The present invention relates to a process for the reinforcement of geological formations and loose rock and earth masses by introducing, into the cavities of the geological formations or rock or earth masses which are required to be reinforced, reaction mixtures based on organic polyhydroxyl compounds and organic polyisocyanates which react to form polyurethanes, which process is characterized in that the polyisocyanate component used is a polyisocyanate mixture containing from about 10 to 80%, by weight, of 2,4'-diisocyanato-diphenylmethane. The present invention also relates to a two-chamber cartridge suitable for carrying out the process according to the present invention. This cartridge comprises two chambers separated from each other, one containing a polyisocyanate component and the second chamber containing a polyol component, the quantitative proportions of the two components being calculated so that when the cartridge is destroyed, a reaction mixture which reacts to yield a polyurethane is obtained, characterized in that the polyisocyanate component used is a polyisocyanate mixture containing from about 10 to 80%, by weight, of 2,4'-isocyanato -diphenylmethane.