33 / 2021-07-27 21:18:34

Modelling of upward surface movement above abandoned coal mines in the Campine coal basin

Ground control,underground coal mining,surface movement

The long-term impact of mining on the environment is an important aspect when evaluating the sustainable strength or weakness of a particular project. This long-term impact also includes the surface movement after the closure of underground coal mines. When applying a total extraction method, like for example longwall mining, the amount of subsidence is largest in the period immediately following the mining. However, the impact of the smaller long-term surface movement rates on the already damaged or weakened buildings and infrastructure should not be neglected. After the closure of an entire coal district and the flooding of the underground infrastructure and surrounding rock mass, a new phenomenon in surface movement has been observed, i.e., the upward movement of the surface or uplift. In the Campine coal basin (Belgium), the range of annual uplift rates is situated between 5 and 15 mm per year. In absolute terms, these rates are of the same order as the residual subsidence rates, prior to the closure.  It is important to note that this uplift is, however, not a simple rebound of the past subsidence. Hence, the deformation imposed on surface buildings and infrastructure is different than during the subsidence phase.



In the past, plenty of research has been conducted worldwide to understand the process of surface subsidence and to model this process. Although subsidence linked to longwall mining with goaf is one of the most complex processes in rock mechanics, it now is well understood, and the prediction of subsidence values is accurate and trustworthy. However, as the process of uplift after the flooding of coal mines is a recently observed phenomenon, there are still questions related to the mechanisms governing uplift. The research presented in this paper answers some of them. While subsidence is mainly driven by a mechanical stress-deformation interaction that is initiated when the roof collapses in the mined-out area, the hydro-geological context plays a significant role too in the process of uplift.



A systematic analysis of all relevant parameters resulted in a framework, allowing, firstly, a better understanding of the mechanisms and processes of the uplift above abandoned coal mines, and, secondly, a method to calculate uplift values. It has been shown that not only the expansion of the softer parts, i.e., the collapsed or goaf material, is important, but the change in pore pressure in the non-collapsed strata layers must be integrated too into the model. It is assumed that the pore pressure values increase to their original values after the mine is closed. The framework developed results in a good correlation between measured and calculated data along linear transects (e.g., North - South), as illustrated in Figure 1. The measured values are collected by satellite observations, i.e., Interferometry with Synthetic Aperture Radar data. To allow an easy comparison, both data sets are presented as a percentage of the maximum value of each set. The shapes of both sets are well correlated, certainly within the mined boundaries (indicated by blue dotted lines).