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A set of geotextile filter design criteria has been developed by examining soil particle bridges on the surface of non-compressible geotextiles. The soil particles were represented by discs in two-dimensional models and by spheres in three-dimensional models. The models examined the relationship between unif onni y coefficient, particle size and pore size, and resulted in two overlapping phenomena being obseived. At uniformity coefficients less than 2. 0, bridge formation is dependant upon particle interlock and packing, whilst at unifonnity coefficients greater than 2.0, large particles act as catalysts for bridge formation. These phenomena resulted in the specification of an upper 090/d90 ratio. Further analysis revealed that certain particles did not usefully contribute to bridge formation, which resulted in the specification of a lower Ogofdgo ratio. Both these concepts have been incorporated on a final design chart, which is then compared to other published filter design criteria.