Triaxial testing of freshwater ice

Peer reviewed: NoNRC publication: Ye

Report to Inco Mines Research on the influence of unloading on rock strength

Ice was used as an analog for rock to simulate cracking behaviour during unloading. Columnar grained ice samples were pressurized and then brought to the melting temperature to cause melting at the grain boundaries, thereby removing any intergranular stress. Then, while pressurized, the samples were allowed to refreeze. Then pressure was rapidly removed. No cracks were observed, in contrast to previous experiments on granular ice samples where air was present at grain boundaries. This suggests that air plays an important role in producing cracks. Compressive strength tests performed immediately after depressurization showed no correlation between strength and degrees of stress drop at depressurization.NRC publication: Ye

High speed testing of freshwater granular ice

High speed uniaxial compression tests were conducted on columnar, granular and monocrystalline ice. Results were consistent with previous work and showed an increasing strength with strain rate in this range. The monocrystals, oriented with the c-axis parallel to the loading axis, were slightly stronger than the other types of ice.NRC publication: Ye

Microcracking during stress-relief of polycrystalline ice formes at high pressure

To study microcracking in a brittle material in response to stress-relief, samples of polycrystalline ice were frozen under hydrostatic pressures up to 19 MPa and subsequently decompressed. Eight cylindrical samples (approximately 56 mm in diameter and 120 mm long) were made by freezing a slurry of crushed ice and degassed water inside a clear Teflon jacket. Four additional tests were performed by reloading samples which were previously unloaded, allowing them to re-equilibrate, and then unloading again. Tests were filmed through windows in the confining cell. Emissions were monitored in some samples. When the stress was released, intense cracking occurred for approximately 0.4 seconds and then tapered off. Thin sections showed that the stress-relief cracks were approximately equal to the facet size (<5 mm), were primarily along grain boundaries, and were distributed throughout the samples with no obvious preferred orientation. Within a few hours of unloading, grain boundaries became cloudy, possibly a result of air trapped at the grain surfaces. Preliminary results from further tests indicate that this air plays a significant role in crack nucleation.Peer reviewed: YesNRC publication: Ye

Studying the Stress Redistribution around the Longwall Mining Panel Using Passive Seismic Velocity Tomography and Geostatistical Estimation

Generally, knowledge of stress redistribution around the longwall panel causes a better understanding of the mechanisms that lead to ground failure, especially to rock bursts. In this paper, passive seismic velocity tomography is used to demonstrate the state of stress around the longwall mining panel. The mining-induced microseismic events were recorded by mounting an array of receivers on the surface, above the active panel. To determine the location of seismic events and execute the process of tomography, double difference method is employed as a local earthquake tomography. Since passive sources are used, the ray coverage is insufficient to achieve the quality images required. The wave velocity is assumed to be the regionalized variable and it is therefore estimated in a denser network, by using geostatistical estimation method. Subsequently, the three dimensional images of wave velocity are created and are sliced into the coal seam. These images clearly illustrate the stressed zones that they are appropriately in compliance with the theoretical models. Such compliance is particularly apparent in the front abutment pressure and the side abutment pressure near the tailgate entry. Movements of the stressed zones along the advancing face are also evident. The research conclusion proves that the combined method, based on double-difference tomography and geostatistical estimation, can potentially be used to monitor stress changes around the longwall mining panel continuously. Such observation could lead to substantial improvement in both productivity and safety of mining operations