Geotextiles and Geogrids for Civil Engineering Projects
Geotextiles, Geogrids, Geomembranes and Geocomposites perform separation, filtration, protection and reinforcement functions in civil engineering projects. These products are factory fabricated in a controlled environment which makes them more cost-effective, durable and safer compared to traditional soil and rock construction methods.
The terminology and testing methodologies of geotextile manufacturers are vastly different from those of civil engineers, which has led to a need for mutual education in the field of specification writing. This guide from Tensar explains the fundamentals of these revolutionary products.
Reinforcement
The largest group of geosynthetic materials are woven, knitted or non-woven textile fabrics that have directionally or randomly oriented synthetic fibres/filaments mechanically or thermally/chemically bonded together. These fabrics are used to add tensile strength to soil masses which improves their overall stability. They are often combined with other geosynthetic materials for specific functions.
These are used to control erosion in river canals and coastal works, reinforce earthen dams and steep soil slopes, provide a filter for drainage in earthen dam construction, behind Geosynthetic material geotextile retaining walls and deep drainage trenches, to protect a roadway embankment from lateral pressure from traffic and as a base for Astro turf and Caselon playing fields. Separators are also routinely used underneath road and railway construction, either alone or as a layer within a geocomposite. For example, a layer of high-strength, permeable geotextile from Tensar’s Basetex range was used beneath a 70-degree railway embankment in the Site of Special Scientific Interest (SSSI) in South Wales.
The SSSI is a highly sensitive environment where the railway embankment had to be built to strict specifications in order to protect wildlife. Using Tensar’s geotextile in the SSSI enabled the project to achieve its 120-year design lifespan and was successful due to the separation, filtration, drainage and reinforcement functions of the material. This is a clear example of the power and effectiveness of geosynthetics in civil engineering projects.
Filtration
The filtration function of geotextiles is to allow water and soil fines to pass through them but to block the movement of coarser particles such as gravel or sand. This function is typically applied to road construction where it helps prevent sand migration into the roadway base, thus improving the strength of the base course.
Protection geotextiles (sometimes called filter geotextiles) are the most common type of geotextile used in civil engineering applications and have the highest tensile strength and puncture resistance. They are made from either natural or synthetic fibers. The most common natural fiber is coir, which is obtained from the exterior husk of coconuts. Coir geotextiles are generally more expensive than the synthetic alternatives but the added cost is often justified due to their durability, low permeability and natural color.
Geocomposites combine two or more of the different geotextile types discussed above. They are most commonly used for drainage and containment applications, and also in some road foundation situations. For example, geogrids are frequently combined with separation/filtration geotextiles to improve their performance in mechanically stabilized walls and slopes, and road embankments.
Separation
The separation function of a geotextile is often overlooked by civil engineers. However, it is a vital function and one of the key reasons why geosynthetics are transforming the way that modern infrastructure works are constructed.
Separation is achieved by placing a flexible geotextile, such as a porous fabric, between two dissimilar materials in order to improve their integrity and functionality. This is a common application for geotextiles like woven non-woven fabrics and thin geocells. It is also commonly used in mechanically stabilized slopes, for reinforcement of granular fills and base course for roads and railways, and behind retaining walls.
Woven geotextiles are particularly good at separating coarse granular soil from fine granular material. This is because the warp and weft threads have different tensile strengths. This means that the yarns are able to hold the coarse granular material without causing the weft threads to buckle. This allows the granular fill to remain intact and prevents slurry pumping of fine soil particles downward into the underlying granular fill layer.
Drainage is another very important function of a geotextile. This is because a geotextile that is capable of passing water (and other liquids or gases) through it into an open space will reduce the risk of damage to the underlying soils and groundwater. The most commonly used drainage geosynthetics are geocomposites, which combine a geonet drainage core with a layer of geotextiles.
Drainage
Drainage is the process of removing moisture and fine particles from an area. Geotextiles used for drainage can prevent fine soil particles from passing into and clogging the drainage system. TenCate Basetex geotextiles are frequently used for this purpose in highway underdrain systems, retaining wall drainage and landfill leachate collection and control systems.
Geotextiles can also function as a filter by separating soil with different particle size distributions and allowing fluid (liquid or gas) to Geosynthetic clay liner flow through while preventing the passage of the soil. They can be designed for this function by altering the structure, thickness, and type of fibers used in their construction.
In contrast to geotextiles, geomembranes are primarily characterized as barriers and liners for liquid or gas migration in human-made projects, structures, and systems. They are generally low-permeability synthetic membranes that are used with other geotechnical engineering materials to control the movement of water or gas.
Each geosynthetic material has been designed and constructed to perform a particular function in association with soil. The primary functions that geotextiles perform are separation, filtration, reinforcement, and drainage. Many geosynthetics are capable of performing multiple functions simultaneously. To help engineers understand the role of each, the table below describes the primary functions, structures, and properties of common geosynthetics.
