Extensive laboratory investigations have resulted in the development of Kaiser Effect Gauging as a practical system to determine the pre-existing in-situ stress by Acoustic Emission (AE) analysis of an extracted core. The principle is based on the phenomenon of Kaiser Effect which is a characteristic AE rate increase as an increasing stress exceeds the previous maximum stress. It is further based on a newly identified AE characteristic which notes that if stress is maintained at a new historic maximum, continuing AE decays exponentially at a rate related to the material''s remaining stable strength.
Des recherches approfondies en laboratoire ont mené au développement de la méthode Kaiser Effect Gauging. Il s''agit d''un système pratique pour déterminer la contrainte in-situ préexistante provenant d''une carotte extraite par l''intermédiaire de l''analyse d''Emission Acoustique (AE). La théorre repose sur le changement de Kaiser Effect de la vitesse AE lorsqu''une contrainte croissante dépasse la contrainte maximum précédente. La théorie est aussi basée sur un caractéristique AE, nouvellement identifié, selon lequel si la contrainte est maintenue au nouveau niveau maximum, le AE restant ralentit de façon exponentielle à une vitesse relative à la capacité restante de la matière.
Ausführliche Laboruntersuchungen haben zu der Entwicklung von Kaiser Effect Gauging geführt, einem praktischen System zur Bestimmung der vorhergegangenen örtlichen Spannung mit Hilfe der Acoustic Emission (AE) Berechnung an einem Bohrkern. Das Prinzip beruht auf dem Phänomen des Kaiser Effects, dem Anstieg der characteristischen AE-Rate sobald eine anwachsende Spannung die vorausgegangene Maximalspannung überschreitet. Es basiert zudem auf einer kurzlich identifizierten AE-Charakteristik die besagt, daβ anhaltende AE mit einer Geschwindigkeit, die mit der verbleibenden stabilen Materialfestigkeit zusammenhängt, exponential abklingt, wenn die Spannung auf einem neuen Maximalspannungsniveau gehalten wird.
The design of an adequate support structure requires two basic data components: the magnitude of the stress to be carried; and the strength of the selected structural material. In a structure such as a mine, the reliability of both of these data components may be in doubt. In order to allow for the resulting reduced confidence in the adequacy of such a structure, it is normal practice to generously size its components. The effect of this conservative practice may be a low extraction ratio and elevated production costs. More readily obtained and reliable data on the stresses to be carried by a structure and a better indication of its capability for carrying more, could result in improved confidence in its structural adequacy. This could lead to improved extraction ratios without sacrificing safety margins. It could also lead to awareness of hazardous situations which had not been otherwise identifiable. A number of techniques are currently available for determining the in-situ stress (14). Among these are: overcoring, hydraulic fracturing, flat jack, stressmeter, and sound propagation velocity. There are also recognized procedures for assessing the load-bearing capacities of materials. A fundamentally new method has been developed for both the determination of in-si tu stress, and the assessment of remaining additional load capacity of the material. This method is titled Kaiser Effect Gauging and is based on the natural phenomenon of Acoustic Emissions (AE). This paper will describe the development of Kaiser Effect Gauging and its application as a routinely workable and practical technology.
The phenomenon of Acoustic Emissions was first identified in the late 1930''s (13,7)-. AE consists of minute ultrasonic pulses that can be detected in substantially inelastic materials when subjected to compressive stress. The wave form is similar to t ha t: of a seismic event but the pulse frequency in the context of this paper is in the approximate range of 50 KHz to 500 KHz with pulse duration in the order of 2 mil seconds (4). The occurrence rate, depending on many variables, can be in terms of thousands per second. They can thus be individually indistinct and might be looked upon as a measurable but non-uniform continuum of events. Although the mechanics at the AE source is not fully understood, (4,12) it is observed that the behaviour is consistent with their being acoustic manifestations of micro-sized inelastic strain occurrences within the material. An early researcher in Acoustic Emission, Dr. Eng. Joseph Kaiser of Munich drew attention to a specific relationship of stress and the resulting pattern of AE (8). He observed that when the stress on a polycrystallized metal is relaxed from a level of historic high, and then restressed, there is a significant increase in the rate of acoustic emission as the stress exceeds the previous maximum. This characteristic increase of AE at the transition from past experience stress into the new experience level has become known as the Kaiser Effect. Various researchers have tested a variety of materials (Table 1) and all have exhibited Kaiser Effect characteristic in their AE response.
