Method of Designing Bentonite/Sand Mixture to Achieve the Target Permeability
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METHOD OF DESIGNING BENTONITE/SAND MIXTURE TO ACHIEVE THE TARGET PERMEABILITY
HITOSHI NAKASHIMA*, TAKASHI ISHII*, TATSURO NAGASAWA* NOBUHIDE OGATA**,AND HIDEO KOMINE** * Power & Energy Project Division SHIMJZU CORPORATION,SEAVANS SOUTH No.2-3, Shibaura 1-chome, Minato-ku, Tokyo 105-07, Japan ** Central Research Institute of Electric Power Industry, 1646 Abiko, Abiko city, Chiba 270-11, Japan
ABSTRACT Bentonite/sand mixture is expected to be an effective component of barriers for radioactive waste disposal facility. Impermeable mixture can be obtained through enrichment of the bentonite content,but it is difficult to estimate a permeability of bentonite/sand mixture before permeability test. At designing of bentonite/sand mixture, we have to decide which sand to be used, and what bentonite content is. But,this means that designing will be a time-consuming task. In this report, we made many kinds of bentonite/sand mixtures using some kinds of sand with different bentonite content, and permeability tests were conducted for these mixtures. After examination of the permeability tests results, the effective void ratio of bentonite/sand mixture which was obtained by compaction test, is an efficient parameter for estimation of the permeability. Last of all, we propose the method of designing bentonite/sand mixture using the effective void ratio.
INTRODUCTION Bentonite/Sand mixture is expected to be an effective component of barriers for radioactive waste disposal facility. Typical characteristics of Bentonite/sand mixture are 1) impermeability, 2) self-healing, 3) long-term durability. It is known that the greater the ratio of bentonite, the higher the mixture becomes in its impermeability[I]. However, permeability of the mixture varies with the difference in varieties of bentonite and sand used, It is then difficult to estimate a permeability of bentonite/sand mixture for which any sand and bentonite were used. This means that we have to pay attention to selection of the bentonite or sand in actual construction of barriers. Although we decide the mix ratio of bentonite and sand in laboratory tests at the stage of planning, we could not avoid examining permeability tests to select which sand to use at the stage of actual construction. Moreover, it is anticipated that still higher mechanical strength would be required of bentonite/sand mixtures if they are to be used in more advanced applications. This in turn means that selecting appropriate materials will be a time-consuming task. In this paper, we carried out permeability tests on bentonite/sand mixtures using several different sands, then showed that permeability may depend on the difference between sands used. We also took note of the "effective void ratio" representing the ratio between void among sands and bentonite in the mixture as a factor governing the permeability. Using test results, we proposed the method of designing bentonite/sand mixture having the target permeability. No test was made to a investigate the difference in the permeability depending on the
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