The myth of hydraulic transmissivity equivalency between geosynthetic and granular liquid collection layers

Design engineers often assume that a geosynthetic liquid collection layer and a granular liquid collection layer having the same hydraulic transmissivity are equivalent. In the United States, such equivalency is often mandated by regulations for leachate collection layers and leakage detection and collection layers used in landfills. This approach is based on the assumption that two liquid collection layers having the same hydraulic transmissivity have the same flow capacity. As shown in the present paper, this is true only in the case of confined flow. A theoretical analysis described in the present paper shows that, in the case of unconfined flow (which is the usual design case), two liquid collection layers that have different thicknesses and hydraulic conductivities (such as granular and geosynthetic liquid collection layers) cannot have the same flow capacity (i.e. cannot be equivalent) if they have the same hydraulic transmissivity. To be equivalent, the thinner liquid collection layer (generally, the geosynthetic liquid collection layer) should have a greater hydraulic transmissivity than the thicker liquid collection layer (generally, the granular liquid collection layer).