|Figure 29. Geotextile material used with riprap allows water movement while preventing movement of soil particles. (Redrawn with permission from Figure 1, Amoco Fabrics and Fibers Company 1994a, p. 1)|
|A geotextile is a synthetic permeable textile material
used with soil, rock, or any other geotechnical engineering related material. Geotextiles,
also called geosynthetics, are generally associated with high-standard all-season roads,
but can be used in low-standard logging roads.
Geotextiles extend the service life of roads, increase their load-carrying capacity, and reduce the incidence of ruts. These benefits are accomplished by separating aggregate structural layers from subgrade soil while allowing the passage of water (see description on Separation).
Geotextiles should be considered for use on any section of road requiring an aggregate (rock) layer for surfacing. Geotextiles can reduce the amount of aggregate required, thus reducing the cost of the road, as well as providing the benefits described in the previous paragraph.
For temporary road construction in environmentally sensitive areas, a biodegradable woven jute geotextile has been developed. This fabric will totally biodegrade after one to two seasons, eliminating the need to remove a synthetic geotextile from under the roadbed. It is economical for use on roads that will be decommissioned after use (Moran 1997).
There are many uses for geotextiles. The geotextile manufacturer can provide help in selecting the correct material for your specific situation. Rather than describe the many potential conditions under which they may be used, the principles governing their use are described in this section. These principles then can be considered in solving your particular drainage problem.
For example, in an erosion control application, rock or other riprap material may be placed over a geotextile along a streambank as shown in Figure 29. The role of the total system is to prevent erosion of soil materials along the channel. The geotextile performs the specific function of filtration, allowing water in the soil to pass through the fabric while retaining the soil particles.
There are a number of classifications for geotextile functions ranging in number from as few as four to several dozen. This document covers four that are closely associated with low standard roads: separation, filtration, reinforcement, and transmission. Not all functions are provided by each type of geotextile, so check before you buy.
The separation function refers to the separation of two dissimilar soils. The primary function of the geotextile is to prevent intermixing of the two soils throughout the life of the structure.
Geotextiles are commonly used for separation when used beneath road-way pavement sections. Although you will not be constructing pavement sections, you may want to use some aggregate over certain sections of the road, in which case the principles described here still apply. Roadway pavements are basically structures for taking the high contact pressures from the vehicle tires and reducing that pressure through the depth of the pavement to a level that can be supported by the underlying soil. Pressure is dissipated down through the various layers of materials within the pavement.
Figure 30. Geotextile material provides separation, which preserves the integrity and extends the life of the road surface layer. (Redrawn with permission from Amoco Fabrics and Fibers Company 1994b)
|Over time, vehicle load pressure causes subgrade soils to
migrate into the aggregate base of the pavement section. Contamination of the aggregate
base by the subgrade results in the reduction of the effective base thickness to less than
originally designed. This concept is illustrated in Figure 30. Reduction of the base
thick-ness results in a decrease in the load-carrying capacity of the aggregate base and a
reduction in the pavement life. Geotextiles prevent the subgrade materials from migrating
into the aggregate base, thus increasing pavement life.
Filtration is one of the functions most widely performed by geotextiles. The filtration function has two concurrent objectives: to retain the particles of the filtered soil, while permitting water to pass through the plane of the geotextile from the filtered soil. These two parallel roles are the key to filtration design (Figure 29).
In both the filtration and separation functions, water is permitted to pass through the geotextile. Occasionally, some confusion arises between the separation and filtration functions in this regard. A distinction may be drawn between the two with respect to the quantity of water involved and the degree to which it influences geotextile selection.
In the filtration function, the volume of water moving through the fabric is a key design element specifically addressed in the design and selection of the geotextile. It must be able to convey a certain quantity of water across the plane of the fabric through-out its design life to prevent the buildup of water pressure.
This is typically not the case with a geotextile used for separation. While water may pass in either direction across the plane of the geotextile, it is not typically an element of design as the quantities of water are relatively small, even in those cases of high groundwater and saturated subgrades.
In the reinforcement function, the geotextile is subjected to a sustained tensile force or load. Soil and rock materials are noted for their ability to withstand compressive forces and their relative low capacity for sustained tensile forces. In much the same way that tensile forces are taken up by steel in a reinforced concrete beam, the geotextile supports tensile forces that cannot be carried by the soil in a soil-geotextile system.
As shown in Figure 31 in a geotextile reinforced levee constructed over soft soils, the geotextile layers are placed across potential rotational failure planes to carry the tensile forces that cannot be carried by an unreinforced soil mass.
Figure 31. Geotextile material under the roadbed reinforces soft soil and preserves the road embankment. (Redrawn with permission from Figure 8, Amoco Fabrics and Fibers Company 1994a, p. 5)
In the transmission function, liquids or gases are carried (or transmitted) within the plane of the geotextile itself. This is distinctly different from the filtration function which involves flow across the plane of the geotextile. This function is often associated with geotextile composites, particularly those that incorporate a drainage net or a permeable core bonded on one or both sides by a geotextile as shown in Figure 32.
Fluid enters the composite through the geotextile and is carried in the channels of the core to a desired location in the application. As shown in Figure 32, a geotextile-drainage core composite can provide drainage adjacent to the face of a retaining wall.
Figure 32. Composite geotextile material allows water flow within the plane of the material, rather than across it, such as behind a retaining wall. (Redrawn with permission from Figure 10, Amoco Fabrics and Fibers Company 1994a, p. 6)
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