Geotextiles are porous fabric manufactured from synthetic material such as polypropylene, polyester, polyethylene, nylon, polyvinyl chloride and various mixtures of these.
They are available in thicknesses ranging from 10 to 300 mils (1 mil = 1/1000 inch) in widths upto 30 ft, in roll lengths upto 2000 ft. The permeability of geotextile sheets is comparable in range from coarse gravel to fine sand. They are either woven from continuous monofilament fibres or non-woven made by the use of thermal or chemical bonding of continuous fibres and pressed through rollers into a relatively thin fabric. These fabrics are sufficiently strong and durable even in hostile soil environment. They possess a pH resistance of 3 to 11.
The use of geotextiles in geotechnical engineering has been growing in popularity for the last many years. Geotextiles can be used in so many ways. They are used as soil separators, used in filtration and drainage, used as a reinforcement material to increase the stability of earth mass, used for the control of erosion, etc. Some of the uses of geotextiles are described in the following sections.
Geotextiles as Separators
A properly graded filter prevents the erosion of soil in contact with it due to seepage forces. To prevent the movement of erodible soils into or through filters, the pore spaces between the filter particles should be small enough to hold some of the protected materials in place. If the filter material is not properly designed, smaller particles from the protected area move into the pores of the filter material and may prevent proper functioning of drainage.
As an alternative, geotextile can be used as a filter material in place of filter soil as shown for an earth dam in Fig-1. The other uses of geotextiles as separator are:
1) Separation of natural soil subgrade from the stone aggregates used as pavement of roads, etc.
2) As a water proofing agent to prevent cracks in existing asphalt pavements.
Geotextiles as Reinforcement
Geotextiles with good tensile strength can contribute to the load carrying capacity of soil which is poor in tension and good in compression.
Geotextiles placed between a natural subgrade below and stone aggregates above in unpaved roads, serve not only as separators but also increase the bearing capacity of the subgrade to take heavier traffic loads. Here, geotextiles functions as reinforcers as shown in Fig-2.
Another major way in which geotextiles can be used as reinforcement is in the construction of fabric-reinforced retaining walls and embankments. This technology is borrowed from the technology for reinforced earth walls. Geotextiles have been used to form such walls which can provide both the facing element and stability simultaneously.
The process of construction of the wall with granular backfill is shown in Fig-3. The procedure is as follows.
1) Level the working surface.
2) Lay geotextile sheet 1 of proper width on the surface with 1.5 to 2 m at the wall face draped over temporary wooden form as shown in Fig-3(a).
3) Backfill over this sheet with granular soil and compact it by using a roller of suitable weight.
4) After compaction, fold the geotextile sheet as shown in Fig-3(b).
5) Lay down second sheet and continue the process as before. The completed wall is shown in Fig-3(d)
The front face of the wall can be protected by the use of shotcrete or gunite. Shotcrete is a low water content sand and cement mixture, often with additives, which is sprayed on to the surface at high pressures in a manner similar to gunite. The design of geotextile reinforced walls is similar in principle to that of reinforced earth walls.
Geotextiles in Filtration and Drainage
Geotextile sheets have been successfully used to control erosion of land surfaces. Erosions of exposed surfaces may occur due to the falling rain water or due to flowing water in rivers, etc. Fig-4(b) shows a schematic sketch for the protection of the banks of flowing water.