This paper discusses the formation of stable arches in frictional soils. A series of laboratory tests are performed to explore the formation of arches in granular materials, either cohesionless or with small apparent cohesion. By considering the stable soil arch as a stress-free surface, the analysis in the framework of continuum mechanics reveals that such an arch can only form in cohesive frictional materials. The shape of the arch mainly depends on the material's friction angle, while the critical width of the arch is primarily dominated by cohesion. The formation of stable arches in cohesionless materials is interpreted by taking into account the discrete nature of the material, with the failure of the arch being considered as buckling of particle columns. It is shown that the width of stable arches in cohesionless materials is generally five to seven times of the particle size. Copyright © 2012 John Wiley & Sons, Ltd.