임계응력 하 거친 암석 균열의 Thermoshearing 수치모델링: 국제공동연구 DECOVALEX-2023 Task G
Numerical Modeling of Thermoshearing in Critically Stressed Rough Rock Fracture: DECOVALEX-2023 Task G
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초록
본 연구에서는 3차원 입자기반 개별요소모델(grain-based distinct element model, GBDEM)을 이용하여 암석의 거친 균열에 대한 Thermoshearing 실험 결과를 모델링하였다. Thermoshearing 실험은한국건설기술연구원에서 수행된 실험으로 임계응력 상태에 놓인 균열에 열을 가하고 열응력의 발생에따른 균열의 점진적 파괴 거동을 살펴본 실험이다. 본 연구에서는 해석 대상을 다수의 사면체 입자의 집합체로 모델링하고, 개별요소법 코드인 3DEC을 통해 암석과 균열면의 열적-역학적 연계거동 특성을 분석하였다. 주요 해석 내용은 가열에 따른 온도 변화, 열응력의 발생, 균열의 전단변위 및 수직변위 등이다. 여기에서는 균열면의 맞물림(matedness) 정도에 따라 두 가지 모델(mated fracture model, unmated fracture model)을 구성하고 그 거동 특성을 비교 분석하였다. 또한, 개별요소 코드의 장점을 활용해, 시간의 경과에 따른 균열면 접촉면적을 지속적으로 업데이트하고 전단거동에 미치는 영향을 살펴보았다. 해석 결과, 균열면의 맞물림 정도에 따라 두 모델은 확연히 다른 전단거동을 나타내었다. 균열면이 완전히 맞물린 모델(mated fracture model)의 경우, 실험 결과를 적절히 모사하지는 못하였지만, 일반적인 암석 절리면의 전단거동에서 관찰되는 정점응력(peak stress)과 잔류응력(residual stress) 단계를합리적으로 재현하는 것으로 나타났다. 또한, 접촉면적 분석을 통해 상기 모델이 정점응력 상태에서 나타나는 급격한 수직팽창과 전단 미끄러짐, 응력연화 현상과 잔류 상태로의 전이 등을 적절히 모사할 수있음을 확인하였다. 한편, 실험 시험편의 형상을 동일하게 재현한 맞물리지 않은 모델(unmated fracture model)의 경우 실험에서 측정된 열전달 특성, 가열에 의한 응력의 증가와 균열의 점진적 전단파괴 프로세스 등을 합리적인 수준에 재현하고 있는 것으로 나타났다. 단, 가열에 의한 전단파괴 시점, 열응력 증분과 변위 크기 등에서는 다소 차이를 보였다. 본 연구는 DECOVALEX-2023 Task G의 일환으로 수행된 연구로서, 향후 국외 연구팀들과의 의견 교류 및 협력을 통해 해석모델을 지속적으로 개선, 검증할 예정이다.
Abstract
In the present study, the thermoshearing experiment on a rough rock fracture were modeled using a three-dimensional grain-based distinct element model (GBDEM). The experiment was conducted by the Korea Institute of Construction Technology to investigate the progressive shear failure of fracture under the influence of thermal stress in a critical stress state. The numerical model employs an assembly of multiple polyhedral grains and their interfaces to represent the rock sample, and calculates the coupled thermo-mechanical behavior of the grains (blocks) and the interfaces (contacts) using 3DEC, a DEM code. The primary focus was on simulating the temperature evolution, generation of thermal stress, and shear and normal displacements of the fracture. Two fracture models, namely the mated fracture model and the unmated fracture model, were constructed based on the degree of surface matedness, and their respective behaviors were compared and analyzed. By leveraging the advantage of the DEM, the contact area between the fracture surfaces was continuously monitored during the simulation, enabling an examination of its influence on shear behavior. The numerical results demonstrated distinct differences depending on the degree of the surface matedness at the initial stage. In the mated fracture model, where the surfaces were in almost full contact, the characteristic stages of peak stress and residual stress commonly observed in shear behavior of natural rock joints were reasonably replicated, despite exhibiting discrepancies with the experimental results. The analysis of contact area variation over time confirmed that our numerical model effectively simulated the abrupt normal dilation and shear slip, stress softening phenomenon, and transition to the residual state that occur during the peak stress stage. The unmated fracture model, which closely resembled the experimental specimen, showed qualitative agreement with the experimental observations, including heat transfer characteristics, the progressive shear failure process induced by heating, and the increase in thermal stress. However, there were some mismatches between the numerical and experimental results regarding the onset of fracture slip and the magnitudes of fracture stress and displacement. This research was conducted as part of DECOVALEX-2023 Task G, and we expect the numerical model to be enhanced through continued collaboration with other research teams and validated in further studies.
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