The mechanical properties of cement mixtures for use as sealing in exploration boreholes in salt and potash mines have been experimentally determined. The mixtures are prepared from commercial grade Portland cement type V mixed with saturated brine. The results indicate that the uniaxial compressive strength and elastic modulus decrease and Poisson’s ratio increases with increasing brine-to-cement ratio. The highest compressive strengths and elastic modulus are obtained from cement-sand mixture. The bending tensile strengths of the mixtures range from 2.36 to 3.61 MPa. The total strain energy at failure has been calculated and derived as a function of brine-to-cement ratio. The Coulomb and strain energy criteria are applied to determine the factors of safety of the materials in boreholes during subsidence in terms of shear strength and bending tensile strength. The factors of safety increase when the depths increase and surface subsidence decreases. The findings can be used to assess the stability of the cement seals in boreholes drilled in the subsidence areas.