Abstract: A method of treating subsurface layers of the earth to either control movement of the subsurface water or build strength of the subsurface layers, or both, characterized by injecting at a predetermined depth and at a plurality of predetermined, spaced apart locations a limefly ash slurry consisting essentially of water, particulate hydrated lime, particulate fly ash and a surfactant. Also disclosed are the preferred proportions, or concentrations, of the components of the slurry; and a finished product; such as, a railroad track subgrade and ballast supporting cross-ties and rails, the subgrade having been injected with masses of lime and fly ash slurry to leave residual masses of lime and fly ash disposed in fissures and any ballast pockets along the railroad track, the masses having a proportion of lime to fly ash in the range of one to 1:1 to 1:3.