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This article presents information on the microbial impact on the stability of cement, clay and steel engineering safety barriers in a deep geological repository (DGR) for radioactive waste in the gneiss crystal massif at the Yeniseisky test site (Krasnoyarsk region, Russia). Based on microbial diversity data collected from groundwater and bentonite clay core samples in the proposed DGR construction area, the most likely microbial processes are steel corrosion, cement degradation and changes in clay barrier properties. This study establishes the synergistic effect of chemical and microbial processes in engineered barrier materials. An important aspect of engineering barrier safety is clay materials, which are simultaneously a source of microflora, organic matter and elements necessary for microbial development. It should be noted that the microflora of clay materials includes predominantly aerobic organotrophic bacteria introduced during the technical preparation of clays. The rate of microbial processes may increase at elevated temperatures or in contact with hydrogen and iron-containing products of steel corrosion. An initial assessment of the risk of intensification of microbial processes in protective barriers has been carried out; biocidal additives suppressing microbial activity in clay material and Portland cement have been proposed. • The microbial impact on clay, cement, and steel engineered barriers was discussed. • The synergetic effect of chemical and microbial processes was established. • Effective biocidal additives for clays and Portland cement have been proposed. [ABSTRACT FROM AUTHOR]