In-situ Stress Measurement Using Non-Destructive Techniques (NDTs)

Rock in depth is subjected to stresses due to overlaying burden and tectonic activities. When a mining activity is undertaken, the stress fields are disrupted, resulting in induced stresses around the openings. In deep underground mines, when these induced stresses exceed rock strength, life-threatening events like rock/coal bursts occur. To manage the rock/coal burst issues, accurate information on the stress magnitudes and their orientation is important. However, the measurement of in-situ stresses in deep mines is challenging as the conventional methods like hydraulic fracturing and overcoring (OC) suffer from inherent complexities and are difficult to apply in great depths and remote regions at a reasonable cost, making them impractical for many mining scenarios. Our research endeavours to address this pressing issue by developing and enhancing Non-Destructive Techniques (NDT), such as Deformation Rate Analysis (DRA) and Acoustic Emission (AE) for in-situ stress measurement from oriented cored rocks.

These methods of stress measurement have been used for many decades. Many researchers question the effectiveness of these methods because they believe that stress memory is affected by the time delay. Additionally, doubts have been raised about the applicability of the method under triaxial conditions. The effect of loading rate, scale, anisotropy, and drilling-induced damages are some of the areas that require further investigation.

In the face of the above challenges, the main objectives of this project were to:

• Perform experiments on the memory effect of rocks, with a specific focus on coal and coal measure rocks, to obtain reliable data for scientific scrutiny;
• Improve the experimental procedures and develop new analysis methods for better interpretation of experimental results;
• Measure in-situ stresses from oriented cored rocks and compare the results with the conventional methods for better stress management.

Four different techniques based on acoustic emission and strain measurements were utilized to measure the applied and in-situ stresses and determine their effectiveness. The results show that the DRA and SMM are the most reliable methods that can be effectively applied using both axial and lateral strains. The AE method is accurate, but it is sensitive and more prone to errors arising from micro-crack generation, making it difficult to get a clear inflection point. The TMM underestimates the stresses and is also affected by time delays, which require further investigation. The AE and the SMM methods are simpler as they require only one loading cycle. Nonetheless, all these techniques are easy to apply, and we recommend using them together for better interpretation of in-situ stress. By combining the power of these techniques, a comprehensive stress measurement approach is offered to prevent rock and coal bursts in mining operations. Additionally, the ability to cross-validate results using these techniques and the borehole breakout method enhances the overall reliability of the method, making it a valuable tool for mining operations.