ボーリングコア側面の非弾性ひずみと初期応力の関係について

Abstract

At three boring sites, principal strain orientations obtained from anelastic strain recovery on core surface were compared with in-situ stress states determined by overcoring or hydraulic fracturing method. Orientations of time-dependent maximum expansion of a core taken from the vicinity of a large mined goaf indicated that they presumably reflected the maximum stress state when the core was retrieved and they were totally different from the stress orientations determined by overcoring technique. Three dimensional principal strain orientations obtained from anelastic strain recovery on surface of granodiorite cores (150mm in diameter) were in good agreement with the in-situ stress state predicted by overcoring method. And anelastic strain recovery measurements on Mannari granite were carried out immediately after the 38 mm diameter cores were removed from 300 m deep borehole, where none of three principal axes of strain coincided with the borehole axis. Laboratory measurements of compressional velocity on relaxed Mannari cores showed that the direction of the slowest velocity correlated with the one of maximum expansion and the strike of microcracks in quartz crystals. Then it is suggested that three dimensional stress state can be determined by combination of hydraulic fracturing test at single borehole section and observation of anelastic strain variations of the core taken from the borehole.