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Drain envelopes and filters

Drain envelopes and filters are two different techniques used to solve different problems. Drain envelopes are permeable materials, such as gravel place around the drains for the purposes of improving flow conditions. Filters for drains are permeable materials, such as geotextiles, placed around the drains for the purpose of preventing fine-grained materials in the surrounding soil from being carried into the drain by groundwater. Drain envelope is a material placed around a drain pipe to provide either hydraulic function, which facilitates flow into the drain, or barrier function, which prevents certain sized soil particles from entering the drain. It is termed as sock also. Drain envelopes are not filters. Filters become clogged over time, but, drain envelopes do not.

Many types of envelope material such as thick gravel and organic fiber to thin geotextiles are in use. The useful life of a synthetic drain envelope is long, if not left in the sun for a long time and exposed to much ultraviolet radiation. Fine-textured soils with a clay content of 25 to 30 per cent are generally considered stable, so they don't need drain envelopes. A geotextile sock is recommended for coarse-textured soils free of silt and clay. These soils are considered unstable even if undisturbed, so that particles may wash into pipes. The need for an envelope in intermediate soils (clay contents less than 25 to 30 per cent) is best left to a professional contractor or soil and water engineer because soil movement is more difficult to predict.

In general gravel, coarse and very coarse sand, silty clay loam, sandy clay and peat soil do not require any filter or envelope materials. Gravelly fine sand, medium sand, fine sand, clay sand, sandy loam, loam and silt require filter materials in low to high degree of urgency. However, silty clay and clay require envelope materials.

 
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