Abstract: A method is described, using a polyurethane resin mixture, for sealing off points through which water enters geological rock formations. The method utilizes an isocyanate component (a) and a polyol component (b), to be intimately mixed in a static mixer. The mixture is introduced under pressure into the rock formation through boreholes or injection lances. Primary or secondary di or polyamines are substituted for a portion of the polyol component (b) in quantities such that the polyol remains in excess. These amines greatly accelerate the gelation time for the resin, resulting in an instantaneous increase in viscosity of the gel.