# A Review of the Impact of Spontaneous Combustion on Slope Stability in Coal Mine Waste Dumps
## 2 Mechanisms of Spontaneous Combustion in Coal Mine Waste Dumps  
The spontaneous combustion process typically involves four distinct phases: (1) the induction phase--heating and accumulation by oxidation (internal temperature increase); (2) slow spontaneous heating--internal temperature increase and the maintenance of a fixed temperature; (3) the acceleration of heating--the drying of internal moisture and a rapid increase in temperature; and (4) spontaneous combustion--high-temperature burning alters material properties and produces gaseous byproducts [19; 41; 42; 43; 44; 45].  
Spontaneous combustion occurs due to the self-heating of combustible materials (coal, sulfide and pyrite) contained in the wastes through low-temperature oxidation. This process is accompanied by an increased production of heat and increased temperatures (Figure 3).  
Figure 2: Slop failure at a coal mine waste dump in southern Poland: (**a**) view from the slope toe; (**b**) view from the slope crest.  
Major factors influencing this process include the following:  
* Coal Rank: higher-rank coals (e.g., anthracite) have lower susceptibility to spontaneous combustion than low-rank coals (e.g., lignite). Coal is a porous material characterized by a complex structure with numerous active sites on its surface. These sites can continuously adsorb oxygen molecules from the surrounding air. During this process, coal undergoes low-temperature oxidation, which results in the gradual release of gaseous products and heat. Under certain conditions--such as restricted ventilation, high ambient temperatures, and sufficient coal mass--this heat can accumulate faster than it dissipates. As a result, the temperature of the coal gradually rises, potentially reaching the critical threshold at which spontaneous combustion occurs [47; 48; 49; 50; 51; 52; 53; 54].
* Particle Size: fine coal particles with an increased surface area are more prone to oxidation. The particle size and porosity of coal significantly influence its specific surface area. A larger specific surface area enhances the contact between coal and oxygen, thereby increasing both the reaction rate and the efficiency of heat transfer. As the particle size decreases, the likelihood of spontaneous heating increases; studies have shown that a reduction in particle size can raise this tendency by approximately 12-14% [55; 56; 57; 58; 59; 60; 13; 14; 15; 52; 13; 16; 52; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 41; 42; 43; 44; 45; 46; 47; 48; 49; 50; 51; 52; 53; 54].
* Moisture Content: field observations and studies have consistently indicated that moisture condensation can enhance the potential for spontaneous combustion. Moderate moisture levels facilitate oxidation, while excessive moisture can inhibit the process [61; 62; 63; 64; 65; 66; 67; 68; 69; 70; 71; 72].
* Ambient conditions: high ambient temperatures and oxygen availability accelerate self-heating [19; 73; 74; 75; 76; 77; 78; 79]. Additionally, external heat sources, such as wildfires, human activities such as the removal/reconstruction of the mine waste dump or wind flow, delivering air to the self-heating cells, can initiate combustion [80; 81; 82; 83; 84].  
Figure 3: Schematics of spontaneous combustion within the mine waste dump [46].