An examination of in situ rock stress estimation using the Kaiser effect

When rock cores are loaded in uniaxial compression, acoustic emission occurs when the stress reaches a level greater than that which the rock has previously experienced. This phenomenon, known as the Kaiser Effect, has been used as an indirect method of estimating the in situ stress field in rock masses. If the procedure is valid, then the Kaiser Effect method has the advantage that the stress field can be estimated through laboratory tests on rock cores, rather than the field drillhole methods of overcoring and hydraulic fracturing. However, the Kaiser Effect method is an enigma: on the one hand, there are good reasons why the method cannot be valid; on the other hand, there are reports in the literature of it being successfully applied. In this paper, we explore the variety of geological and mechanical factors involved and report on a case example for the Olkiluoto crystalline rock site in western Finland. We conclude that the Kaiser Effect method for stress measurement is only likely to be successful if it is supported by key geological and other stress measurement information and if certain procedures are followed as explained in the paper.