Evaluating long-term strength of rock under changing environments from air to water

Evaluating long-term strength of rock under changing environments from air to water

Significance Statement

The long-term stability of a rock mass should be accounted for during the design and construction of subsurface structures within a rock mass. Such subsurface structures include: underground power plants, repositories for radioactive waste, caverns for storage of natural gas or liquefied petroleum gas. It therefore becomes a necessity to understand the time-dependent fracturing of a rock and its direct influence on the strength. More so, time-dependent fracturing has been invoked as an important mechanism responsible for the increase in seismicity preceding earthquake ruptures and volcanic eruptions. It is known that evaluating the long-term strength of a rock is important in ensuring the long-term stability of a rock mass, considering the design and construction of various structures within it. The long-term strength of rock is normally evaluated under the same environmental conditions. However, in practice, the environment is persistently changing and must be accounted for in evaluating the long-term strength of the rock.

Researchers led by Professor Yoshitaka Nara at Kyoto University in Japan developed a new technique to evaluate the long-term strength of rock under changing environmental conditions, with a focus on the influence of water on subcritical crack growth in rocks. They aimed at providing clarity on the influence of water on the long-term strength of the rock and the long-term integrity of a rock mass surrounding various structures. They also hoped to provide further insight on the time-dependent deformation and fracturing in rock to evaluate its long-term strength by utilizing subcritical crack growth, which provides insight into the weathering of a rock mass over the long term. Their research work is now published in the Engineering Fracture Mechanics.

The research team conducted their tests by investigating the effects of water on the long-term strength of various rock samples including: gabbro and sandstone in both air and water medium. They first analyzed the mineral composition of the two rocks to be used in order to understand their cleavage and crystal morphology. Porosities, Young’s modulus, uniaxial compressive strength and the fracture toughness of the rocks were also obtained.

The research team observed that the long-term strength of the rocks decreased rapidly when the environmental conditions changed from air to water. They further observed that when the environmental conditions were changed repeatedly from air to water medium, the long-term strength achieved was identical to that obtained in a continuous water environment. In totality, the authors of this paper agreed that the water environment had the most significant effect on the long-term strength reduction in rock.

Knowledge exists that subcritical crack growth in rocks accelerates most in under water environments. The remarkable decline in the long-term strength of a rock in a water environment indicates a significant acceleration of crack propagation in the rock. Therefore, to ensure the long-term stability of a rock mass surrounding various structures, it is essential to understand the effects of water in which crack propagation is accelerated for the long-term strength estimation in aquatic environs. Effects of water should therefore be considered in the long-term use of rock structures.

 Thank you to Author

Yoshitaka Nara is currently Associate Professor in the Graduate School of Engineering, Kyoto University, Japan. He has completed Ph.D. in March 2004 in Hokkaido University, Japan. From April 2004 to March 2011, he has been a Postdoc in Hokkaido University. From October 2008 to October 2009 and from March 2010 to August 2010, he has stayed in University College London, UK, as a Postdoc. From April 2011 to March 2014, he has belonged to Kyoto University, Japan, as Assistant Professor. During this term, he has stayed in University of Minnesota, USA, from August to October in 2011, and University College London from March to May in 2012, as a visiting researcher. From April 2014 to March 2016, he has belonged to Tottori University, Japan, as Associate Professor. From April 2016, he has been in his current position.

His research interests concern the studies of subcritical crack growth, fracture toughness, crack distribution, and permeability of rocks to consider the application to rock engineering projects, such as geological disposal of radioactive wastes, carbon capture and storage, constructions of underground power plants, etc.

Reference

Yoshitaka Nara, Mayu Tanaka, Tomoki Harui. Evaluating long-term strength of rock under changing environments from air to water. Engineering Fracture Mechanics volume 178 (2017) pages 201–211.

http://www.sciencedirect.com/science/article/pii/S001379441630443X