# A Review of the Impact of Spontaneous Combustion on Slope Stability in Coal Mine Waste Dumps
## 3 Thermal Effects on Geotechnical Properties  
The environmental impacts of spontaneous combustion are well-documented and include air pollution, acid rain, and greenhouse gas emissions. However, the geotechnical impacts, particularly on mine waste dumps, have received less attention. The combustion process can alter the physical, chemical and mechanical properties of coal and its associated wastes, leading to changes in slope stability. The stability of any slope is heavily influenced by the geotechnical conditions of the site including the physical and mechanical properties of the built-up materials and hydrological conditions. Coal combustion generates substantial heat, affecting slope stability through various mechanisms: thermal expansion, material degradation, and gas generation, significantly altering the geotechnical properties of mine waste materials, which are critical for slope stability. Key impacts include:  
Reduction in Shear Strength: In the middle of the slope, where combustion occurs, the temperature can reach several hundred degrees Celsius [20; 43; 85; 86; 87]. Various research studies have pointed out the impact of the temperature of soils and rocks strength [88; 89; 90]. Temperature gradients within the dump generate thermal stresses, which can lead to material cracking. In clay-rich dump materials, these temperature variations can induce significant fractures, compromising structural integrity. Elevated temperatures accelerate the decomposition of thermally sensitive minerals, such as pyrite, leading to the release of acidic byproducts and further weakening the material. Additionally, thermal expansion contributes to material degradation, weakening the bonds between particles and reducing the shear strength of the waste material, including the cohesion and friction angle. This reduction in shear strength is a primary factor contributing to slope instability, increasing the risk of failure in waste dumps and other natural/man-made earth structures. Thermal Stress Accumulation: Combustion within the mine waste dump generates gases that accumulate over time, leading to an increase in pore pressure and a subsequent reduction in effective stress. As the pressure builds up, these gases eventually escape through the dump, creating pathways that often manifest as surface fractures and/or cracks (Figure 4). This process induces volumetric expansion, which can weaken the structural integrity of the slope, potentially triggering progressive failure mechanisms and ultimately leading to a landslide. Additionally, the formation of an extensive fracture network further compromises slope stability by facilitating water infiltration, accelerating weathering, and reducing overall material cohesion. An example of 2D slope stability analysis showed the significant influence of fractures orientation and length on slope stability (Table 1). Figure 4: Occurrence of cracks/fractures on the slope surface caused by coal combustion within the dump body. Many other studies have also pointed out the significant impact of discontinuities on slope stability in various regions worldwide [101; 102; 103; 104]. Changes in Density and Compaction: The formation of voids within the mine waste dump after the combustion process is primarily due to the thermal degradation and oxidation of carbonaceous materials, leading to volume reduction and structural collapse. When self-heating or spontaneous combustion occurs within the dump, organic matter, sulfides and/or carbon, and other combustible materials undergo exothermic reactions, producing gases. As these gases escape, they create cavities within the dump. Additionally, the combustion process results in the thermal decomposition of minerals, altering their physical and mechanical properties. Clays and sulfates may dehydate, while carbonates can decarbonate, further reducing the material volume.