Protocols for evaluating the stability of induced cavities in underground thin seam coal deposits created by in-situ borehole mining

The economic extraction of thin-seam coal deposits are often problematic due to several significant limitations associated with conventional mining methods, operating practices, and equipment. Mines with low-seam heights are endemic of operations that possess low labor productivities, high operating costs, and relatively small production capacities. Furthermore, the ability to implement new equipment and automation in order to efficiently exploit these thin-seams is hampered by the limited cash-flow positions of most of these operations and the inability to amortize their high costs over a sufficiently large resource base. Consequently, these mines are usually small, labor intensive, and rely extensively on used and rebuilt equipment modified to operate in these challenging work environments. It appears that the most prudent way to extract these resources in a more economical way is through the development of technology to remotely extract these resources from the surface. Displacing workers from the underground work environment will eliminate the inherent hazards of mining these deposits and reduce the direct costs associated with ventilation, support, and equipment.;Despite several potential benefits, there are a number of technical challenges that must be overcome to advance the concept of in-situ borehole extraction of non-soluble resources to a commercially viable stage. Paramount among these include the continued technical advancement of drilling and excavation systems, the mechanisms used to crush, bail, and transport cuttings from the borehole, the required instrumentation to effectively control and monitor the mining process, and a technical understanding between cavity formation and stability for a given set of operating characteristics and geomechanical rock properties. While each of these areas are important to the overall success of the technology, understanding the structural dynamics of these cavities is a key element in designing a mining system capable of sufficient resource recovery to economically justify the capital investment. The unintended collapse of these cavities could potentially result in the detrimental loss of mineral reserves, as well as surface subsidence, the incidence of significant dilution, and the loss of equipment. In addition, adverse alterations in cavity geometry caused by failure in the surrounding host rock will significantly hamper the ability to recover and bail fragmented mineral from the borehole. Given these factors, borehole placement, the excavation strategy, extraction ratios, production rate, and overall project economics are all highly influenced by the stability of these subsurface excavations. Cavity stability, in turn, is the product of a complex set of multi-dimensional variables that include in-situ stresses, rock properties, cavity geometry, time, and the rate and manner of excavation. A number of additional confounding issues associated with the proposed excavation methods (e.g., fluid pressurization of the cavity) may also adversely influence the stability of these cavities, where their potential effects need to be quantified.;Several models were analyzed using Flac2D to perform a parametric study of factors that could potentially impact the cavity stability during borehole mining process, whereas information derived from the literature was useful in identifying several parameters that could possibly affect cavity design and the mining process. The results and observations of these studies (numerical modeling and literature search) led to a proposed protocol design for the creation of stable cavities during borehole mining.