Increasing environmental pressure against waste disposal, particularly fine waste surface storage and concern about mining damages have resulted in an increase in the popularity of a fly ash, tailing and binding agent mixture used as compaction grout of roof fall rocks in a gob area of longwalls. Backfilling of voids forming as a result of exploitation with the fall of roof with mixtures containing fine-grained industrial wastes is a common practice in coal mines. It is aimed at achieving numerous technological and ecological advantages as well as at controlling mining hazards. Research on hydraulic transport of fine-grained slurry conducted to date focused mainly on issues related to the analysis of the conditions related to pipeline transportation. The processes concerning the propagation of mixtures within the gob, on the other hand, remain largely unknown. The process of flow of fine-grained slurry through the caving is subject to a series of factors related, among other things, with the properties of the applied wastes and mixtures, the characteristics of the gob as well as the variability of these properties during the flow through the gob and in time. Due to the lack of sufficient knowledge pertaining to the changes taking place in the gob and in the slurry while it penetrates the gobs, no methods allowing for the design and optimization of the gob grouting process have been established so far. The paper presents the selected results of laboratory tests regarding the flow of ash and water mixtures in a model of a gob, pertaining to two selected types of fly ash produced in hard coal combustion, particularly concerning the impact of the type of the ash and the density of the slurry on the effectiveness of the gob grouting process.

The shortage of investment areas may be at least partially satisfied by the development of reclaimed post-mining areas. These are often subsidence zones levelled with hard coal mine waste or reclaimed sub-level old dumps of this waste. From the geotechnical point of view, such grounds represent anthropogenic grounds containing mine waste, and they are considered as possessing unfavourable properties in terms of the foundation of building structures. The paper initially presents the analysis of the properties of waste from the hard coal mining industry, emphasising that they expose several beneficial properties enabling their safe use. The second part of the article is devoted to the determination of soil density using the DPSH probe. It has been found that the applicable standards lack complex relationships that would allow for a reliable interpretation of the measurement results in a wide range of soil types. The last part presents exemplary results of measurements made with the DPSH probe at a construction site. The obtained results allowed for the formulation of several conclusions regarding the possibility of building on a ground made of hard coal waste and the use of dynamic sounding to assess the geotechnical properties of such anthropogenic soil.