# Study on the \"Two-Zone\" Heights in Lower Slice Mining Under Thick Al Alluvium and Thin Bedrock
## 1 Introduction  
Coal mining leads to the fracturing of the overlying strata, resulting in the formation of cracks. These longitudinal cracks can develop and extend upward, creating water-conducting channels. These channels connect to the overlying aquifer, allowing water to flow into the goat and even the working face, which can cause significant damage to the underground environment [1]. In addition, surface water or water from water-bearing formations flows into the goat through the fracture zone. The alteration of the rock state due to water immersion can also lead to surface subsidence, negatively impacting ground buildings [2]. Therefore, studying the developmental laws governing water flow in fractured zones during coal mining is essential for ensuring safe mining underwater bodies in coal mines [3]. The development height of the water-flowing fractured zone is influenced by various factors. To accurately determine the development height of the 'two zones' under theseconditions, it is essential to fully consider the water richness and occurrence conditions of the aquifer, the lithology and structure of the overlying strata of the coal seam, the thickness of the coal seam and the mining method [4; 5; 6; 7]. In recent years, the common methods for studying the development law of water-conducting fracture zones in mining underwater-bearing strata mainly include numerical simulation, theoretical analysis, similar simulation and field measurement. Many scholars have conducted extensive research using these methods. [PERSON] et al. established a numerical model to simulate the development height of an inverted water flowing fractured zone by using UDEC 7.0 software, based on the five factors that affect the development of these zones. They also verified the model's validity [8]. [PERSON] et al. used borehole peeping combined with borehole TV to observe the development height of the water-conducting fracture zone [9]. [PERSON] et al. simulated the development law of water-conducting fracture zones following coal seam mining through a similar simulation experiment and monitored the dynamic evolution of fractures using VIC-3D technology [10]. [PERSON] et al. utilized an enhanced Winkler elastic foundation beam model to calculate the height of the water-flowing fractured zone. They verified the development height of this zone, induced by mining, through numerical simulation [11]. At present, with advancements in computer technology, an increasing number of scholars are predicting the development of water-flowing fractured zones using self-developed program algorithms. Through the use of the BP neural network [12; 13], multi-group genetic algorithm [14], radial basis function neural network [15; 16], improved cuckoo search algorithm [17], grey theory model [18], and random forest regression algorithm [19], the height of water flowing fractured zone is predicted with greater accuracy. The aforementioned research has significantly contributed to predicting the height of water-conducting fracture zones. However, most of these studies focus on mining under general geological conditions. There is a relative scarcity of research on the development law of mining fractures in specialized geological conditions, such as thin bedrocks and thick loose layers, and the methodologies employed are relatively singular. Zhaogu No. 1 Coal Mine is characterized by typical thin bedrock and a thick loose layer. To fully exploit the coal deposits covered by the thin bedrock and enhance the safety and reliability of mining in the lower-layer working face, it is crucial to determine the development height of 'two zones' of overlying strata [20; 21; 22]. In order to accurately obtain the development law of overburden rock failure after mining in a layered working face under the condition of a thick loose layer and thin bedrock, this paper analyzes the failure characteristics of overburden rock through theoretical analysis, numerical simulation, and field measurements.