Analysis of Fracture System in Granitic Rock

Abstract

In order to evaluate the phenomena of masstransportation by groundwater flow in granitic media, it is necessary to make clear geological, hydrogeological and geochemical characteristics of fracture distributed in rock mass.Characteristics of fracture pattern, fracture filling material and alteration in Upper Cretaceous granite are investigated by core logging, mineralogical study, geophysical logging and hydraulic testing. This report describes the results which were obtained in the interval from the ground surface to 500m at Tono area in the central part of Japan.The results are summarized as follows ;(1) Fractures are classified into four groups on the basis of fracture pattern; Planar group (P), Irregular group (I), Curved group (C), and Stepped group (S).(2) Most of fractures belonging to P group and S group have filling materials composed of chlorite, calcite and sericite. Some of the fractures belonging to I group have been filled with materials composed of montmollironite, sericite and chlorite. Fractures of C group hardly have filling material.(3) Some of alterated fracture zones are consisted of P group fractures.(4) As a result of analysis on the fracture pattern and alteration pattern, three segments are recognized; Segment I (from 16.8m to 300m), Segment II (from 300m to 420m), and Segment III (from 420m to 500m). Especially, there are marked differences in fracture pattern and alteration pattern between Segment I and Segment III.