Have you ever heard of road sections, usually near or in swamps failing time and time again by sinking into the marsh? The Kabale-Katuna Road near the Uganda-Rwanda border at Katuna may be one such example. The sinking mass gives the layman the feeling that something should have been mixed with the soil and stone to provide the strength and stop the sinking. Yes, and geotextiles do this, among other things.
What they are
Geotextiles, according to Wikipedia, are permeable fabrics which, when used together with soil, have the ability to separate, filter, reinforce, protect or drain. Typically made from polypropylene or polyester, geotextiles come in three basic forms: woven, needle-punched and heat-bonded. There are composites of geotextile products like geogrids and meshes. However, the interesting important facts about geotextiles is that they are not easily biodegradable, that is, they are durable and can withstand many things, and even soften the fall on an object on impact!
The basic principles of incorporating geotextiles in civil works like on marshland are the same as those used in the design of reinforced concrete by incorporating steel bars which are strong in tension to strengthen the concrete which is weak in tension even though it is strong in compression. Geotextiles are used in providing tensile strength for the earth mass in locations like marshland where shear stress would be generated. Moreover, to allow rapid dewatering of the roadbed, the geotextiles need to preserve the roadbed’s permeability unaltered by the mechanical loading.
Geotextiles have become quite popular, especially over the past 15 years. They owe their success in more than 80 applications mainly due to their resistance to biodegradation.
Geotextiles are indeed textiles, though not in the traditional sense like other natural materials such as cotton, wool or silk. They are synthetic fibres that can be made into a flexible, porous, non-woven felt fabric. They are porous to the flow of water, in varying degrees.
Non-woven geotextiles resemble felt and provide planar water flow. They are commonly known as filter fabrics, although woven monofilament geotextiles can also be referred to as filter fabrics. Typical applications of non-woven geotextiles include aggregate drains, asphalt pavement overlays and erosion control.
A woven geotextile is a planar textile structure produced by interlacing two or more sets of strands at right angles. There are two types of strands: slit films, which are flat; and monofilaments, which are round. Woven slit-films are generally preferred for applications where high-strength properties are needed and filtration requirements are less critical. These fabrics reduce localized shear failure in weak subsoil conditions and aid construction over soft subsoils. Woven monofilaments are preferred for applications where both strength and filtration are a concern, such as in shoreline rip-rap applications.
Geotextile-related materials such as fabrics formed into mats, webs, nets, grids, or formed plastic sheets are not the same as geotextiles. These would fall under the more general category of geosynthetics.
There are five different applications of geotextiles as hereunder
- Separation: The geotextile is laid between two distinct layers of different materials. This could be two different types of soil, old and new pavements, or soil and new construction. Separation is nearly indistinguishable from stabilization, but there are some distinct variations to achieve stabilization. So why use a geotextile separator?
Wet soils are weaker than dry soils and fine soils are weaker than coarse soils. Therefore, a suitable geotextile can:
Prevent the reduction of load-bearing capacity which may have been caused by the mixing of fine-grained subgrade soil with the aggregate base.
Increase the load-bearing capacity by preventing the migration of aggregate or armour blocks into the soft subgrade. The use of a geotextile can increase the degree of compaction.
Reduce the deterioration of roads through frost heave effects.
The segregation prevents adjacent materials from mixing, thus maintaining the integrity and stability of the respective materials and structures.
- Drainage: Geotextiles efficiently collect superfluous water such as rainwater or surplus water from the soil or the structure, and discharge it. They are thus used in various drainage applications such as: (a) Typar and French drains, (b) vertical drains, (c) agricultural and pipe drains, (d) blanket drains in roads and sports fields, (e) road and civil engineering drainage, (f) side drains, and (g) wall drainage.
- Filtration: Geotextiles are an ideal interface for reverse filtration in the soil adjacent to the geotextile. In all soils, water allows fine particles to be moved. Some of these particles will be halted at the filter interface. Some will be halted within the filter itself while the rest will pass into the drain. The complex needle-punched structure of the geotextile enables the retention of fine particles without reducing the permeability of the drain.
- Reinforcement: Heavy geotextiles can be used to reinforce earth structures by means of fill materials. Thanks to their high-soil fabric friction coefficient and high-tensile strength, they are an ideal reinforcement solution. Along with their advantages, geotextiles may be used for various reinforcement applications in such places as: steep slopes, retaining walls, waterworks, earth dam slopes (erosion control), river and lake embankments, land reclamation areas (using hydraulic fill), and embankments on compressible soils.
- Protection: Geotextiles are an ideal protection from erosion of earth embankments by wave action, currents or repeated drawdown. A layer of geotextiles can be placed so as to prevent leaching of fine material. They can be used for rock beaching or as mattress structures. They can even easily be placed underwater. Advantages include: increased road durability (less rut), better compaction and aggregate saving. This ensures longer service life and less maintenance costs.
Use of geotextiles in civil engineering
Geotextiles are used in civil engineering earthworks to reinforce vertical and steep banks, to construct firm bases for temporary and permanent roads and highways, to line ground drains so that the soil filters itself and stops the drainpipes from filling up. They also prevent erosion of stones on river banks. Civil engineering works where geotextiles are employed can be classified into the following categories:
- Roadworks: The basic principles of incorporating geotextiles into a soil mass are the same as those used in the design of reinforced concrete by incorporating steel bars. The fabrics provide tensile strength in the earth mass in locations where shear stress would be generated. Moreover, to allow rapid dewatering of the roadbed, the geotextiles need to preserve the roadbed’s permeability without losing its separating functions. Its filtration characteristics must not be significantly altered by the mechanical loading.
2. Railway works: The woven fabrics or non-wovens are used to separate the soil from the sub-soil without impeding the circulation of the groundwater where the ground is unstable. Enveloping individual layers with fabric prevents the material wandering off sideways due to the shocks and vibrations from the running trains.
3. Works in river canals and coasts: Geotextiles protect riverbanks from erosion by currents or lapping. When used in conjunction with natural or artificial enrockments, they act as a filter. For erosion prevention, the geotextile used can be either woven or nonwoven. The woven fabrics are recommended in soils of larger particle size as they usually have larger pore sizes. Non-woven fabrics are used where soils such as clay silt are formed. Where there is hydrostatic uplift, the fabrics must have sufficient high permeability.
4. Drainage: In civil engineering, the need for drainage has long been recognized and has created the need for filters to prevent in-situ soil from being washed into the drainage system. Such wash-in soil causes clogging of the drains and potential surface instability of land adjacent to the drains. The use of geotextiles to filter the soil and a more or less single-size granular material to transport water is increasingly seen as a technically and commercially viable alternative to the conventional systems. Geotextiles perform the filter mechanism for drainages in earth dams, roads and highways, reservoirs, behind retaining walls, deep-drainage trenches, and agriculture.
- Agriculture: It is used in mud control. For the improvement of muddy paths and trails, those used by cattle or light traffic, non-woven fabrics are used. They are folded by overlapping to include the pipe or a mass of grit.