Mining Contract: Assessment of Technology for Non-destructive Testing of In-situ Underground Mine Seals
Mine seals are a critical component to keeping an explosion that occurs in a sealed area from entering into the active parts of a mine. An ongoing concern of coal industry stakeholders is the structural integrity of coal mine seals over prolonged exposure to the mining environment; however, no technology currently exists to evaluate the integrity of these structures once built. Currently, visual inspection is the only means for evaluating the integrity of an as-built seal, making it impossible to know if the structure meets the regulatory requirements or continues to meet the regulatory requirements over time. Therefore, it is difficult to determine if a faulty seal is present and should be repaired or replaced.
Contract Status & Impact
This contract is complete. To receive a copy of the final report, send a request to OMSHR@cdc.gov.
The proposed research aimed to design and construct MUSSEL, a mobile underground laboratory, and employ it to assess the feasibility of utilizing several types of existing technology for non-destructive testing of the integrity of in-situ seals, with the objective of identifying damage due to improper construction or damage that occurs after installation due to movement of equipment, ground conditions, or chemical reactions.
This project assessed four different non-destructive testing (NDT) technologies for detecting imperfections such as cracks, voids, and composition variations, which could affect the structural integrity of an as-built coal mine seal. A proven NDT method should provide a clear diagnosis of a seal’s condition and provide guidance to mine operators on whether to replace or repair a faulty seal, or do nothing to a competent seal. The four NDT methods assessed were ground penetrating radar (GPR), ultra-sonics (US), radio frequency identification tags (RFID), and passive tracer gas (PTG). All of these methods have been used successfully in various industries with various applications, but not necessarily in underground mining, where structural integrity of seals is a critical safety issue.
It was found that the RFID technique could not penetrate more than one foot into typical seal materials. Although the GPR technique was able to detect anomalous features at depth within four-foot test blocks, features lying deeper in the seal material were not able to be detected with sufficient accuracy. The US technology results were more positive, in that the technology showed good resolution and accuracy, but when it was tested in the mining environment, large amounts of ambient noise prevented the small-scale experiment test results from being successfully transferred to large-scale demonstrations.
Of the four technologies tested, PTG results showed the most promise for non-destructive testing of underground mine seals, with a high level of accuracy to this technology. Test results showed that perfluoromethylcyclohexane (PMCH)—due to its heavy molecular weight—is an effective tracer gas because of its ability to move through fracture paths in seal material as well as along macro-cracks. Both small- and large-scale experiments support the claim that PMCH can be used as a tracer gas to indicate an increase in the discontinuities or fracture network found within the seal material. However, test results also indicated that tracer gases are not likely to identify a specific structural defect within a seal, but rather to give a global indication of whether the seal is truly serving as an effective barrier between the sealed environment and the active area of the mine.
To allow for easy comparison of the four technologies, the final report includes a matrix table including such factors as cost, relative difficulty in proving or improving the technology, maintenance of the technology, and required skill level to use it.