The strength measures of expansive cement grout borehole plugs cast in welded tuff cylinders is investigated as a function of the degree of saturation of the plugged rock cylinder and of borehole size. Details on experimental procedure regarding rock cylinder and cement grout preparation, sample curing conditions, experimental apparatus, sample loading, mechanical characterization of the rock, and cement grout, along with procedures for the determination of the sample saturation assuming uniform saturation, and strength measures are presented. The extrapolated axial strengths to a plug radius of 100 mm show that the more saturated samples show higher strengths as compared to the dry samples. The strength measures decrease with increasing plug radius, obeying a power law. (C) 2000 Elsevier Science Ltd. All rights reserved.