Long-Term Stability and Time to Failure of a Single Pillar in a Flooded Abandoned Gypsum Mine

Assessment of the long-term stability of abandoned underground mines is a difficult task especially when working areas are located under groundwater table. As the mine is flooded upon closure, the wetting (saturation) of the left-in-place pillars leads to a decrease in strength and some stability problems. In evaporate deposits such as gypsum, a further decrease in the strength of pillars occurs due to dissolution and reduced pillar width. The rate of strength reduction and the time to possible failure in these mines depends on the dissolution rate which is strongly affected by the rate of groundwater movement through the mine. In areas where groundwater is stagnant, the dissolution stops as the groundwater becomes saturated with dissolution products. In areas where groundwater continues to flow through the abandoned mine, however, dissolution will continue and can affect the long-term strength of the mine's support structures. In this paper, a simplified method is proposed to evaluate the long-term stability and time to failure of a single gypsum pillar in an abandoned mine flooded by flowing groundwater. To develop such a model, the rates of pillar width and strength reduction due to gypsum dissolution were investigated experimentally, using core samples in the lab. A simplified analytical model was developed to predict the change in specimen radius with time due to the dissolution. A finite volume model was also developed to model the strength reduction of specimens due to both saturation and dissolution. The model was then used to estimate the long-term stability of pillars in an abandoned gypsum mine located in Grand Rapids, Michigan. Finally, using the dissolution models along with the results of the numerical simulations, the time to possible failure of a single pillar in the flooded abandoned mine was estimated under different flow rates. The results showed that flow rate and the concentration of gypsum dissolution products in flowing groundwater are key factors controlling the time to failure of a single gypsum pillar in an abandoned mine flooded by flowing groundwater. The time to failure of the pillars with minimum diameter in the studied abandoned mine could change from 40 years to more than 120 years depending on the flow rate and the concentration of gypsum dissolution products in flowing groundwater.