However, the shear failureby loading is not expected to occur, because the load acting to the buŠer material is not so high under the plan ofthe project. As a general tendency, the strength ofbuŠer material becomes low with the increase of watercontents and sand contents. The study about swelling pressure has been also carried out by some researcherslike Komine and Ogata (2003).The strength of the buŠer material has been also studied and was found as a function of the degree of saturation, dry density and the mixture ratio of sand (Takajiand Suzuki, 1999). (1995), Suzuki and Fujita (1999), Shibutani et al. The properties related to the permeability and thesolute transport have been intensively studied by manyresearchers, e.g., B äorgesson et al. The main function ofthe buŠer material is considered to prevent the transportof the leaked radioactive waste into the surroundingrocks. The buŠer material,therefore, has to have two main properties of large swelling capacity and low permeability. When the water content became large during seepage, the strength forhydraulic fracturing reduced.Key words: bentonite, dry density, hydraulic fracturing, laboratory test, nuclear waste disposal, swelling, water content (IGC: D4/D5/D6)portant properties as a barrier. While the tensile failure was dominant, the specimen having a low dry density might be failed initially by the shear failure. However a long period of low-pressure supply was neededto recover the strength at the failed parts. The fracture made byhydraulic fracturing was ˆxed through the supply of water. The swelling pressureof the buŠer material worked as a constraint stress for the strength for hydraulic fracturing. As a result, it was found that the strength for hydraulic fracturing of the buŠer material increased withthe increase of initial dry density, decrease of sand-mixture ratio and decrease of water content. The observed phenomena were analytically examined.
Thirdly, the constantpressure test was carried out to observe the change in strength during seepage. Secondly,the cyclic pressurize test was carried out to examine the self-sealing function as a buŠer material. Firstly, the constant pressurize rate tests were carried out for the material having various speciˆcations.
2008CHARACTERISTICS OF STRENGTH FOR HYDRAULICFRACTURING OF BUFFER MATERIALAKIRA KOBAYASHIi), KIYOHITO YAMAMOTOii) and SHOHEI MOMOKIiii)ABSTRACTThe fundamental characteristics of strength for the hydraulic fracturing of highly compacted bentonite were studied. 表示 SOILS AND FOUNDATIONSJapanese Geotechnical SocietyVol. Characteristics of Strength for Hydraulic Fracturing of Buffer MaterialĪkira Kobayashi・Kiyohito Yamamoto・Shohei Momoki
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