ABSTRACT

A new application takes advantage on acoustic emission in order to seek out the "stress memory" of rock via Kaiser Effect. The method is based on one highly similar method that is currently in commercial use at Western Australian School of Mines. It utilizes multi-directional secondary samples which are tested for Kaiser Effect in uniaxial compression. Acoustic emission signatures from the tests are then analyzed, and the normal stress values thus obtained are reassembled in order to form the complete stress tensor. This method has been researched at TKK since 2005, at first on Master's thesis level and continued afterwards as an independent project. From 2006 on, acoustic emission research will continue in the form of post-graduate studies, which aim to produce an ISRM Suggested Method for acoustic emission measurement of in situ stress. Another non-standard in-situ stress measurement testing method called deformation rate analysis (DRA) will also be tested in the laboratory. The aim is to do DRA tests on sub samples taken from CSIRO HI cell overcoring sample and compare the results in order to verify the suitability of DRA tests as an in-situ stress measurement method for hard rocks in Finland. Third non-standard test experimented is the strain rate stepping test. The main goal of conducted strain stepping experiments was to provide well-documented laboratory test cases for comparison with data from numerical models. The results of strain rate tests can also be utilized directly for evaluation of time-to-failure of brittle rock in triaxial stress state by comparing the inelastic strain rates at different total axial strain rates.

1 INTRODUCTION

The upgraded rock mechanical testing facility at the Laboratory of Rock Engineering in Helsinki University of Technology (TKK) has been mainly utilized since its procurement in 1992 for standard uniaxial and triaxial compression tests, as well as indirect tensile (Brazilian) tests. In addition, several non-standard and experimental testing programs have been conducted during past years. The initial system has been gradually augmented in 1990-ies during these projects; a strain gage measurement system was purchased in 1994 and an acoustic emission system in 1999. In years 2005 and 2006, three new testing programs, which utilize the potential of all available equipment, have been running. Two out of these three methods are in essence indirect methods for measurement of in-situ states of stress, which are both currently considered as potential methods without large-scale acceptance. The first one is called the Kaiser effect (KE) of acoustic emission, and is based on the "memory effect" of rock.The memory is recalled by monitoring acoustic emission under uniaxial loading of a specimen, and the threedimensional stress tensor is reassembled from six memorized stresses in independent directions. The method is similar to the one used by Villaescusa et al. (2002). The second testing method, deformation rate analysis (DRA) also uses the stress memory of rock, but seeks it out with a different indicator.

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