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 technology for gob-side entry retaining in steep coal seams is still in the development stage. The
analysis results of the caving structure of main roof, low influence of gateway’s stability because of long
filling distance and weak dynamic effect of the gateway, and the low stress redistribution environment
indicate that using this technology in steep coal seams has significant advantages. Moreover, to reinforce
the waste rock and the soft floor and to better guard against the impact of the waste rock during natural
filling, a rock blocking device and grouting reinforcement method were invented, and theoretical calculations
result show that the blocking device has high safety factor. In addition, we also developed a set of
hydraulic support devices for use in the strengthening support zone. Furthermore, because the retaining
gateway was a systematic project, the selection of the size and shape of the gateway cross section and its
support method during the initial driving stage is a key step. Thus, first, a section the size of bottom width
and roof height of a new gateway was determined to meet any related requirements. Then, according
to the cross sections of 75 statistical gateways and the support technique, it chosen a trapezoidal cross
section when the dip of the coal seam is 35° < α ≤ 45°, a special and an inclined arch cross section when
45° < α ≤ 55°. Eventually, a support system of bolts and cables combined with steel mesh and steel belts
was provided. The support system used optimized material and improved parameters, can enhanced the
self-bearing ability of the surrounding coal and rock masses.
The longwall mining system with fall of the roof is still the most common hard coal extraction system in Polish mining. Its utilization for selective coal seams’ mining results in the development of post-extraction gobs at different depths. Methane desorption phenomena from the coal seams in the stress release zones and migration of gas towards the area of operations, result in methane accumulating also after completion of coal exploitation. Methane which is not exploited from the gobs can migrate directly to the atmosphere e.g. through overlying layers, faults, workings or directly via an operated ventilation grid of an adjacent coal mine – contributing to the Greenhouse Gas effect. One of the methods to capture methane (Abandoned Maine Methane) from abandoned coal mines is to drill vertical wells through several post-extraction gobs from the surface.
This paper presents the results of drilling operations at the AGH-Wieczorek-1 well, where first time in Poland, down-the-hole-hammer (DTH) with casing-while-drilling (CwD) technology were used to drill through several post-extraction gobs. The AGH-Wieczorek-1 well with 440.0 m MD was successfully drilled without any complications. Finally, three post-extraction gobs and two coal seams were drilled. Additionally, results from drillability tests, which were performed during drilling operations, are presented.
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