報告書番号

N04039

タイトル（和文）

地下水流動における断層破砕帯影響評価手法 --地下水化学的調査の適用と系統的評価手段の提案--

タイトル（英文）

Evaluation methodology for fault effect on underground water flow-Application of hydrochemical investigation and a proposal for systematic evaluation methods-

概要 （図表や脚注は「報告書全文」に掲載しております）

断層破砕帯の水理的評価は地下構造物建設のみならず地下水保全からも重要となっている。従来、断層の水理特性は原位置試験やコア試験で得たが、広域や深部地下水流動における断層影響は岩石-水反応に基づいた地下水化学的な手法が必要である。このため本報告では地化学的方法の適用性を示すとともに、調査法について検討することを目的とした。神岡鉱山ではガウジ部のコアガスの4He濃度は損傷帯の湧水に比較して100-10000倍の濃度となり地下水流動が非常に遅く低透水性を示唆した。十勝中央断層の調査では降水が浅部でCa-HCO3型地下水となり、これが岩石中のNa＋とのイオン交換により深部地下水（Na-HCO3型）を形成する一方で深部地下水と塩水（Na-Cl）の混合したと解釈された。この塩水は化石海水で高間隙圧を動力として断層沿いに上昇したと考えられる。断層の水理性状は透水性、間隙率、ポテンシャル分布が支配要因であり、直接試験で得るほか上記の地化学的方法でも検討できる。さらに報告書では当所調査技術による断層水理評価手法をとりまとめた。

概要 （英文）

Hydraulic assessment of fault zones in rockmass is investigated for safety of underground construction works. Recently, such as assessments are gaining important for underground water resources management. The effects of faults on groundwater flow have been estimated by permeability studies using in-situ hydraulic tests or core hydraulic tests. Geochemical methods based on long-term water and rock interaction for estimating the effects in regional areas or deep strata are required. This report discusses the applicability of geochemical methods for assessing the effects of faults on groundwater flow by field studies, and considers methodologies for obtaining fault-related hydraulic properties. A geochemical study of the Kamioka mine demonstrated that the dissolved 4He gas in a fault gouge is high-density, and it is suggested that groundwater age in a fault gouge is 100-10000 times older than that in the seepage of a fault dameged zone. This suggests that groundwater flow in a fault gouge is very slow and permeability is very low. A geochemical study of the Tokachi Chyou fault showed that infiltrating rainwater that had changed to Ca-HCO3 type groundwater in the shallow aquifer, underwent ion exchange with the Na+ in the rockmass, thus producing Na- HCO3 type groundwater in the deep aquifer. It was further shown that this groundwater then became mixed with Na-Cl saline water, old seawater that has been forced up the Tokachi Chyou fault by high pore pressure from deep strata. These studies show that the hydraulic characteristics are governed by the permeability, the porosity and distribution of the hydraulic potential. These properties can be obtained from field tests and laboratory measurements, and geochemical methods. Our investigation methods to obtain these properties are summarized and are presented in this report.

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