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[en] The application of horizontal slicing-and-caving in No.3 ore body of Benxi uranium mine is discussed in this paper. The parameters are determined. It has been proved that this method is very suitable to unstable orebody and rock condition
[en] In this study, a comprehensive assessment of the overlying strata caving characteristics of mental mines is conducted for the non-pillar sublevel caving method, especially considering the geological features and rock mass characteristics. In general, the overlying strata caving process can be divided into four stages: the intermittent roof caving stage, the crack extension and surface subsidence stage, the karst collapse stage, and the mining-induced collapse stage. Roof caving is an intermittent process, with higher caving occurring in stronger rock masses, and occurs more easily in weaker rock masses. Due to gaps generated in the overlying rock mass, the bulking coefficient of the overlying strata is much greater than the laboratory-measured value; it also varies with the mining advance lines. During the crack extension and surface subsidence stage, the surface deformation is less extensive when the rock strength is greater, and the gradient of surface deformation is lower when the rock mass integrity is lower. Thus, higher mining height and lower rock mass quality result in greater surface deformation. Furthermore, weak planes, such as faults and joints, can help to generate fissures in weathered rock masses; these fissures create passageways for groundwater flow, which accelerates crack extension and connection in reverse. In addition, surface collapses can be divided into karst collapses and mining-induced collapses. Because a large number of fissures and a high karst rate create favorable conditions for karst collapse, significant variations in groundwater levels can induce karst collapse at the surface, with high negative pressure forming in the karst caves. Mining-induced collapses are mainly related to the mining depth, mining height, and the properties of the overlying rock mass. Underground mining is a major factor in surface collapse, and heavy rainfall can induce a delayed but sudden collapse.
[en] Highlights: • We developed a method for predicting radon diffusivity tensor for fractured rocks. • We applied this method to predict principal and cross diffusivity tensor for fractured rocks. • We found that the fracture density affects diffusivity, which characterizes radon transport through fractured rocks. - Abstract: This study develops a numerical model for predicting radon effective diffusivity tensor for fractured rocks using a two dimensional discrete fracture network (DFN) model. This is motivated by the limitations of existing techniques in predicting the radon diffusion coefficient for the fractured zones of cave mines. These limitations include access to the fractured zones for the purpose of conducting field studies as well as replication of the degree of fracturing in these zones for laboratory studies. The caving of a rock mass involves the fracturing and breaking of intact and naturally fractured rock, which creates migration pathways for radon gas trapped within uranium-rich rock. Therefore, this study develops a stochastic DFN model with equations derived from radon transport to predict diffusivity. Our simulation results reveal the establishment of a representative elementary volume (REV) for diffusivity tensor; approximately equal principal and cross diffusivity magnitudes for each of the DFN domain; a range of diffusivity with porosity (calculated based on the fractures in the domain); and a significant effect of fracture density on diffusivity tensor. These results are essential in developing proactive measures for mitigation of radon gas in cave mines.
[en] To determine the age of a cave we have to determine the age of elements which were created at the same moment when the cave was. Usually a cave is dug out by infiltration of acid waters. During this alteration process rocks free some aluminium and potassium ions. In Carlsbad and Lechuguilla caves, scientists have found alunite, this aluminium and potassium sulfate can be dated by using the carbon method of age determination. (A.C.)
[en] The measurement of radon in underground cavities is not codified by a normative text. However, a specific methodology has been developed by I.R.S.N. in the area of studies devoted to the characterization of radon content in different underground cavities. These underground cavities can be classified in three categories: firstly, the natural cavities descended from the dissolution of carbonates or sulfates by water circulation. secondly, anthropogenic spaces, out of mines (underground quarries relative to the materials extraction for construction, marl pits, basements, civil construction works (tunnels, aqueduct) and military ones (war sapping), thirdly, the mines (coal, metals, salts). The present document presents the methodology applicable to the radon detection in a natural underground cavity. It is founded on the approach initiated by the AFNOR NF M60-771 norm for the realisation of detection and on the results of studies realised by I.R.S.N. in the karstic caves and the french agricultural underground cavities. The implementation of this methodology requires knowledge relative to radon and cavities. It is thus the responsibility of relevant agencies. (N.C.)