The surface movement in an area of about 22 km² above the underground coal mine of Houthalen was analyzed based on Interferometry with Synthetic Aperture Radar (InSAR) measurements. After its closure in 1992, a residual subsidence was observed over a period of several years, followed by an uplift of the surface above and around the past longwall panels, whereby the rate of movement was, in absolute terms, of the same order for the two types of movements. The processes behind these movements are different. The process of subsidence is caused by the caving of the roof above the mined-out area and is mainly a mechanical stress-deformation process, including time-dependent aspects. However, the process of uplift is most probably caused by the swelling of the clay minerals in the argillaceous rocks in the coal strata after the flooding of the underground workings. Hence, the areas in which there is the greatest risk of damage to the surface infrastructure are not the same for the hazards linked to subsidence and uplift. For example, the zone in which the maximum uplift occurs clearly is at a different location from that of the zone with the maximum residual subsidence. There is no clear sign that the amount of mining underneath affects the residual subsidence, and there is no indication that the process of uplift is linked directly to the mining characteristics. It is more likely that uplift as the result of flooding is initiated at, or close to, the vertical shafts.

This paper covers the entire lifetime of an underground coal mine, including the start-up of the mining operations and the period after the closure and sealing of the underground workings. Measurements of the vertical surface movement are presented for three neighbouring mines in the Belgian Campine coal basin, where the longwall mining method was applied. First, the monitoring of the initial phase showed that the impact of the first panel was smaller than the impacts of the following two neighbouring panels. When the second panel was mined, a change in the induced surface tilt was observed in the immediate proximity of the panels. Second, data for a transect with a length of about 2.5 km were presented, covering a total time of 54 years, of which the first 31 years were during the active life of the mine. Average subsidence rates of up to − 0.3 m/year were observed. After the mine was closed and the pumping facilities were stopped, the average uplift rate was situated mostly between 4 and 16 mm/year, but a value of 141 mm/year also was measured. Thirty years after closure, the upward movements are still occurring. Third, the phase of uplift is analyzed in more detail over a full north–south transect. A clear narrow maximum was observed for the uplift, corresponding to a total vertical movement of 0.2 m over approximately an 18-year period. All these phases are relevant, for example when analysing damage to buildings and infrastructure. Damage becomes visible when the cumulative movement and loading exceed critical limits and not necessarily when the largest rate of movement is observed, for example.

Since the 1990s, remote sensing data have been available to monitor the surface movement for long periods of time. The analysis of satellite data shows that there is still residual subsidence (i.e., with average rates of about −10 mm/year) several decades after mining longwall panels in an area. Several years after the underground infrastructure was sealed, the surface started to move upwards. In the past, it often was claimed that movement of the surface was limited in time, i.e., a few years after mining a longwall panel. This is not the case for the conditions of the Campine coal basin, Belgium. This knowledge is important when one wants to design new operations in deep coal seams, but also when planning to stop the underground mining and to seal the access to the mine.

Rock strength is an essential parameter in the design of any underground excavation, and it has become even more relevant as the focus increasingly shifts to sustainable excavations. The heterogeneous nature of rock material makes characterising the strength of rocks a difficult and challenging task. The research results presented in this article compare the impact on the strength when the classic stress paths in laboratory experiments are applied versus when in situ stress paths would be applied. In most laboratory experiments, the rock specimens are free of stress at the beginning of the tests, and the load is increased systematically until failure occurs. Opposite paths occur around an underground excavation; that is, the rock is in equilibrium under a triaxial stress state and at least one stress component decreases while another component may increase. Based on discrete element simulations, the research shows that different stress paths result in different failure envelopes. The impact of this finding is evaluated in the application of wellbore stability (e.g., the minimum or maximum mud weight), whereby it is concluded that failure envelopes, based on stress paths closer to the in situ stress paths, result in a more accurate design. Although the most critical location along the circumference is not different, the required density of the mud is significantly different if the rock strength criteria are based on the more realistic in situ stress paths. This means that a change in the way the strength of rocks is characterised improves the sustainable design of all underground excavations.

The strength of rock is a non-intrinsic property, and this means that numerous parameters influence the strength values. In most laboratory experiments, specimens are free of stress at the start of the tests, and the load is increased systematically until failure occurs. Around excavations, the opposite path occurs, i.e., the rock is in equilibrium under a triaxial stress state and at least one stress component decreases while another component may increase. Hence, the stress paths in classic laboratory experiments are different from the in situ stress paths. In the research presented, a first step was made to evaluate with an open mind the effect of these different stress paths on the failure process and failure envelope. The research was based on distinct element models, allowing the simulation of micro-fracturing of the rock, which is essential to correctly model rock failure. The micro-fracturing when loading rock (from zero or low stress state) until failure was different from the micro-fracturing when unloading rock (from the in situ stress state) until failure. And, hence, by this difference in weakening processes, the failure envelopes were significantly different. The conventional loading resulted in the largest strength and, thus, overestimated the rock strength in comparison to the real in situ behaviour. This finding, after being confirmed by further lab experiments, will have a direct effect on how one characterises rock material and on the design of rock excavations.

The environmental impact of deep underground coal mines using the longwall mining method is diverse, e.g., short- and long-term subsidence, damage to surface infrastructure, disturbance of the hydrogeological conditions, and the quality of groundwater and surface water. The study presented focusses on the long-term surface movements after the closure of an entire coal district. Due to the flooding of the underground infrastructure and rock mass, an upward surface movement or uplift is observed. For a specific site in the Campine coal district, Belgium results are presented of satellite data (radar-interferometry). However, the main aim of the study is to better understand the process of uplift and to determine the various mechanisms that are involved. For this purpose, an analytical framework was developed recently, and it was applied successfully in a relatively easy case. The case study of the paper is more challenging, but the usefulness of the analytical framework is clearly confirmed. The most important conclusions are that (i) the uplift is induced by an increase in water pressure after the closure, i.e., re-establishing the original hydraulic gradient, (ii) the expansion of both the goaf volumes and the volumes of the non-collapsed rock mass must be considered, and (iii) the assumption of a linear decrease of water pressure variation from the top to the bottom of the mined area at the end of the mining phase provides the most realistic results. However, the next step in the analysis should focus on a more advanced hydrogeological model of the complex underground environment.