Two-dimensional mesoscale experiments and a theoretical analysis show that the formation of clusters in a geotechnical filter is associated with fine particles that form arches between the grains of the matrix. The analysis of the stability of such arches indicates two critical parameters. The characteristic size of a cluster is obtained as a function of the uniformity of the matrix. If the matrix is uniformly spaced, the clusters occupy large domains, whereas if the matrix is non-uniform the clusters form in small pores.