Geosynthetics — GeoTechnical Solutions

Geosynthetics are polymer-based products used in contact with soil, rock or waste to perform defined engineering functions in a civil structure. BS EN ISO 10318 sets out the terminology and the functions: separation, filtration, drainage, reinforcement, containment, protection and surface erosion control. Almost every geosynthetic on a UK site is one of seven product families, and choosing correctly starts with naming the function required rather than the product.

The six functions

Separation keeps two dissimilar materials apart, most commonly a granular capping layer from a soft subgrade, so the aggregate is not lost into the soil. Filtration allows water to pass while retaining soil particles, which is the role of the fabric behind a revetment or around a filter drain. Drainage conveys water within the plane of the product itself, as a drainage geocomposite does behind a retaining wall. Reinforcement adds tensile strength to a soil mass that has none, which is what a geogrid does in a reinforced soil wall or a basal reinforcement layer. Containment provides a barrier to liquid or gas, the role of a geomembrane or a geosynthetic clay liner. Erosion control holds surface soil in place while vegetation establishes. Many products perform two or more functions at once, and the design must state which of them is the primary function being relied on.

The product families

A geotextile is a permeable fabric, woven or non-woven, used for separation, filtration, protection and light reinforcement. A geogrid is an open grid of tensile ribs whose apertures interlock with granular fill, used for reinforcement and base stabilisation. A geomembrane is an effectively impermeable sheet used for containment. A geonet is a stiff open mesh whose function is in-plane flow, almost always supplied as part of a geocomposite. A geocomposite bonds two or more of these into one product, most often a geonet or drainage core between two geotextile filters. A geocell is a three-dimensional honeycomb that confines fill to create a stiff mattress. A geosynthetic clay liner is a bentonite layer contained between geotextiles that hydrates to form a very low permeability barrier.

Specification, standards and durability

Harmonised European product standards define the characteristics that must be declared for each application: BS EN 13251 for earthworks, foundations and retaining structures, BS EN 13252 for drainage systems, BS EN 13253 for erosion control, and further parts for railways, roads, reservoirs, canals and waste containment. Specification should be by declared performance — tensile strength and strain, characteristic opening size, water permeability normal to and in the plane of the product, static puncture and dynamic perforation resistance — rather than by mass per unit area, which correlates poorly with performance. Durability is declared against a stated design life and exposure, and UV exposure before covering is a real limitation: most products have a maximum permitted period of exposure measured in weeks.

Getting the selection right

Two failure patterns dominate. The first is specifying by product name when the design needs a function — a non-woven separator asked to perform reinforcement, or a woven reinforcement fabric asked to filter. The second is ignoring the filtration compatibility between the fabric opening size and the soil it is in contact with, which either blinds the fabric or lets fines pass. Both are avoided by writing the specification around the declared performance characteristics required for the function.

Related ViaCon solutions

ViaCon supplies the full geosynthetics range for UK infrastructure projects. See our geotechnical solutions. Glossary entries for each product family: geotextile, geogrid, geomembrane, geocomposite, geonet, geocell and geosynthetic clay liner.

Frequently asked questions about geosynthetics