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Theoretical investigation of the kaiser effect manifestation in rocks after true triaxial pre-loadin |
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Theoretical investigation of the kaiser effect manifestation in rocks after true triaxial pre-loading Author: A. Lavrov
Kaiser effect takes place in rocks when they are cyclically loaded with the peak stress magnitude increasing from cycle to cycle. The effect consists in non-reprodicibility of acoustic emission activity at stress values lesser than the maximum previously applied ("memorized") stress. As soon as this "memorized" stress level is attained, acoustic emission activity increases dramatically. It allows to estimate the pre-stress level relatively simply. For practical application of the Kaiser effect to geo-stress measurements, it is necessary to know its features in rock samples which were under triaxial state of stress in situ. So far, all experimental and theoretical investigations of the Kaiser effect have been limited to the cases of uniaxial or axisymmetric triaxial pre-loading (crl a2 = 73). Here, the first attempt is made to study the Kaiser effect in rock samples which were under true triaxial stress state in situ ((71 r2 r3). The behaviour of a rock sample containing 1000 arbitrarily oriented penny-shaped cracks was stimulated. The rock sample was "loaded" in two cycles. The first cycle was true triaxial compression with maximum principal stresses different from each other: a[ u\ ffy The second cycle was a uniaxial compressive test as it is normally carried out on rock samples extracted from the rock mass. During this test, the samples was "loaded" in the direction of the first-cycle rj-axis. As a result, curves "acoustic emission activity versus stress" were obtained for the second-cycle loading for various combinations of the first-cycle principal stresses. Processing AE curves has shown that the Kaiser effect manifestation in rock samples, previously subjected to true triaxial in situ stress state, is far more complex than in the samples after axisymmetric triaxial pre-loading. In the former, the Kaiser effect is lesser distinct, the AE begins to increase from the beginning of the uniaxial loading and continues to rise to a1 = a\-(k+\)a\, and a sharp increase in AE activity is not observed. From the shape of the AE curve only, it is hardly possible to conclude about the type of the in situ stress state and to estimate absolute values of in situ principal sresses. The intermediate and the minimum in situ principal stresses have a pronounced influence on the shape of the AE curve and on the Kaiser effect stress value. This and other results of thw simulation are to be always taken into account when using the Kaiser effect for stress measurement in rocks.
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2009