The surface of rock or soil, newly-exposed, for example, by tunnelling, and having the tendency to become friable on exposure to the atmosphere, can be protected by forming thereon a film by the process of combining on the surface at least one coagulable material and a coagulating agent therefor. The preferred coagulable material is an aqueous polymer dispersion, preferably one which has some degree of elastomeric properties, and preferably one which additionally is anionically stabilized.