Three different sieving methods were put forward by national delegations to determine the characteristic opening size of geotextiles. A comparison of these methods was carried out by laboratories in Europe together with laboratories in China and North America.
The interlaboratory trials showed that wet sieving gave results with good agreement within narrow limits in all involved laboratories. The dry sieving procedure showed a larger spread of results, mainly due to several laboratories not respecting the rules laid down in the test procedure, moreover dry sieving is not appropriate for very fine textured geotextiles (O90 less than 100 μm). The results of the inter-laboratory hydrodynamic sieving showed a somewhat larger variation than the wet sieving procedure although sets of results were more or less of the same order.
The main constraints of the hydrodynamic sieving procedure were that before any results are available one or more days must pass (sieving time is 24 hours) and the apparatus which is not commercially available, is expensive and non-standard. As a result apparatus used for the hydrodynamic sieving all had slightly different constructions while for the wet sieving complying sieve apparatus was used by most laboratories.
Wet sieving has therefore been put forward as the preferred method for a European EN-standard. A detailed description of the advantages and some constraints of the wet sieving procedure and analysis will be given. Also the acceptance of the O90 as the determination criteria for pore size rather than the O95 is discussed.