Distinct element modeling for Class II behavior of rock and hydraulic fracturing

岩石のクラスII挙動及び水圧破砕のための個別要素法解析

清水 浩之*; 小山 倫史*; 村田 澄彦*; 石田 毅*; 千々松 正和*; 藤田 朝雄 ; 中間 茂雄 

Shimizu, Hiroyuki*; Koyama, Tomofumi*; Murata, Sumihiko*; Ishida, Tsuyoshi*; Chijimatsu, Masakazu*; Fujita, Tomoo; Nakama, Shigeo

本研究では、個別要素法(DEM)を用いて新たに構築した数値解析法を示し、岩盤工学における二つの問題についての物理現象及びメカニズムの解明のためのDEM解析を実施した。まず、半径方向のひずみを制御した一軸圧縮試験のDEM解析を行い、岩石のクラスII挙動を解析した。その結果、DEM解析によりクラスII挙動が再現可能であること、岩石の載荷条件がクラスII挙動に重要な役割を果たしていることがわかった。次に流体を連成させたDEM解析により水圧破砕の解析を実施した。その結果、水の浸透挙動は水の粘性に依存することを示した。粘性が低い時は、水は亀裂にすみやかに浸透し、粘性が高い時は、亀裂の発生・伸展の後、亀裂へゆっくり浸透した。さらに従来の理論と同様にDEM解析においても引張亀裂が優先的に発生した。しかし、岩石の引張強度は圧縮強度よりも小さいため、引張亀裂から放出されるエネルギーは小さい。このような小さなAEイベントはノイズと見分けることが困難である。よってAE観察においては、大きなエネルギーのせん断型のAEイベントが優先的に観察された。

In this research, newly developed numerical approaches using the Distinct Element Method (DEM) were presented, and a series of DEM simulations were performed for better understanding the physical phenomena and mechanism for the following two fundamental issues in rock engineering field. The first issue is the Class II behavior of the brittle rocks under uniaxial compression. The radial strain control method for uniaxial compression tests was introduced in the DEM codes and the Class II behavior of rocks was simulated. The simulation results suggest that the DEM can reproduce the Class II behavior of the rock successfully and revealed that the loading condition of rocks will play an important role for the Class II behavior. The second issue is the hydraulic fracturing behavior in rocks. A series of simulations for hydraulic fracturing in rock was performed by using the flow-coupled DEM code. Simulation results clearly show that the fluid infiltration behavior depends on the fluid viscosity. The fluid infiltrates into the fracture immediately, when a low viscosity fluid is used and the fluid infiltrates slowly into the cracks after the fracture generation and propagation, when a high viscosity fluid is used. Moreover, the tensile cracks are dominantly generated in the DEM simulations as expected in the conventional theory. However, the energy released from tensile cracks becomes smaller due to the fact that the tensile strength of rock is usually smaller than the compressive one. Such a small AE events is not distinguishable from noise and hard to recognize during laboratory experiments. Therefore, in AE measurements, shear type AE events with large energy are dominantly observed.