2.8.1 Geotextile to soil contact
When geotextiles are used in earth structures to restrain soil migration and to retain particles in water three distinct filtration mechanisms are prevalent: first, the geotextile itself, which is in contact with the soil; secondly, particles in suspension and, thirdly, the migration of soil particles under cyclic loading.
The drainage of a saturated soil is usually achieved by constructing a trench and subsequently lining the trench with a geotextile. A stone drainage medium, in conjunction with a perforated pipe, is installed and the entire system is enveloped with the geotextile. The entire system is buried where it performs its drainage function.
16 2.7.6 Drainage in tailings dams
Figure 2.13 shows that thick nonwoven geotextiles are often used a filter between the in-situ and imported tailings materials. Because of the fine nature of tailings, it is critical to dimension the filter correctly to prevent potential clogging of the geotextile filter and subsequent failure of the drain.
Figure 2.13: Geotextile used as a filter medium below the toe drain of tailings dam (Kaytech)
2.8 GEOTEXTILE FILTRATION MECHANISMS
2.8.1 Geotextile to soil contact
When geotextiles are used in earth structures to restrain soil migration and to retain particles in water three distinct filtration mechanisms are prevalent: first, the geotextile itself, which is in contact with the soil; secondly, particles in suspension and, thirdly, the migration of soil particles under cyclic loading.
The drainage of a saturated soil is usually achieved by constructing a trench and subsequently lining the trench with a geotextile. A stone drainage medium, in conjunction with a perforated pipe, is installed and the entire system is enveloped with the geotextile. The entire system is buried where it performs its drainage function.
16 2.7.6 Drainage in tailings dams
Figure 2.13 shows that thick nonwoven geotextiles are often used a filter between the in-situ and imported tailings materials. Because of the fine nature of tailings, it is critical to dimension the filter correctly to prevent potential clogging of the geotextile filter and subsequent failure of the drain.
Figure 2.13: Geotextile used as a filter medium below the toe drain of tailings dam (Kaytech)
2.8 GEOTEXTILE FILTRATION MECHANISMS
2.8.1 Geotextile to soil contact
When geotextiles are used in earth structures to restrain soil migration and to retain particles in water three distinct filtration mechanisms are prevalent: first, the geotextile itself, which is in contact with the soil; secondly, particles in suspension and, thirdly, the migration of soil particles under cyclic loading.
The drainage of a saturated soil is usually achieved by constructing a trench and subsequently lining the trench with a geotextile. A stone drainage medium, in conjunction with a perforated pipe, is installed and the entire system is enveloped with the geotextile. The entire system is buried where it performs its drainage function.
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The water flow rate within the soil can be estimated using Darcy’s law:
Q = k (ΔH / ΔL) A
where:
Q = water flow rate in m3/s
k = soil permeability in m/s
A = soil cross sectional flow area in m2
H = hydrostatic head in m
L = water flow path length in m
The permeability and pore size distribution of geotextiles are selected to restrain particle migration while allowing the water to percolate through. These properties are a function of the structure of the geotextile and must remain constant throughout the service life of the system.
The long term performance of geotextile filters can be affected by soil conditions, the design of the system, installation procedures and hydraulic conditions. It is therefore important to understand that the successful performance of a geotextile filter includes it encouraging the formation of a natural upstream filter. This usually occurs by either one or a combination of the following two mechanisms:
2.8.1.1 Self-filtration
Under hydraulic gradient, soil particles will migrate towards the drain. The particles with smaller diameter than the geotextile pore opening size and located adjacent to the geotextile lining the drainage trench will be carried into the geotextile by the flowing water. These particles will either be flushed into the drainage pipe system or will remain trapped permanently between the fibres. As the finer particles are removed from the soil, the coarser soil fraction will migrate towards the geotextile where it will be stopped. This is assuming that the coarser fraction is larger than the pore opening size of the geotextile. These larger particles will, in turn, stop finer particles from migration, which will in turn stop even smaller particles. As a result, coarse particles will form a layer at the geotextile interface and the soil migration will be stopped. This phenomenon is favoured in well graded soils (Rollin and Lombard, 1988).
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2.8.1.2 Vault network formation
In soils that are not well graded, the geotextile can be properly selected to favour vault formation (Figure 2.14). Particles adjacent to the geotextile can rearrange themselves as they migrate toward the filter interface to form vaults. This is believed to be as a result of the electrical and adsorption forces between the organic lubricant or anti-static agent on the geotextile fibres and soil particles and also between the soil particles themselves. Upon formation of this vault network, the geotextile will stop particles with a smaller particle size than that of the geotextile, from migrating through it (Rollin & Lombard, 1988). This is favourable, as no further piping of finer soil through the geotextile can occur.
Figure 2.14: Upstream soil particles forming vaults or arches over geotextile openings (after McGown 1985)
2.8.2 Particles in suspension
Besides subsurface drainage applications, geotextiles are also frequently used as filters in silt fences, sea barriers, tailings storage facilities and other places. Geotextiles are in these instances used to protect rivers and lakes from being contaminated by silt.
If care is not taken during installation, fine soil particles may be carried in by the run of water and settle on top of, or adjacent to, the geotextile filter. A soil particle carried in suspension will remain so if its velocity is higher than that of the gravitational forces acting on it. Clay and silt particles will remain in suspension if their velocity is greater than 0.01 m/s. These particles reach the geotextile interface at relatively high velocity. In this instance the filtration mechanism reacts totally differently than in the case previously mentioned, as the formation of an impervious layer on the upstream side of the geotextile is highly probable (Rollin & Lombard, 1988). This phenomenon is known as blinding or blocking, in the cases of nonwoven and woven geotextiles respectively.
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Under relatively high flow rates, particles will block the pores of the geotextile and slowly start building up an impervious layer on the adjacent upstream side of the geotextile. Over time the geotextile filter will inevitably stop functioning and will need replacement. In these cases the filter behaves similarly to the way it would in industrial filtration. In this instance, geotextile filter selection should be based on the life expectancy of the structure or system (Rollin & Lombard, 1988).
2.8.3 Soil particles migration under cyclic loads
In applications such as railways and access roads, cyclic loads are exerted onto the geotextile due to the passage of heavy trains and vehicles. Under these conditions, smaller particles tend to migrate towards the geotextile due to the pumping effect on them. This effect is a function of the soil type, particle distribution, soil water content, and the amplitude and frequency of the applied load. For silty and silty clay soils, the geotextile must be able to stop the flow of slurry, consisting of the in-situ water and the soil that will flow as a result of cyclic loading (Rollin & Lombard, 1988).