Permeability of stratified sands

Summary The permeability of graded sands was studied in the laboratory to determine the influence, if any, of stratification order on permeability (i.e. the position in which finer or coarser layers occur in the flow path). The evidence suggests that the order of stratification does not influence the order of magnitude of permeability of a sand. This order of magnitude is determined by the fine fraction of the sand. However, the order of stratification does have an influence on the finer details (i.e. variation within the same order of magnitude) of permeability. In this regard, an unsorted sand (all fractions mixed together) has the lowest permeability while a sorted one that has the fine fraction at the end of the sequence has the highest vertical permeability. It is also concluded that empirical formulae that use only the effective size, D₁₀, to estimate permeability yield results that are comparable with those that incorporate sorting in their formulae. However, experimental results show that these formulae do not yield reliable results when used for more denser sands.