The article presents the results of tests on SHC-40 hydraulic props equipped with two types of valve blocks: standard (with spring steel cylinder) and BZG-2FS (with gas spring). The research was conducted using impact mass of 4,000 kg and with extreme dynamic load of free fall impact mass of 20,000 kg released from different heights h. The dynamic tests involved a camera with the speed of image capture up to 1,200 frames/sec, which made it possible to register the stream of liquid at the dynamic load and to determine the valve opening time. The study conducted on SHC-40 NHR10 props equipped with two types of valve blocks: a standard and the BZG-2FS fast acting relief, showed that the prop with the BZG-2FS block is more suitable and more effective in the case of areas with high risk of mining tremors and rapid stress relief of a seam. Research methodology developed in the Central Mining Institute combines digital recording technique of pressure in a prop and fast registration of the images, and allows to acquire more accurate analysis of dynamic phenomena in the prop during testing.

This article focuses on the difficulties in ensuring longwall stability resulting from the wrong geometric form of the structure of powered support sections. The authors proved, based on the in-situ measurements and numerical calculations, that proper cooperation of the support with the rock mass requires correct determination of the support point for the hydraulic legs along the length of the canopy (ratio), as well as the inclination of the shield support of the section of the powered roof support. The lack of these two fundamental elements may lead to roof drops that directly impact the production results and safety of the people working underground. Another matter arising from the incorrect geometric form of the construction are the values of forces created in the node connecting the canopy with the caving shield, which can make a major contribution to limit the practical range of the operational height of the powered roof support (due to interaction of powered support with rockmass) in terms of the operating range offered by the manufacturer of the powered support. The operating of the powered roof support in some height ranges may hinder, or even in certain cases prevent, the operator of powered support, moving the shields and placing them with the proper geometry (ensuring parallelism between the canopy and the floor bases of the section).

The dynamic characteristics of the hydraulic leg are essential for determining the safe working range of roof supports operating in seams threatened by rock mass tremors. The systematic increase in the support of the hydraulic legs due to deteriorating geological-mining conditions has increased their diameters, which currently exceed 0.32 m for the 1st hydraulic stage. Evaluation of the dynamic properties of the roof support and the hydraulic legs are carried out by the Central Mining Institute through calculation methods as an implementation of the Regulation of the Minister of Energy on occupational safety and health. However, the issue of validating the calculations concerning natural scale studies still needs to be addressed. There are significant limitations in this area due to the technical and metrological capabilities of the testing stations. This paper presents an attempt to evaluate bench testing of a hydraulic leg with 0.32 m of the 1st hydraulic stage diameter for the validation of computational and test methods. Results of previous studies affecting the evaluation of the research methods used are also cited. According to the authors, the optimal and economically justifiable direction is to undertake model tests using numerical analyses and to validate these results, based on the study of models of hydraulic legs that are in use at a reduced scale. The construction of testing stations to ensure adequate dynamic loading for the support of the largest diameter hydraulic legs is currently not economically viable. The problem presented, however, is important given the constantly deteriorating geological-mining conditions and the associated threat of rock mass tremors.

The stability of longwall mining is one of the most important and the most difficult aspects of underground coal mining. The loss of longwall stability can threaten lives, disrupt the continuity of the mining operations, and it requires significant materials and labour costs associated with replacing the damages. In fact, longwall mining stability is affected by many factors combined. Each case of longwall mining has its own unique and complex geological and mining conditions. Therefore, any case study of longwall stability requires an individual analysis. In Poland, longwall mining has been applied in underground coal mining for years. The stability of the longwall working is often examined using an empirical method. A regular longwall mining panel (F3) operation was designed and conducted at the Borynia-Zofiówka-Jastrzębie (BZJ) coal mine. During its advancement, roof failures were observed, causing a stoppage. This paper aims to identify and determine the mechanisms of these failures that occurred in the F3 longwall. A numerical model was performed using the finite difference method - code FLAC2D, representing the exact geological and mining conditions of the F3 longwall working. Major factors that influenced the stability of the F3 longwall were taken into account. Based on the obtained results from numerical analysis and the in-situ observations, the stability of the F3 longwall was discussed and evaluated. Consequently, recommended practical actions regarding roof control were put forward for continued operation in the F3 longwall panel.