Time-Dependent Behavior of Coal Measure Rocks: Adsorption Rate and Strength Degradation

摘要

The time-dependent behavior of coal measure rocks was investigated for implementation in stability analysis and safety evaluation of in-mine rock structures. The initial study focused on moisture adsorption rate and strength degradation of shales in Pennsylvanian coal-bearing strata in the Illinois Basin. A reaction of clay minerals in Illinois shales with atmospheric water vapor was hypothesized as the adsorption and strength degradation mechanism. Part of the energy exchange between shales and the environment was attributed to a free energy controlled by pore pressure of the rock and partical pressure in the environment. The relationships between adsorption versus time and strength degradation versus moisture content were derived and correlated with the experimental data. Partial saturation at various levels of relative humidity occurs within approximately 2 weeks at room temperature. The strength of shales tested decreases linearly with increasing moisture content. When the moisture exceeds 7 pct of the dry weight, these Illinois shales become unstable. To this end, the moisture content is a good predictor of rock strength for shales.