Controlled disposal sites generally rely on impermeable barrier layers to prevent groundwater contamination. Because bentonite has low hydraulic conductivity and high swelling pressures in the presence of water, soil-bentonite mixtures are commonly used with geomembrane to construct hydraulic barriers. If inexpensive materials could be used instead, the potential cost reduction is dramatic. Sedimentary rock is widely distributed in Japan, and it is common for such rock to be excavated from the disposal site itself. This study evaluated the water-shielding performance of bentonite mixtures made with friable Neocene mudstone and commercially available graded sand. The hydraulic conductivity of the friable Neocene mudstone mixture compared favourably with that of the purchased sand mixture, even at half of the standard bentonite mixing ratio (5%). The results also confirmed our hypothesis that the friability of the mudstone could be used practically to improve mixture performance. This study demonstrated the fundamental properties of an efficient construction method that utilised roller compaction to ensure an effective seal. Construction of hydraulic barrier layers using a method that takes advantage of the friability of in situ excavated sedimentary rock, without adjusting its grain size, has the potential to greatly lower construction costs.