Comparative Tests for Evaluating Permeability Changes of a Compacted Bentonite/Sand Mixture During Shear

Abstract

ABSTRACTA compacted mixture of Kunigeru VI bentonite and D-sand is being considered for use as an engineered barrier in low-level radioactive waste disposal facilities in Japan. An important issue is the maintenance of the retardation property of the mixture during shear that might be induced in such barriers by earthquakes and/or gradual tectonic deformations occurring over the design life of the facility. To investigate this issue, comparative tests on a bentonite-sand mixture and kaolin-sand mixture were conducted by means of a recently-developed coupled shear and permeability testing apparatus under temperature controlled condition. In addition to permeability, the specific storage of bentonite-sand specimen during shear is also systematically evaluated with the new analytical theory for the constant flow permeability test. The present study reveals that 1) temperature control is preferred for measuring the permeability of extremely-low permeability materials with the constant-flow pump method; 2) both the permeability and specific storage of the mixture of Kunigeru VI bentonite and D-sand (bentonite to sand weight ratio = 15: 85) were not significantly influenced by shear strains up to 3% whereas the permeability of the kaolin-sand mixture increased almost linearly with the increment of shear strain; 3) the swelling of bentonite in the mixture under low confining stress decreases both the permeability and specific storage of bentonite-sand mixture. This feature of bentonite helps maintain the long-term retardation property of the mixture; and 4) the constant flow permeability test method, with the newly derived theoretical analysis, promises to become a very effective means of investigating, rapidly and systematically, the permeability and specific storage of extremely-low permeability materials with relatively-low (i.e. close to in situ) hydraulic gradients. This is necessary for the design and long-term performance assessment of engineered barriers.