포화유무에 따른 KURT 화강암의 균열손상 기준 및 수정 파괴인성 측정 (Level II Method)
Evaluation of Stress Thresholds in Crack Development and Corrected Fracture Toughness of KURT Granite under Dry and Saturated Conditions
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본 연구의 목적은 KURT 화강암 시료의 포화유무에 따른 균열손상 기준과 파괴인성의 변화를 측정하는것이다. 이를 위하여 일축압축시험을 이용한 소성체적변형률을 통해 KURT 화강암의 균열손상 기준을도출하였다. 또한 암석의 파괴인성을 보다 신뢰성 있게 측정하기 위해 암석의 비선형적 변형에 대한 보정(Level II Method; ISRM, 1988) 을 통해 포화유무에 따른 KURT 화강암의 수정 파괴인성(corrected fracture toughness)을 측정하였다. 시험결과 건조시료의 평균 균열개시 응력(σci)과 균열손상 응력(σcd)은91.1 MPa과 128.7 MPa이었으며, 포화시료의 평균 균열개시 응력(σci)과 균열손상 응력(σcd)은 58.2 MPa과 68.2 MPa이었다. 건조시료에 비해 포화시료의 균열개시 응력은 36% 감소하였으며 균열손상응력은 건조시료 대비 47%나 감소되는 결과를 나타내었다. 균열손상 응력(σcd)이 상대적으로 더욱 감소하였음을 감안할 때 시료의 포화로 인해 더 낮은 응력조건에서 구조물에 대한 손상이 쉽게 발생할 수있음을 알 수 있다. KURT 화강암의 비선형성을 고려한 수정 파괴인성은 0.811 MPa·m0.5이었으며 포화시료의 수정 파괴인성은 0.620 MPa·m0.5이었다. 즉 암석의 비선형성을 고려함으로써 파괴인성의 증가를 확인할 수 있었으며, 암석의 포화시 수정 파괴인성은 24% 감소하였다. 따라서 지하수 포화로 인해암석 내 균열의 생성과 진전에 대한 저항성이 감소함을 알 수 있다.
Abstract
The objective of this study is to evaluate the stress thresholds in crack development and the corrected fracture toughness of KURT granite under dry and saturated conditions. The stress thresholds were identified by calculation of inelastic volumetric strain from an uniaxial compression test. The corrected fracture toughness was estimated by using the Level II method (Chevron Bend specimen), suggested by ISRM (1988), in which non-linear behaviors of rock was taken into account. Average crack initiation stress(σci) and crack damage stress(σcd) under a dry condition were 91.1 MPa and 128.7 MPa. While, average crack initiation stress(σci) and crack damage stress(σcd) under a saturated condition were 58.2 MPa and 68.2 MPa. The crack initiation stress and crack damage stress of saturated ones decreased 36% and 47% respectively compared to those of dry specimens. A decrease in crack damage stress is relatively larger than that of crack initiation stress under a saturated condition. This indicates that the unstable crack growth can be more easily generated because of the saturation effect of water compared to the dry condition. The average corrected fracture toughness of KURT granite was 0.811 MPa·m0.5. While, the fracture toughness of saturated KURT granite(KCB) was 0.620 MPa·m0.5. The corrected fracture toughness of rock in saturated condition decreases by 23.5% compared to that in dry condition. It is found that the resistance to crack propagation decreases under the saturated geological condition.
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