The present invention relates to a method of strengthening geological formations in underground workings or mines by introducing reaction mixtures containing a polyisocyanate component (a), a polyol component (b) and optionally auxiliary agents and additives (c) into the formation which is required to be strengthened and reacting the mixtures to form polyurethanes, wherein polyol component (b) contains mixtures of (i) organic polyhydroxyl compounds having a hydroxyl number of about 100 to 800 and (ii) monovalent alcohols having a molecular weight of 32 to about 600, component (ii) being present in an amount of about 0.01 to 20% by weight, based on the total quantity of component (b).