Constriction Size of Geotextile Filters

Filtration performance of nonwoven geotextiles strongly depends on pore opening constriction size, i.e., the minimum opening size of flow channels across the geotextile. Currently available methods of pore opening characterizations do not provide accurate information about the constriction size. This paper presents the constriction size distributions of eleven nonwoven geotextiles commonly used as filters based on a probabilistic approach coupled with image analysis, named constriction size (CONS). The image analysis method was developed using mathematical morphology operations. Randomness in the structure of geotextile media was modeled using Markov chain processes. Two characteristic constriction sizes, C95 and C50, were determined and compared with the available results from laboratory tests, theoretical equations, and manufacturerʹs reported apparent opening sizes. The CONS-based C95 constriction size was comparable to C95 determined using the bubble point test. On the other hand, the CONS-based C95 was consistently lower than the manufacturerʹs reported apparent opening size, which is typically determined on the basis of the dry sieving test. Similarly, the CONS-based C95, was highly comparable with the value suggested by a theoretical equation. The observed differences in the constriction sizes determined by CONS versus the available physical tests may be reflective of the nature and indirectness of the physical tests rather than inaccuracies associated with CONS, which is based on direct observation of pore openings.