Abstract This paper assumes that an explosion under a runway causes a large void, a camouflet, to be formed. It considers the effects of a single wheel load moving along the length of the runway and determines the vertical displacement of the runway both with the unfilled void and with the void filled with polystyrene. In particular it considers the effects of the resulting subgrade stresses on the runway's fatigue life. It was found that filling the void made little difference to the displacements or to the fatigue life of the runway or subgrade. Once a camouflet has been identified, the authors recommend that the camouflet should be dug out and refilled to improve the runway's fatigue life.