The paper presents a macroscopic constitutive model for rockill that includes the effect of water on compressibility and collapse phenomena. Breakage of rock particles and fracture propagation are basic underlying mechanisms controlled by the relative humidity of the air filling the rockill voids. A conceptual deformation model based on these mechanisms is first proposed and discussed. The results of oedometer tests on a quartzitic slate rockill, in which the air relative humidity was controlled, are then presented. A significant finding is that bringing the relative humidity within the specimen to its maximum (100% RH) leads to a collapse strain equal to that observed in flooded specimens. An elastoplastic constitutive model, consistent with the basic deformation framework, is developed. Its parameters have a clear physical meaning. Guidelines for parameter determination are given. Model performance is finally compared with the results of the experimental programme.