All soils with structural instability create low soil permeability. The upward force of water entering a drain pipe may exceed the buoyant soil particle weight and result in instability and soil movement unless natural bridging occurs. Particles longer than 0.25 mm generally have enough mass to withstand the usual forces produced by soil water flow and tend to be stable. Instability would be greater for fine silts and clays except these soils exhibit cohesive inter-particle forces which bind them together. The critical tractive force must be exceeded for a particle to move.
Intermediate diameters move at lower values of tractive force.
Indeed, these soils may clog an envelope material if one were used. Another common criterion is that envelopes are required for soils where the plasticity index, PI, is less than 10. Once the grain size distribution has been determined, graphs are available for establishing the effectiveness of an envelope for a particular soil. At the same time, the Atterberg Limits for the soil must also be determined in order to make a proper classification to assess the appropriateness of an envelope. Cut the centre from the plastic lid to leave a 10 mm retaining ring.