Filter Criteria

Aggregates and geotextiles employed in drainage systems have to operate next to soil or aggregate that surrounds them. To achieve good performance, they must re­main permeable, retain the surrounding ground or aggregate in place and not clog. These requirements are met by defining specific performance criteria. The first of these is the non-sedimentation criteria, which is usually provided by aggregates with no plasticity and a limited amount of fines (usually no more than 5%).

The following filter criteria should be fulfilled:

• F15 (filter material)/D85 (soil) < 5

• F15 (filter material)AD15 (soil) > 5

• F50 (filter material)/D50 (soil) < 25 where the number, n, following F is the size of the sieve through which n% of a filter soil will pass and the number, m, following D is the size of the sieve through which m% will pass of the soil being drained. This terminology was introduced in Chapter 2, Section 2.4.1.

Typically, literature about geosynthetics also defines the following filter crite­ria (e. g. Bergado et al. (1996) who give a long list of references on the subject, Christopher & Holtz (1985) or Koerner (2005)). Firstly, to ensure that the filter holds the soil in place and doesn’t let it through – the “retention criterion” – the following requirements are, typically, set:

• for fine-grained and erodible soils

• for fine-grained cohesive soils

• for problematic (gap-graded) soils

• for coarse-grained and well graded soils where O90 is the effective opening size – 90% of the openings in the geosynthetic are smaller than this value, and Cu is the Uniformity Coefficient (see Chapter 2, Eq. 2.4).

Next, to ensure that particles of soil don’t enter the filter’s pores and clog it – the “clogging criterion” – the following requirements are, typically, set: • O90 < 5D90 although, for cohesive, soils O90/D90 may be larger.

There will usually be a “permeability criterion”, too, as the geosynthetic should remain at least as permeable as the protected soil throughout its lifetime. For this reason, the permeability of the geosynthetic should be N times greater than that of the soil it is filtering at the outset, where N typically varies between 10 and 100. To achieve this criteria such as the following may be set if permeability values are not obtainable:

• for fine-grained cohesive soils O90 > 0.05 mm

• for coarse-grained and well graded soils O90 > DJ5 and O90 > 0.05 mm.

Where a filter is provided between the soil and a grooved, slotted or perforated medium such as a porous plastic pipe, the relationship between the dimension of the filter material and the grooves or the tube perforation size should be as follows:

• F85 > 1.2 times the grooves’ width, or

• F85 > 2 times the perforations’ diameter.

Where it is difficult to know the best design solution, drains can be filled with aggregate wrapped in a geotextile which has a coefficient of permeability 10 times more than the surrounding soil’s coefficient of permeability.

The characteristics required of geotextiles and geotextile-related products for use in drainage systems is contained in European Standard EN 13252 (2000). InEurope, all geosynthetics used in drainage systems should be CE marked.

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