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ABSTRACT:In order to reduce the barrier performance deterioration in bottom clay liners at waste containment facilities, this study investigated the effectiveness of (1) using manufactured modified bentonites, (2) prehydrating natural bentonites, and (3) confining natural bentonites at high effective pressure based on the hydraulic conductivity tests. The results showed that a modified bentonite of "multi-swellable bentonite" exhibited a low hydraulic conductivity of < 1.0×10 -8 cm/s for NaCl solutions with molar concentration of ≤ 1.0 M, and a manufactured modified geosynthetic clay liner (GCL) of "dense-prehydrated GCL" exhibited an extremely hydraulic conductivity of <1.0×10 -10 cm/s for any CaCl 2 solutions tested. Prehydrated bentonite, which is naturally prehydrated by absorbing the moisture in the underlying base layer, also exhibited a low hydraulic conductivity of < 1.0×10 -8 cm/s for the permeation of any CaCl 2 solutions tested. Although the permeant solutions with high concentration and ionic valence increased the hydraulic conductivity of natural bentonite, higher confining pressures decreased the void ratio of the bentonite even for the chemical solutions so that the confined bentonite could exhibit a low hydraulic conductivity of < 1.0×10 -9 cm/s even to the NaCl solutions and CaCl 2 solutions.