The Kaiser effect of Shirahama sandstone was investigated using the three-dimensional source locations of the acoustic emissions (AEs) detected during two kinds of uniaxial cyclic loading in elastic stage. In the experiments AEs were detected using eight transducers in order to elucidate the relation between the migration of the AEs' sources in the rock and the appearance of the Kaiser effect. Based on the data relative to the AE source location, the Kaiser effect can be interpreted as follows: newly formed microcracks grow to an equiblibrium configuration (e.g. some characteristic length) corresponding to the previously applied stress and this stress would be memorized in the rock through microcrack configuration.
L'effet Kaiser du qre Shirahama a ete etudie par le sondage tridimensionnel des sources'd'emission acoustique (EA), phoenomene pouvant être detectee par les deux types d' essai repetitif de charge axiale en etat elastique. Au cours de ces essais, les 8 capteurs assurent la deplacement de sources EA dans les roches et l'apparition d'effet Kaiser. Les resultats d'essai sur les sources EA nous permet d' expliquer cet effet: les nouveaux micro- fissures se produisent en configuration d'equilibre ayant une lonqueur caracteristique en fonction des contraintes precedemment appliquees du fait provablement que celles-ciont ete retenues dans les roches a travers l'emergence des micro-fissures.
Der Kaiser-Effekt von Shirahama- Sandtein wurde unter Verwendung der wahrend zwei Arten von einachsigen zyklischen Belastungen m der elastischen Stufe entdeckten dreidimensionalen Quellenorte der akustischen Emissionen (AE) untersucht. In den experimenten wurden AEs unter Verwendung von acht Wandlern entdeckt, um den Zusammenhang zwischen der Wanderung der AEQuellen im Fels und der Erscheinung des Kaiser-Effekts aufzuklaren. basierend auf den Daten relativ zum AE-Quellenort kann der Kaiser- Effekt wie folgt interpretiert werden: Neu geformte Mikrorisse wachsen zu einer Gleichgewichtskonfiguration (z.B. eine charakteristische Lange) entsprechend der vorber angelegten Spannung, und diese Spannung wird im Fles durch die Mikroriβkonfiguration gespeichert,
Estimating in-situ stress is an important step in the analysis and the design of any underground rock excavations. Several methods have been proposed and developed before to measure the in-situ stress in the rock. However, the cost and time restriction associated with each measurement has prevented the extensive use of the established stress measurement techniques which include the overcoring method and the hydraulic fracturing method. An alternative method is to estimate the stress by utilising the drilled cored rock. One of the stress measurement methods to use cored rock which can be collected from the borehole is the AE method to measure the in-situ stress using the Kaiser effect of AE. Since the Kaiser effect was discovered in 1948(Kaiser,1948), there have been many attempts to use the Kaiser effect to determine in-situ stress (Goodman,1963; Yoshikawa and Mogi,1978,198l: Kurita and Fujii,1979; Hayashi et al.,1979; Kanagawa et al,198l; Murayama et al.,1984; Momayez and Hassani,l992; Seto et al.,l992; Holcomb; 1993a, b). However these studies have not actually explained thoroughly the Kaiser effect itself until now. We think that the Kaiser effect of the rock has close relations with microcracking in the rock. We, therefore, made experimental studies to understand the relations between the Kaiser effect of AE and microcracking in the rock as the fundamental study of the Kaiser effect of the rock. The purpose of this paper is to show the relations between the Kaiser effect of AE and microcracking in the rock using the three dimensional distribution of hypocentres of AE generated during the cyclic loadings. In the experiments two different kinds of loading patterns were applied to the sandstone specimen together with the AE measurement which includes event counting and waveform recording.
The rock tested was commercially called Shirahama sandstone and the uniaxial compressive strength is 60MPa. The specimen tested was a rectangular one with 30mm length, 30mm width and 60mm height. As Shirahama sandstones tested were collected at the depth of maximum 80m below the surface of the earth, they had already the stress history corresponding to the depth. The parallelism between ends was set within ±1/50 mm. Generally, sandstone has bedding planes, and P wave velocity is minimum in a direction perpendicular to the planes. P wave velocity in our specimen under atmospheric pressure were 2.94l±0.007km1sec in a direction perpendicular to bedding planes and 3.161 ± 0.096km/sec in a direction parallel to the plane, respectively.
