In soils where there is a chance of heavy movement of soil particles into the tile drains and consequently clogging them, filter materials are placed around the tiles. The filter or envelope materials prevent the entry of the relatively coarser particles as well. They act as bedding material and thus allow the tiles to take greater loads. They also increase the effective diameter of the tiles and allow quantities of water to flow into the tiles. A variety of materials have been used as envelope materials in tile. These consist of gravel coarse sand organic materials like straw, coir matting etc. However, gravel has been the most widely used material because of its; efficiency and also of permanent nature.
The function of a drain envelope is to improve permeability in the surrounding of the drain. For this reason, the envelope material should have a hydraulic conductivity 7 times higher than the base material. Since envelopes are not designed for their filtration capacity, they do not need to be well graded. The gravel envelope should Prevent movement of soil particles into the tile drains. The thickness should be the same as the sand and gravel filter (i.e. 100 mm around the pipe). All the envelope material should be smaller than 38 mm, D90 < 19 mm and the D10 > 0.250 mm.
The gravel envelope should be designed taking into consideration the nature of the soil around the tile drains. The gravel should be clean; free from organic matter and bentonite clays. The United States Bureau of Reclamation recommends the following criteria for the design of gravel envelopes. For uniform soils, the ratio of D50 of envelope material and D85 of soil should range from 5 to 10. However, the ratio ranged from 12 to 58 for graded soils.