An assessment of the impact of the degree of the filling of shallow voids on the possibility of sinkhole formation on the surface

The issues covered by the work are important and topical as sinkholes that develop in large num-bers over shallow mining excavations pose a great threat to public safety. In Upper Silesia (Poland), the formation of sinkholes can be observed even for a period of over 100 years following the termi-nation of mining works. An effective method of risk elimination consists of filling the voids with a binding material with strength properties similar to those of the rocks surrounding the void. The application of fly ash is very suitable for this purpose, the use of which also has an ecological aspect. The literature studies presented in the paper indicate the possibility of making mixtures with the use of fly ash that has the required strength parameters. The compressive strength of the mixtures after solidification is up to 3 MPa, or even up to 7 MPa, and in some cases, up to 15 MPa. Most of the voids at shallow depths are found in coal seams, in which the compressive strength at shallow depths amounts to approx. 5 MPa. Thus, by filling the void with such material, we can ensure conditions similar to those prevailing before the excavation was made. The paper presents a case study involving the formation of a sinkhole above a dog heading and an ex post forecast made with the use of two selected methods. These methods yielded results affirming that the development of a sinkhole in the considered conditions is certain. Then, using the said methods, the impact of the filling level of the void on the possibility of sinkhole development was analyzed. The obtained results indicated the necessity to fill the void to around 90% with the use of one of the methods and its complete filling with the use of the other method.