An envelope is placed around drain pipes in order to rapidly conduct water from the soil to the drain inlets. Backfill material 2 m Figure 2. Trench with a drain and gravel envelope. Particle gradation in the envelope material. Envelope material gradation requirements are presented in the USBR manual on pages 212-214.
Gradation refers to the size distribution of the particles in the envelope.
The base material is the soil surrounding the drainage trench. Velocity at the interface between the base and the envelope material has to be sufficiently low so that the fine textured material can not be moved into the envelope even under high leaching conditions. Thus, materials will not be carried into the envelope material for finer textured soils if water entrance velocity is high. However, envelopes surrounded by base material with coarse texture must prevent movement of fines into the envelope. Depending on the type of base material, the USBR specifies gradation requirements for the envelope material.