International Journal of Rock Mechanics & Mining Sciences 41 (2004) 927–938 Combined effects of increasing temperature and confining pressure on the fracture toughness of clay bearing rocks T. Funatsu a , M. Seto b, *, H. Shimada a , K. Matsui a , M. Kuruppu c a Department of Earth Resources and Mining Engineering, Kyushu University, Fukuoka, Japan b Institute of Advanced Industrial Science and Technology, Ibaraki, Japan c Western Australian School of Mines, Curtin University of Technology, Kalgoorlie, Australia Accepted 19 February 2004 Abstract In order to understand the effects of increasing temperature and confining pressure on the fracture toughness of clay bearing rock, fracture toughness experiments were carried out at temperatures elevated from room temperature up to 200  C using single edge- notched round bar in bending (SENRBB) and semi-circular bend (SCB) specimens of Kimachi sandstone and Tage tuff. This paper describes the methodology for evaluating level 1 fracture toughness and the crack growth resistance curve. The crack growth resistance curve is shown to yield true fracture toughness even when under-sized specimens are employed. The under-sized specimens refer to those which are too small to produce valid fracture toughness value using the standard methods. The fracture toughness of Kimachi sandstone did not vary significantly with temperature up to 125  C, but above that point, it increased with temperature. SENRBB tests showed that the level 1 fracture toughness increased by approximately 40% at 200  C over its value at room temperature. The fracture toughness of sandstone and tuff was found to be significantly affected by increasing confining pressure. For example, in the arrester orientation, the fracture toughness of Kimachi sandstone increased by approximately 470% at 9 MPa confinement over its value at atmospheric pressure. A quite similar variation of fracture toughness is caused by the combined effects of temperature and confining pressure. For example, under a confining pressure of 7 MPa, the fracture toughness of sandstone decreased with temperature up to 75  C, and then increased between 75  C and 100  C. r 2004 Elsevier Ltd. All rights reserved. 1. Introduction Fracture mechanics is becoming increasingly impor- tant in problems relating to rock strength in rock engineering. For example, fracture mechanics is utilized in the analysis of surface and underground mine structures. Determination of the strength and stability of rock structures is particularly important for nuclear waste containment in proposed underground reposi- tories. For the long-term use of an underground rock mass structure, the design must satisfy a high degree of structural integrity. For such an application, both creep behavior and stress corrosion must be considered in addition to strength. Microcracks usually exist in a rock mass, and they can grow in response to underground environmental conditions and in situ stress. As a consequence, the stability of a rock mass structure can be expected to change over time. Therefore, it is desirable to investigate the fracture toughness, which is an important rock property that characterizes the resistance of fracture initiation and propagation. In this investigation, the fracture tough- ness of two rock types was determined under different pressures and temperatures. A number of test specimen configurations and methods have been suggested to determine the fracture toughness of rock materials. The International Society for Rock Mechanics (ISRM) has incorporated the chevron-notched bend specimen [1] and the short rod specimen [2] into the standard method for measurement of the fracture toughness of rock [3]. The semi-circular bend (SCB) specimen proposed by Chong et al. [4] and the single edge-notched round bar in bending (SENRBB) specimen [5] are complementary to the standard method. Most rock fracture toughness tests have been performed under ambient temperatures and pressures. ARTICLE IN PRESS *Corresponding author. Tel.: +81-3-5501-0830; fax: +81-3-5501- 0855. Email-address: m-seto@aist.go.jp (M. Seto). 1365-1609/$-see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmms.2004.02.008