Friday Haggler

A reminder of last Friday’s Haggler

The picture below shows failure in a strong, unfractured rock due to insufficient radial stress (mud density) in a vertical hole. Which is the minimum horizontal stress, A or B? Bonus: where was this hole excavated?

The Answer

Short Answer: A is the minimum stress and the hole is located approx. 120km N.E. of Winnipeg, Manitoba.

Long Answer: First off, I need to fess up to a mistake; this is not showing a vertical hole, it actually shows a section of a 46m tunnel that was excavated at level 420 (420m TVD) of the Underground Research Laboratory (URL) established by the Atomic Energy of Canada Limited (AECL). The 46m long, 3.5m diameter Mine-by Experiment test tunnel was excavated between 1990 and 1992 with the objective of studying the processes involved in progressive rock failure for research into atomic energy waste disposal. It showed that even very strong rock will fail if the imposed stresses exceed the mechanical strength of the formation. The tunnel was excavated parallel to the intermediate principal stress so that the differential stress was very high, promoting the maximum development of stress-induced damage. There is a great paper on the experiment at the link below:
https://inis.iaea.org/collection/NCLCollectionStore/_Public/27/065/27065127.pdf?r=1