The purpose of this paper is to present a detailed project that may be helpful to others undertaking the building of an earth dam. Results obtained in an impermeable earth dam are presented; the impermeability was attained by sodium hydroxide treatment of its central core; the effect on the impermeability of the dam was determined from undisturbed samples collected during and after the construction period. The position reached by the saturation line in the external slope was determined in order to evaluate the effect of the treatment on the impermeability of the dam.