Filters can be either geotextile or well graded gravel and sand. Filters are necessary only where there is something in soil that needs to be filtered out, namely fine sand. If sand is not present, filters are not necessary. Not only do filters add to the cost of drainage but they also constitute an additional barrier to inflow of water and can, therefore, reduce the effectiveness of the drain. Few soils present the danger of sand particles clogging the drainage system. Most soils contain sufficient amounts of clay or organic matter to form relatively stable aggregates of individual soil particles. Filters in these soils are of no benefit and may have reduced drain performance. Instead of filters, a porous envelope is appropriate to ensure good flow conditions at all times at the interface between drain and soil. Drain rock, pea gravel and similar materials meet these requirements and are used extensively in some drainage applications. The first step in design of a proper filter system, either geotextile or sand and gravel, is to perform a particle size analysis of soil at drain depth in field. Usually soils with more than 30% clay content do not require a filter. Geo textile filters

Suitable filters may be used to restrict entry of fine particles of silt and sand from entering the drains in soils with poor cohesion. A properly designed filter stabilizes the soil around the drain and allows free entry of water. There are two basic types of geotextile filters, knitted and non- woven. Knitted geotextiles are usually made of polyester or polypropylene filaments that are knitted or woven together. The most common type has a thickness of 1 mm, a weight of 150 g/m2 and an Apparent Opening Size (AOS) of 300 microns. For applications requiring more filtration capacity, a sock knitted with velour or pile on one side, that is thicker (< 2 mm), heavier (250 g/m2) and has an AOS of about 100 microns is needed.

Non-woven geotextiles are made from several layers of randomly distributed fibers that are rolled pressed and usually interconnected by needle punching. It has been found that fabrics about 2 mm thick are very good for silty soils. The general guide for designing a geotextile filter is that the ratio of O95 Fabric material and D50 Soil should be =< 2.5. Where O95 is the apparent opening size (AOS) of the geotextile filter D85 is the size of which 85% of the particles are finer. Gravel and sand filters

In arid areas sand and gravel filters are used to some extent instead of geotextile filters. Drains usually run deeper and the sand and gravel filters also act as an envelope to improve bedding and permeability characteristics. Filter materials should be well graded. If more than one gradation is used, the layers should be from coarsest to finest material, starting at the pipe. A minimum thickness of 100 mm is recommended for each layer of the filter.

Limits for the filter material are such that ratio of D50 Filter material and D50 Base material ranges between 12 to 58 and that of D15 Filter material and D15 Base material from 12 to 40. D50 (lhe size of which 50% of particles are passing through the screen) of the base material times 12 and 58 will yield the lower limit and upper limit for D50 filter. Provided

the filter has no more than 5% finer than 0.074 mm and is relatively well graded.

The chosen filter material should be checked for stability by adopting the criterion that the ratio of D15 Filter material and D85 Base material should be < 5. The D85 size of the filter material with respect to the opening of the drainpipe should be verified using ratio of D85 Filter material and Maximum drainpipe opening as > 2.

A well graded filters material is required for good performance. A filter material is considered well graded when all particle sizes from the largest to the smallest are present in a balanced way. As given below, the coefficient of uniformity can be used to verify how well graded the material is.

where Cu = Coefficient of uniformity

where, Cc = coefficient of curvature.

In general, for well graded filter materials maximum size of aggregates should be 38 mm. However, D90 upto 19 mm and D10 upto 0.25 mm. Cu >4 for sand and >6 for gravel may be recommended. However, Cc might range in between I and 3 for well graded filter materials.

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