Pavement structures consist of material layers with different grain size gradations and different mechanical as well as permeability properties. As water will flow through the structure it is important that migration of a portion of the fines from one layer to the next will not take place. To achieve this, the principles of fil — tration/separation must be applied at each interface (Cedergren, 1977). This is of special interest where water flows from fine grained material into a coarser grained material on its way out of the structure.
In applying the filter criteria the material to be drained is frequently referred to as the base material and the new material to be placed against it the filter material. The filter criteria stipulate that the filter needs to fulfil two functions:
• water needs to drain freely through it (filtration function or permeability criteria); and
• only a limited quantity of solid particles are allowed to move from the base layer into or through the filter layer (separation function or piping requirement). This criteria is set as otherwise the coarse grained material could be filled or clogged with time.
These two functions are in conflict with each other as filtration requires a high discharge through the filter while separation requires this to be small. The conditions of these two requirements can be expresses as:
—f —f
—5 > 4 to 5 & —5 < 4 to 5
D?5 — Ї5
where —15 is the diameter in the particle-size distribution curve for the filter material corresponding to 15% finer and — b5 and — b5 are the diameter in the particle-size distribution curve for the base material corresponding to 15% and 85% finer respectively. The application of these issues is presented in Chapter 13, Section 13.3.9.
