Sand arch. The arch of sand created outside of an orifice or perforation on the [[formation]] side of casing or liner. For example, in [[unconsolidated]] sandstone the rock must have the property of [[dilatancy]], or the property to expand, before the arch will form. The resulting arch, caused by the interlocking and wedging together of sand grains, prevents the flow of sand through a perforation. Cohesion is imparted to the loose grains forming the arch when the wetting phase is at [[irreducible saturation]].