Abstract: Nonwoven geotextile is used in engineering as filter materials. When the geotextile is subjected to biaxial tension, the pore size of geotextile can be changed. Based on a 3D pore network model for the nonwoven geotextile, the method for calculating the number of structural layers in Rawal’s formulation is improved and a theoretical solution is proposed for the pore size distribution of nonwovens under equal biaxial tension. Two geotextiles are uniformly stretched in bi-directions to strains of 3%, 5% and 10%, respectively, using a self-developed bi-directionally stretching machine, and the pore-size distributions of the stretched geotextiles are determined under strain-controlled condition using the dry sieving method. By comparing the theoretical solutions and the measurements, it is shown that the theoretical method overestimates the pore sizes, and yields more reasonable results for the thinner geotextiles. In addition, the pore sizes increase with the biaxial tensile strain，and the characteristic opening size O95, O50, and O30 increase approximately linearly with the tensile strain.

Key words: biaxial tensile strain, nonwoven geotextiles , analytical solution of pore , pore size distribution curve, dry sieving method