Journal of the Japan Society of Engineering Geology

Original Article

Characterization of Water Conducting Fracture and their Long-term Behavior in Deep Crystalline Rock :

A Case Study of the Toki Granite

Masayuki ISHIBASHI, Tomomi ANDO, Eiji SASAO, Takashi YUGUCHI, Shoji NI ...

2014 Volume 55 Issue 4 Pages 156-165
Published: October 10, 2014
Released on J-STAGE: April 27, 2015

DOIhttps://doi.org/10.5110/jjseg.55.156

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Long-term history of water conducting fracture is key issue to evaluate deep geological environment for geological disposal of high-level radioactive waste. Thus, we carried out study on the geological features and the long-term behavior of water conducting fractures based on the data obtained at －300 m levels in the Mizunami Underground research laboratory (MIU), central Japan.
Total 1,670 fractures were described in underground gallery at the －300 m levels. Water conducting fractures occupy about 11 % of all fractures. They are divided into two groups： grout filling fractures and low inflow-rate fractures. The former is filled with calcite without conspicuous host rock alteration around fractures. The letter possessed similar geological character with the sealed fractures which are not acted as flow-path.
The geological character of fracture filling and host rock alteration around fractures indicate the history of three stages; a) intrusion and emplacement of host granite, b) fracture filling at hydrothermal event, and opening and/or elongation during exhumation, c) fracture filling at inflow of meteoric water.
In conclusion, the present water conducting fractures were formed by opening or elongation of pre-existed fractures, which were filled at the hydrothermal event, at the time of exhumation.

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Fault Exposure of Dainenjiyama Fault Across Urban District of Sendai City, Northeast Japan

Hideki KOSAKA, Atsushi MIWA, Toshifumi IMAIZUMI, Hideki INAGAKI, Shuic ...

2014 Volume 55 Issue 4 Pages 166-176
Published: October 10, 2014
Released on J-STAGE: April 27, 2015

DOIhttps://doi.org/10.5110/jjseg.55.166

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Dainenjiyama fault is a back thrust of the Nagamachi-Rifu fault forming a distinct fold (flexure) scarp across the urban district in the Sendai city, NE Japan. This paper aims to identify detail location of fault, fault feature and displacement of the Dainenjiyama fault, which are important for a disaster prevention perspective, based on the observation of fault outcrop exposed by construction and surrounding fault topography. F1 and F2 faults, displaced sand and gravel layer, are appeared on this outcrop. Accumulated displacement of the gravel layer suggests that the F1 fault has moved at least twice. The net displacement of the Dainenjiyama fault is estimated to be 0.8 m by the vertical displacement (1.6 m) and fault history of the F1 fault. The height of the fold (flexure) scarp on the Uemachi terrace is 5-6 m, which is much greater than displacement of F1 fault and F2 fault. This implies the presence of other fault in addition to these two faults. On the basis of these results, we conclude that the Dainenjiyama fault would branch into multiple in the fault scarp. Thus activity of the Dainenjiyama fault would cause surface ruptures which of triggering a serious disaster to infrastructure in the Sendai city.

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Case Study of the s-Q Curves Obtained by Step Drawdown Tests

Kazushi MORI, Eiichi ABE, Kouta IIZUKA

2014 Volume 55 Issue 4 Pages 177-184
Published: October 10, 2014
Released on J-STAGE: April 27, 2015

DOIhttps://doi.org/10.5110/jjseg.55.177

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In determinating the producibility of a well, the s-Q curve obtained by step drawdown test can be approximated in many cases by the Jacob's empirical equation of s＝BQ＋CQ², and there is no discontinuity in the curve.
Mathematical maximum curvature point in Jacob's empirical equation is given by 0.5126B/C when plotted on log-log graph, but it cannot be said to the critical pumping rate of a well.
In some cases, an obvious discontinuity appears on a s-Q curve that cannot be approximated by the Jacob's empirical equation. Some of these cases, it is common that high pervious unconfined aquifer is underlain by impervious formation and well screen is installed also in the impervious layer. This discontinuity indicates the limit of pumping rate of a well and varies with water table fluctuation.

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