Analysis of the Influence of Macro Fractures on Underground Coal Mine Ventilation Seals

In April 2008, the Mine Safety and Health Administration (MSHA) issued a final rule to reduce the likelihood of seal failures and minimize the risk of explosion in abandoned areas of underground coal mines. The rule dictates different aspects of mine seals such as design, strength, construction, maintenance, repair, and other considerations associated. Commonly, materials used for the construction of the mine seals are concrete (reinforced and unreinforced) and cementitious foams. In most of the MSHA approved mine seals under the final rule, the designers use numerical (finite elements FEM) or analytical (plug design formula) methods for the analysis. One fundamental assumption in the analysis of mine seals is that the seal’s materials (especially for those using concrete or pumpable materials) are continuous and homogeneous solids. However, in most of the cases, to harden, the seal’s mixture goes through a process that generates an exothermic reaction (hydration). The hydration, in some cases, produces the shrinkage of the mixture, generating cracks, fissures, and fractures. To date, there is not a study to assess the effects of fractures (generated during the curing process or in any other stage) in the structural behavior of mine seals. The proposed research investigates, through testing and numerical modeling, the generation of macro fractures and their effects on the structural behavior of mine seals. Objectives and Research Approach The proposed research focuses on investigating the generation and effects of macro fractures on mine seals’ structural behavior and integrity. The fractures of concern are those fractures generated during the shrinkage of the cementitious materials used for seals’ construction. The project includes the following objectives by phases: Objective I – Mine seals inventory and fractures’ generation assessment • To inventory and analyze, according to the MSHA database, the different mine seals designs approved, • To build representative samples of mine seals to collect information relevant to fracture generation during the curing process (heat, strains), • To assess the characteristics of the macro fractures generated during the curing process (location, geometry). Objective II – Structural and integrity assessment of the effects of fractures on mine seals • To subject, mine seals samples to methane and coal dust explosions, • To do a parametric study using numerical modeling, considering fractures. Objective III – Proposal for alternative materials solutions • To propose to the Alpha Foundation alternative materials for mine seals construction. Project’s expected outcomes There are several outcomes expected from this research. The following list includes the most important: • Analysis of MSHA database regarding mine seals, • Trend or relationship between curing parameters (heat, strains) and fracture generation initiation and density, • Characteristics of macro fractures generated during curing (geometry, length, and aperture), • Static and dynamic material’s properties of materials used for mine seals, • Results of explosive tests (methane explosions) on the seals, • Numerical methodology for the analysis of mine seals, • Parametric relationships between mine seal integrity and macro fractures generated by curing or other processes such as stress conditions change.