# Liquidation of Shallow-Lying Post-Mining Excavations
## 4 Case Study  
An example of the treatment of rock mass disturbed by shallow hard coal mining can be the work carried out in one of the towns in the Malopolska province, Poland, where we are dealing with the effects of shallow historical mining. As a result of the increasingly frequent occurrence of discontinuous sinkhole phenomena on the surface of the area in the form of sinkholes, a decision was made to conduct a site reconnaissance using geophysical methods. As a result of the conducted microgravimetric, georadar, and resistivity electrometry studies, the occurrence of anomalies indicating a change in the continuity or density of the soil-rock medium was found. Next, based on the obtained research results, drilling works were planned to be carried out, the aim of which was to treat the subsoil and protect against further formation of sinkholes. In the lithology of this area, Quaternary layers can be distinguished, lying horizontally above the Carboniferous formations. Drilling and backfilling works consisted of drilling holes to the depth of the mine floor. In order to properly secure the ground surface, it was planned to drill a series of holes in a regular grid with a side of 15 m and in places where, based on the results of geophysical surveys, it can be concluded that the bearing capacity of the terrain has significantly weakened. An example profile of the hole at the place of drilling is shown in Figure 5. The strength parameters of individual rock mass layers have been presented in Table 1.  
Figure 5: Lithological profile occurring at the site of the reclamation works. The drilling and injection works were carried out in several successive stages [27]:  
* Dry-drilling of a hole using a 160 mm diameter auger drill, reaching the layer of clay or clayey sands of the Carboniferous (8 \(\div\) 10 m);
* insertion of a PVC casing pipe with an external diameter of 120 mm;
* cementing the annular space between the wall of the hole and the casing pipe;
* waiting for at least 48 h for the cement slurry to set;
* drilling a hole using the rotary method with the right circulation of drilling fluid to the depth of the bottom of the post-mining void in the coal excavation;
* measuring the occurrence of the static water table in the holes within a given location;
* running the drill string to a depth of approx. 0.3 m above the bottom of the void and starting to inject the cement-ash mixture;
* pulling the drill string up several times if the hole is found to be lacking in absorption (as a result of the local formation of a cement slurry flow cone that clogs the outlet of the drill string);
* ending the injection of the slurry when the rock mass loses its absorption capacity;
* waiting at least 7 days for the slurry to set;
* checking by placing the drill bit on the injection cone of the drill bit together with the drill string;
* liquidating the borehole by injecting it into the top cement slurry. Backfilling works were carried out using cement-ash mixtures developed by the AGH University of Krakow, Faculty of Oil and Gas Drilling, Department of Drilling and Geoengineering, Laboratory of Geoengineering and Sealing Slurries, which were to be characterized by [28; 29; 30; 31] as follows:  
* density similar to the medium of application;
* the highest possible flowability;
* low sedimentation (below 10%);
* beginning and end of setting time adjusted to the works carried out;
* mechanical strength greater than the surrounding medium;
* low unit price;
* inert impact on the soil and water environment. The flow parameter is particularly important in the aspect of obtaining a large radius of grout spreading in the drilled voids. It should have a value greater than 160 mm [32]. Flow can be increased by the amount of mixing water used, but this worsens the operating parameters of the fresh and hardened grout. A beneficial solution is to use a liquefier, which, in this case, was an appropriately selected compound from the polycarboxylate group.