## 1 Introduction  
Nowadays, coal provides about a quarter of the world's energy generation. The transition to renewable energy sources is inevitable. However, in reality, the pace of this transition turned out to be slower than expected. The European energy crisis that was caused by the war in Ukraine has shown that in the near future, abandoning coal as an important source of energy is not possible. At the same time, certain types of coal are important raw materials for the metallurgical and chemical industries. Therefore, coal mining will be quite relevant in the near future. To reduce the negative impact of mining on the environment, governments of various coal-mining countries are introducing increasingly stringent restrictions and environmental regulations. Also, every year, safety requirements in mining become more and more stringent. This leads to an increase in the cost of coal, which stimulates the search and implementation of new technological solutions. A major part of the world's underground coal is mined from longwall. The US, China, Australia, Poland, and Ukraine use the longwall method. The modern trend in longwall mining is the pillarless mining technology. Its advantages are the following:  
* Reduced expenses to mine roadway excavation and, therefore, reduced coal cost;
* An increase in the pace of longwall panel preparation;* Improvement of the coal recovery ratio;
* Application of effective Y-shaped ventilation systems in the panel, which allows longwall production to be increased by the ventilation factor. The pillarless method has been known for over 70 years. Experience in its application has been gained in traditional coal-mining countries. Scholars from Germany and Great Britain have made a great scientific contribution to the development of this method [1; 2]. Most progress was achieved through the implementation of goat filling technology and the installation of filling walls. In the 1950s-1960s in Germany, quick-setting materials based on phosphogypsum were developed, from which filling walls were constructed. However, pillarless systems were initially applied with an advancing-type layout of longwall in which gateroads were formed as the coal face advanced (Figure 1a). The majority of such methods were used in thin coal seam mining. However, the absence of a pre-prepared panel and the complexity due to simultaneous roadway excavation and longwall mining in the same panel result in a reduction in the pace of coal mining. Therefore, small coal pillar methods have become more widely used (Figure 1b). In the 1970-1980s, traditional wide pillars were reduced from 20 m to 2-3 m and were applied in many coal basins of Europe and China [3; 4]. In the 1970s, Ukrainian mines completely abandoned wide pillars. The elaboration of technologies for the creation of new quick-setting materials has ensured a gradual increase in non-pillar method application (Figure 1c) [5]. Many scholars have researched the efficiency of the filling wall body of gob-side entry retaining in fully mechanized longwalls [6; 7; 8]. Additional possibilities for using pillarless methods have appeared in recent years thanks to Chinese scientists. [PERSON] and [PERSON] [9] proposed new pillarless coal-mining technologies. Among them, the N110 construction method is currently the most used pillarless coal-mining technology, which has many advantages [2; 10; 11]. The essence of this technique is to cut off the connection between the partial roof of the gob and the roof of the  
Figure 1: Evolution of thin coal seam longwall mining methods: (**a**)—pillarless method with advancing mining type; (**b**)—small coal pillar method; (**c**)—modern pillarless method. roadway by directional roof cutting so that the roof in a certain range above the roadway forms a short-arm beam structure [12]. The widespread application of pillarless coal mining is restricted mainly by the difficulty of ensuring the stability of gob-side entry [13; 14].