites the use of a geotextile as a separation layer between a topsoil growing medium and a granular material below, when the desired result is to stop fines in the topsoil being washed into the coarser-grained material below. But what tends to happen, he says, is that within a very short period of time, in some cases within just a few months, the fines block the geotextile, which in effect becomes an impermeable barrier. This leads to poor-draining saturated soil and poor plant growth.
While O’Hare argues that it is better to use a traditional coarse grit blinding layer, the correct specification geotextile fabric could do the job equally well.
In a similar vein, US horticulturist Dr Linda Chalker-Scott at Washington State University recently published a technical note titled ‘The Myth of Landscape Fabric’ which challenges the perception that lightweight landscape fabric-grade geotextiles offer permanent weed control. ‘Like the perpetual dieter searching for a permanent weight loss pill, so we as landscape professionals seek permanent weed control solutions,’ she says. ‘Unfortunately there is no such fix, weed control fabrics will decompose and can hinder plant health.’
Nevertheless weed-control fabrics remain popular with contractors because they cut down on maintenance in the short term. Long term it is a different story, because the fabrics make subsequent maintenance, such as aeration, more difficult.
When selecting a geosynthetic, think about the basic purpose and ensure that the product and its performance characteristics are appropriate for the soils/materials you are working with and the conditions in which it is being used. If you are unsure, speak directly with the manufacturer.
Correctly specified, geosynthetics can provide a cost-effective solution to many workaday problems that landscape professionals encounter; and with a little investigation can significantly unlock the designer’s imagination.
And of course geosynthetics can be used in ways the manufacturers never intended. In Philadelphia the Not Garden/Not Again project uses ‘customised geotextiles’ for experimental greening techniques on abandoned housing plots. Philadelphia has over 60,000 abandoned properties and over the last decade, the Neighborhood Transformation Initiative has provided a land management programme to provide relatively low-cost ways to limit blight. Developed by local practice PEG office of landscape + architecture, the more than 3,000 ‘Not Garden’ prototypes used laser-cutter fabrication to precut customized geometric patterns out of geotextile, which was laid on site and seeded. This produced diverse configurations with very low-investment, effort, installation expertise, or need of long term care.
The Dutch manufacturer TenCate offers a product called GeoDetect, which it describes as “the first sensor-enabled geotextile on the market to provide soil reinforcement, structural health monitoring and an early warning system into one package.” In essence it structurally strengthens and physically monitors the landscapes it is buried within. It comprises a geotextile embedded with fibre optics, which could potentially relay information from a wide source of sensors, although to date monitoring has been restricted to strain (ie movement) and temperature. Although applications at present are very much within engineering, it potentially opens up huge possibilities for the creatively minded.
Paul Wheeler is director of Base Cities. He studied geotechnical engineering to masters level and is a former editor of Ground Engineering.
Lightweight geotextiles are frequently
used for suppressing weeds. The principle is that they allow the passage of water, oxygen and nutrients while blocking weeds. The geotextile is installed on the
top surface of the soil and covered with a layer of bark chippings, stone chippings, pebbles or gravel.
Separation and drainage
Geotextiles are frequently used to separate material and prevent mixing in a wide range of applications such as footpaths, hardstandings, SuDS, land drains and living roofs. Correctly specified they will allow the flow of water and oxygen through the soil, without any clogging.
Various geosynthetic products can be used to spread loads to protect tree roots from damage by, for instance, vehicles. Geocell is often used, as it creates a honeycomb-like cellular confinement system in which the interconnected cells are filled with a granular fill and loads are spread laterally rather than vertically. This limits mechanical damage and also minimises compaction of the surrounding soil, preventing the roots from being starved
of moisture and oxygen.
Protecting structures from roots
There are instances when the protection is needed the other way round, ie when tree roots need to be isolated or directed away from paths, walls, shallow foundations, pipes, grass and so on. An HDPE wrapping or barrier will effectively inhibit root development and penetration, but will create a water barrier too. Heavy-grade geotextiles can be used where land drains need to be protected from roots. This won’t give total protection and care is needed to select the correct geotextile for the surrounding soil, otherwise fines from the soil could block the fabric, rendering it impermeable and the drain redundant.
Protecting grassed surfaces
Thin plastic meshes can provide reinforcement on grass areas prone to wear. The porous mesh is supplied on a roll and laid directly on the grass, which grows through creating a natural appearance. A more sophisticated solution is offered by Terram’s Advanced Turf, which incorporates high-strength plastic mesh elements within the root zone to produce a very strong, yet free-draining reinforced grassed surface. It has been used in the grassed ‘event areas’ outside London’s City Hall (see opposite) and can also be used on sculpted slopes and other areas where grass needs to be invisibly reinforced.
Geotextiles can help to temporarily stabilise swales and river banks to allow vegetation to become established. A geocell blanket can improve resistance to erosive forces such as rainwater run-off on steep or unstable slopes.
Engineered reinforced-soil slopes, usually formed with horizontal layers of geogrid, can provide an alternative to hard-engineered retaining structures. These will include some form of surface protection to resist water and wind erosion, at least until vegetation is established.
Porous paths and hardstanding
Interlocking grid pavers made from recycled plastics can be infilled with a granular material to form a free-draining pavement. They are available at different grades making them suitable for footpaths and cycle tracks right up to coach and HGV car parking areas.
The art of the possible
Charles Jencks’ human landform sculpture, Northumberlandia, incorporates geogrid reinforced soil slopes, gabion retaining walls, erosion control blankets and other geosynthetics in what is claimed to be the world’s largest human form sculpted into the landscape. The 30m-high and 400m-long reclining female form is made from coal mining spoil and comprises the centrepiece of a new park near Cramlington. The chin, nose, and eyebrows, in particular, make extensive use of geosynthetics and associated products.