Abstract: A method of completely filling mines and underground cavities in such a way as to reinforce the strata and ground thereabove in order to prevent collapse or subsidence. The method comprises introduction into mines and cavities, expandable plastic materials which are incorporated into a chemically catalysed foam reaction and strongly bonded thereby. The heat required to expand foamable plastic materials many-fold is provided by the chemically exothermic polymerization reaction of polymeric isocyanate with polyols and epoxides by basic catalysis which promotes highly exothermic urethane/isocyanurate polymer formation in the presence of suitable blowing agents and surfactants.