The objective of this study is to conduct a laboratory investigation of AE in rocks and explore the possibility of its application to evaluate the damage in rock mass. The emphasis of this paper will be on the verification of the Kaiser effect in different stress levels and the influences of stress state on the Kaiser effect. And, we suggest a new parameter, which was produced by AE measurements of differently damaged rocks, in order to evaluate the rock damage.

In the experiment, rock specimens were uniaxially or triaxially loaded under compression by means of a servo-controlled testing machine. Average uniaxial compressive strength of Gosford sandstone used in this study was 40 MPa in dry condition. This study gives better understanding of the behaviour of rock under different stress conditions, eventually this can be used to evaluate the rock mass damage. The following main results can be obtained:

1. The breakdown of the Kaiser effect in rock can be represented quantitatively by the felicity ratio that is defined as the ratio of the AE-onset stress to the maximum previous stress. A high felicity ratio means that the rock is of good quality. The felicity ratio remains at the level of near 0.9 from the low stress level to about 80% of the ultimate strength, but it decreases rapidly to around 0.4 when the stress approaches to the strength.

2. The felicity ratio is likely to be one of the effective parameters to evaluate the damage of rock, if it is possible to know the maximum stress previously applied stress. Practically, however, the information which we can obtain from the rock specimen are "strength", "applied stress in the test", and "the onset of AE increase" in the test. Therefore, we suggest the rock damage parameter ( RDP), expressed by the following equation, to evaluate the rock damage.
   

   where,

   _AE: stress of AE onset

   S_c: uniaxial compressive strength of the specimen

   _applied: applied stress in the test

   The parameter decreases dramatically with the maximum stress level. When the maximum previous stress is less than 50% of the strength, which means it is within elastic range, the parameter is higher than 1.0. A high RDP, higher than 1.0, means that the rock is of good quality.

3. We applied the new parameter to the rock triaxially preloaded. The evaluated result (RDP=1.11) is consistent with the previous loading condition, axial stress 20 MPa and confining pressure 10 MPa, which is within elastic range.