## 1 Introduction  
There are various types of rocks forming the hard coal seams in Poland. They include claystones, coal shales, mudstones, sandstones and, where the ongoing exploitation is divided into layers, also hard coal. All of these rocks have different strength properties [1; 2]. So far, the strength properties of the rocks surrounding mine headings have been considered primarily in terms of their impact on the capacity to maintain the stability of mine headings [3; 4; 5]. When coal is exploited with caving, the analysis also encompasses their capacity to transform into the state of caving and to maintain the roof in a longwall heading (classification by [PERSON] [6]). The strength properties of these rocks were, therefore, mainly examined in terms of rock mass mechanics. From the perspective of ensuring safe exploitation in longwalls, the impact of the strength of roof rocks should be considered not only in terms of maintaining the stability of the mine headings, but also in terms of the occurrence of ventilation hazards, and--more precisely--the risk of spontaneous coal combustion (endogenous fires) in the caving goaves of longwalls. Spontaneous combustion of coal is a phenomenon that commonly occurs in hard coal mines [7; 8; 9; 10; 11]. It results from the self-ignition of coal following its self-heating in a mine heading or its immediate surrounding, e.g., in the goaves with caving. Goaves with caving are a space formed after coal extraction, filled with rock rubble from the collapse of roof rocks hanging over the exploited seam. The degree to which this space is filled once the coal has been extracted depends on a number of factors. However, the most important of these is the type of roof rocks forming the goaves [12; 13; 14]. The parameter that is critical for the propensity of these rocks to transform into the state of caving is their tensile strength [15]. It determines the filling (tightness) degree of the cave-in rubble, and hence its porosity and permeability. Strong and solid roof rocks separating from the rock mass fill goaves with caving to a lesser extent than weak and brittle rocks. Therefore, goaves with caving filled with roof rocks of low tensile strength provide better filling of the space created after the mined coal (due to their lower permeability). However, regardless of the type of rocks forming the goaves with caving, such goaves always include void spaces not filled with any rock material. These spaces, representing open contacts between chaotically arranged blocks of fractured roof rocks, form a particular type of a porous medium which allows for the flow of gases, including the air migrating from the longwalls. The flow of air through goaves with caving results from its migration from the area of the longwall to which a stream of ventilation air is supplied. This stream is supplied to longwalls for their ventilation. There are several longwall ventilation systems, with the most common being the U-type ventilation system from the borders and the Y-type ventilation system [16] (Figure 1). Analysing both of these systems, it is possible to see a clear difference in the flow of air through the exploited headings. The choice of a longwall ventilation system depends on a number of factors. The ventilation system must ensure proper chemical composition and temperature of the atmosphere [17; 18; 19]. One of the most important is the level of natural hazards, including gas-related risks, which occur in the region of active exploitation. In practice, there is no system that would be beneficial in the event of simultaneous occurrence of the self-combustion risk and the methane risk for the longwall under exploitation. Figure 1: Diagram of the U-type (**a**) and Y-type ventilation systems (**b**). A system advantageous in the case of spontaneous combustion risk is less suitable for areas with methane hazard and vice versa. In longwalls with a high methane hazard which is typical of mines, it is increasingly common to use Y-type ventilation systems with air discharge along the goaves.