In the 1980's HTM experiments in Stripa, Climax, Hanford, CSM/Terra Tek and Sandia heated block tests and Yucca Mountain, there were unexpected results caused by joint over-closure when heated. The effect needed JRC to be significant. 'Roughness profiles' apparently fitted together better when heated. Moduli, thermal expansion coefficients, Vp, apertures, permeability, shear strength were each affected. We have not yet tackled thermal over-closure in UDEC-BB. See review in Barton, 2007 if concerned, as we should be. In the case of nuclear waste disposal it is a source of risk when cooling, if joint sets of different roughness are present.
A little more detail. Deformation moduli much higher than expected (2:1 even 4:1) due to relative easier closure of rough joint sets when heated. So Vp, aperture etc affected. When cooling the rough joints stay closed (as measured) while the smoother ones will open more to compensate - therefore exaggerating increased permeability and reduced shear strength. This is not unfortunately, isotropic continuum behavior. But it IS reality that we cannot ignore. More lab and in situ thermal cycle
testing needed before we can write appropriate software.
Thermal overclosure? There is the local thermal gradient and overburden pressure to start with. In the case of a deep nuclear waste repository, then what is the likely increase in temperature compared to the imposed by the existing thermal gradient? Hydrated minerals in serpentinised ultrabasic rocks may alter as their composition may become unstable. The resultant hotter fluids may then start to ciculate upwards. Then again, it is known that joints become tighter with depth and temperature as many minerals will tend to deform in a more plastic manner. Hence the temperature difference caused by the nuclear waste may be of some importance. The subject is definately very exciting. Thanks Nick for sharing it !
Georgios! The over-closure phenomenon is specifically to do with how rock joints respond to stress and/or temperature change. If - without heat - you load a rock joint to 4 x or 8x the intended normal stress and perform direct shear tests, you will get three different strength envelopes (assuming plenty of similar JRC, JCS samples). This nearly caused me to fail in my 1971 Ph.D. studies, as steep excavated slopes in jointed (tension-fractured) models with 40,000 blocks almost refused to fail. All the steep joint sets had been over-closed in relation to the reduced normal stress prior slope failure - in the case of 'green-field' excavation of a new pit. The O-C over-closure effect is accentuated with temperature change in the case of rough joints. Just cycling temperature gives improved closure, temporarily reduced normal stiffness, permanently increased modulus (following the closure), and reduced aperture, increased Vp.
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