Laminar and non-laminar flow in geosynthetic and granular drains

Hydraulic transmissivity tests on common geosynthetic and granular drainage materials (e.g. geonets and gravel) show that the hydraulic transmissivity of these materials often depends heavily on the hydraulic gradient, which indicates that the flow is non-laminar. Despite the non-laminar nature of flow in these materials, Darcy's equation and equations derived from Darcy's equation are extensively used for the design of geosynthetic and granular drainage systems, even though these equations are strictly valid only for laminar flow. Therefore it is important to identify the drainage materials and flow conditions for which the flow is laminar in order to evaluate the applicability of Darcy's equation. In classical hydrodynamics, the conditions for laminar flow are generally described in terms of a limiting Reynolds number. This paper provides guidance for Reynolds number calculation in geosynthetic and granular drains, and presents a methodology to establish the conditions for laminar flow as a function of the Reynolds number. Numerical applications of the methodology show that, for typical hydraulic gradients used in hydraulic transmissivity tests in the laboratory and encountered in drainage layers in the field, flow is generally laminar in needle-punched nonwoven geotextiles and sand, whereas it is generally non- laminar in geonets and gravel. However, in the case of geonets adjacent to geotextiles (such as in geocomposites), the flow becomes closer to laminar conditions as the geotextile progressively intrudes into the geonet channels under increasing values of the applied normal stresses. Practical recommendations are given for the use of Darcy's equation and equations derived from Darcy's equation to obtain approximate solutions when flow is not laminar.