INTRODUCTION
The deformation and failure of rock are associated with the growth and coalescence of cracks. The concept of damage is more useful than the formalism of plasticity in describing brittle rocks [ 1 ]. Acoustic emission (AE) generated in a loaded rock is a reflection of the cracking process in the material. Laboratory experiments [2] have demonstrated that AE was absent in rock until the load exceeded the previous maximum load that had been imposed to rock before. This is the socalled Kaiser effect. In the tests, rock specimens were uniaxially loaded and unloaded repeatedly. The maximum stress level was increased for each loading cycle, generating further damage in the specimens. The development of damage in the specimens was reflected by an increased AE activity. It has been suggested that the onset of AE could be used to detect and assess the amount of damage that has developed in a rock [3 ].
The objective of this study is to explore the possibility of assessing the in-situ damage of rocks by the Kaiser effect. Cores were drilled from a hard rock runnel. The specimens prepared from the cores were uniaxially compressed in laboratory and the AE emitted from the specimens was recorded. The onset of the increasing AE was taken as an indication of the degree of damage in the rock after blasting. The results showed that the AE-onset stress decreased with the distance from the tunnel contour. The range of disturbance caused by blasting could be estimated using the curve of the AE-onset stress.
A DESCRIPT? MODEL OF DAMAGE IN ROCK SURROUNDING BLASTING
Underground blasting causes damage to the surrounding rock. The degree of damage decreases with an increase in the distance from the blasting point. The surrounding rock can, in general, be divided into three zones based on the degree of damage, see Figure 1. They are the strength-reduced, disturbed and un-disturbed zones, respectively. Heavy damage occurs in the strength-reduced zone. This damage may include both the growth and coalescence of pre-existing cracks and creation of new cracks in the rock. With further increase in distance, the degree of damage decreases, while the strength increases and reaches the level of the virgin rock at point B in Figure 1. In the disturbed zone, the strength of rock is not reduced, but a certain damage has occurred in the rock. The damage in this zone is likely to be dominantly represented by the growth of pre-existing cracks rather than the creation of new cracks and coalescence of cracks. Beyond point C in Figure 1, the blasting stress causes no damage in the rock. The damage occurring in the disturbed zone is similar to the progressive damage of rock under uniaxial compression in the pre-peak stage, and might be detectable using the Kaiser effect in AE tests. As stated before, the AE-onset stress is a measure of the damage in rock. The higher the AE-onset stress is, the more damage the rock has suffered. Since the blasting stress would be attenuated rapidly, the degree of damage in this zone is also reduced with an increase in the distance.
