Overcoring is a measurement method used for estimating the state of stress in a rock mass. The method is based on measuring the strains that develop when a sample of rock, by drilling with a hollow core bit, is released from the surrounding rock and the stresses acting upon it.
During the current work to find a site suitable for a future underground repository for final disposal of spent nuclear fuel in Sweden, the overcoring stress measurement method was used in the site investigation programme. In some of these tests temperature effects have been noted, which affect the reliability of the results. The causes and consequences of these effects have not been investigated though. This thesis work was conducted to address part of the issue of temperature effects during overcoring.
The main part of the thesis consisted of a study of which of the parameters (i) logger, (ii) glue, (iii) strain gauges and (iv) material type that were most sensitive to temperature effects during overcoring. The temperature sensitivity was determined by measuring the strains recorded by the logger when heating each object in different test cases.
All of the factors logger, glue and material showed a response dependent on temperature. The response of the logger was so small in magnitude that it could be neglected on basis of the small magnitude of the absolute error it caused on calculated stresses.
The glue showed a response of fairly great magnitude regarding temperature change when not fully cured. When subjected to temperature increase, the curing of the glue was reactivated, causing shrinkage of the material and affecting strain readings to seem smaller in magnitude than the true value.
The rock cores tested showed a temperature response of great magnitude. The response was not that of an ideal material as the cores did not expand and contract as expected when subjected to temperature change. The response could thus not be quantified based on the tests performed
It is not known if these factors are independent or dependent of each other and hence it is hard to tell which factor that is most sensitive to temperature increase during an actual overcoring. The temperature effects of the glue and rock were difficult to quantify and thus also difficult to compensate for. Therefore they should be avoided by minimizing the temperature increase caused by overcoring. This can be done by better flushing and temperature control, by a larger overcoring diameter or possibly by a different overcoring feed speed or different drill bit.
Author: Bertilsson, Rebecca
Source: Lulea University of Technology
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