ABSTRACT

Irreversible deformation in brittle rocks is accompanied by acoustic emissions. In sequential loading series with increasing load one can estimate the previously applied maximum load by using the so-called Kaiser effect: An abrupt rise of acoustic emissions can be observed, as soon as the applied load exceeds the former maximum load. So far there is only little knowledge about in which way a prestress in a different direction than the applied load affects the occurrence of the Kaiser effect. An experimental campaign was carried out with granite parallelepiped specimens to win valuable clues about the influence of an axis rotation on the acoustic emissions and the rock mass properties. For this purpose we compared the acoustic emissions of specimens with and without a perpendicular preload. The results of the tests indicate that neither the rock strength nor the acoustic emissions are influenced by a preloading orthogonal to the reloading direction. The results are in accordance with those of other researchers (e.g. Stuart et al., Lavrov et al.) stating that no Kaiser effect can be detected in specimens uniaxially and sequentially loaded in orthogonal directions. Other interesting results on the acoustic emissions were achieved at low stress levels, which can be explained by some crack propagation theories.

INTRODUCTION

Design and operation of underground excavations require accurate information about in-situ stresses. As an alternative to conventional stress measurement techniques, the acoustic emission technique in combination with the Kaiser effect is being explored since the past decades. This method is based on the ability of brittle material to accumulate, retain and reproduce II1formation about the stresses experienced in the past, the so-called "stress-memory".

Explanations for these phenomena can be made with known crack propagation theories.

Acoustic Emission-technique

The Acoustic Emission technique is a non-destructive method allowing the detection and the monitoring or the micro-structural changes in an object. Under the action of compressive stresses or tension brittle rocks progressively accumulate damage as micro-cracks nucleate, grow and coalesce. The AE-technique relies on the fact that cracks usually form dynamically, emitting packets of high frequency elastic energy that Can be detected by using the appropriate (piezoelectric) transducer (Cox, 1991).

The Kaiser effect

The so-called Kaiser Effect can be observed in brittle materials, such as rocks. If a rock sample is loaded (Fig. I), (σ1), acoustic emissions (AE) can be detected (I). If the sample is unloaded and then reloaded, no

(Figure in full paper)

Crack growth

The Kaiser effect can be well explained with the growth and propagation of cracks in the material. It is well known that small microcracks exist in brittle materials. At a certain stress level ali the weaker elements in the structure begin to fail and wings cracks emerge at the tips of existing cracks (Fig. 2(a) and (b)). If the sample is then unloaded, the cracks remain in the rock mass (Fig. 2(c).

During a reloading, no new cracks will be created until the former stress level σ10 the weak elements have already failed. is reached, because

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