Geotextile filter

The synthetic materials that best meet in-plane flow rate regulations are called geonets. Geonets require less space than perforated pipe or granular material, promote rapid transmission of liquids, and, because of their relatively open apertures, are less likely to clog. They do, however, require geotextile filters above them and can experience problems with creep and intrusion. 

With a liquid flowing across the plane of the material, as in a geotextile filter, the permeability perpendicular to the plane can be divided by the thickness, T, to obtain a new value, permittivity (see Figure 26.23). In crossplane flow, T is in the denominator for planar flow, it is in the numerator. Crossplane flow is expressed as... 

Geotextile filter design parallels sand filter design with some modifications. The three elements of adequate flow, soil retention, and clogging prevention remain the same. 

The second part of the geotextile filter design is determining the opening size necessary for retaining the upstream soil or particulates in the leachate. It is well established that the 95% opening size is related to the particles to be retained in the following type of relationship. 

In recent years geotextile wicks have replaced sand drains. Wicks consist of a thin corrugated core surrounded by a thin geotextile filter jacket. Typically, wicks are about 1/4 inch by 4 inches in cross section. They are placed by insertion into a similarly shaped mandrel (tube) whose bottom is closed with a plate to which the wick is attached. The mandrel is pushed or driven into the soil to the desired depth. When it is withdrawn, the bottom plate stays in place, holding the wick in place. 

The soil retention function of the geotextile separator can be seen as similar to the geotextile filter, in that for a suitable separator the AOS of the geotextile must be related to the particle size distribution and, as stated under the filtration function, this aspect will be considered in more detail later when describing the relationship between structure, properties, and functionalities of geotextiles. 

High performance textiles for geotechnical engineering 309 Table 8.5 Summary of soil retention requirements for geotextile filters... 

Zone 2 soils consist of coarse silts and fine sands (dg5 > 0.075 mm and djo < 0.6 mm) and these exhibit an increased tendency for the movement of fine soil particles. They are considered to be problem soils with regard to geotextile filters. Table 8.5 shows that the AOS requirement for retention has a smaller range than for Zone 1 soils. 

When a geotextile filter is to be used for environmental applications, it becomes necessary to design the filter with resistance to biological clogging. Biological clogging, initiates with the development of a network of... 

Additional to the criteria for particle retention, the permeability requirements of the geotextile filter for various soil types must be considered. The permeability of a geotextile filter in severe soil and hydraulic conditions can be reduced by an order of magnitude over its fife. To ameliorate this, it is important that the geotextile maintain or exceed its index permeability while under load i.e. any re-orientation of the fibres should not decrease or increase the permeability. Criteria against clogging have also been recommended for the permeability as well as for the retention properties of a geotextile filter. 

Mackey, R. E and Koemer, G. R. (1999), Biological clogging of geotextile filters a five year study. Geosynthetics 99, Conference Proceedings, Vol. 2, pp. 783-798. 

Geotextile filters might present themselves as an alternative to the very complex, costly, and uncertain filter made of multiple layers of granular material. However, geotextile mats are not only difficult to install under waves and currents, but also may introduce a shear surface which might be detrimental to the stability of the armor layer. 

Granular and/or geotextile filters can protect structures subjected to soil erosion when used in conjunction with revetment armor such as riprap, blocks and block mats, gabions and mattresses, asphalt or concrete slabs, or any other conventional armor material used for erosion control. 

BAW, Code of practice Use of geotextile filters on waterway, Bundesanstalt fiir Wasserbau, Karlsruhe, Germany (1993). 

MAG, Code of Practice — Use of Geotextile Filters on Waterways (Federal Waterways Engineering and Research Institute (BAW), Karlsruhe, 1993), www.baw.de. 

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