When a granular liquid collection layer is replaced by a geosynthetic liquid collection layer, it is often assumed that two liquid collection layers having the same hydraulic transmissivity are equivalent.  In the United States, this approach is often mandated by regulations for the case of leachate collection layers and leakage detection and collection layers used in landfills.  This is true only in the case of confined flow (i.e. if the liquid collection layer is completely filled with liquid).  In reality, liquid collection layers should be designed for unconfined flow, as demonstrated in the paper by Giroud, Zhao and Bonaparte (2000).  To be equivalent under the unconfined flow condition, the geosynthetic liquid collection layer should have a greater hydraulic transmissivity than the granular liquid collection layer.

Formula

     where: