2.1. Investigation and Characteristics of Geological Disaster Secondary Coal Mine Accidents
2.1.1. Distribution Characteristics of Geological Disasters in China
Geological disasters refer to the disasters related to geological processes, including rock avalanche, landslide, debris flow, ground collapse, ground fissure, and land subsidence, which are caused by natural factors or human activities and endanger the safety of people’s lives and property. Among them, rock avalanches, landslides, and debris flow are the most serious disasters, which were characterized by wide distribution, strong disaster occurrence and destructiveness, concealment, and easy chain disasters, and caused huge economic losses and casualties every year.
The term landslide (
Figure 1) refers to the natural phenomenon in which a soil mass or rock mass on the slope slides down the slope as a whole or dispersedly along a certain weak surface or zone under the effect of gravity due to river scouring, groundwater activity, rainwater immersion, earthquake, artificial slope cutting, and other factors.
Debris flow (
Figure 2) refers to a special torrent that carries a large amount of sediment and stones due to landslides caused by rainstorms, snowstorms, or other natural disasters in mountain areas, deep ravines, or steep terrain areas. Debris flow was characterized by suddenness, fast velocity, large flow, large material capacity, and strong destructive force.
Rock avalanche (
Figure 3) refers to the geological phenomenon that the rock and soil mass on the steep slope suddenly break away from the parent body, collapse, roll, and accumulate at the foot of the slope under the action of gravity.
2.1.2. Statistics of Typical Geological Disaster Secondary Coal Mine Accidents
According to the analysis in the
Table 1, 118 people died in 25 accidents, and ground buildings were severely damaged. Geological disasters pose a serious threat to coal mine safety.
From the perspective of the inducements of secondary coal mine accidents caused by geological disasters, there were 18 landslide accidents, accounting for 72%, 3 debris flow accidents, accounting for 12%, and 4 collapse accidents, accounting for 16%. Landslide accidents are the most likely to induce secondary coal mine accidents.
From the perspective of disaster-bearing bodies, geological disasters secondary coal mine accidents mainly caused damage to surface workers and buildings, totaling 23, accounting for 92%, with a relatively small threat to underground workers and buildings.
From the perspective of the causes of geological disasters, due to the vertical and horizontal gullies in the mining area, severe weathering, and erosion, favorable terrain has been created for the occurrence of geological disasters. With the increasing human engineering activities in the mining area, human engineering activities have become a huge force to change the landform of the mining area. It was intertwined and superposed with natural geological processes, resulting in a relatively stable geological environment tending to be unstable. In addition, the annual distribution of precipitation in some mining areas is extremely uneven. More than 85% of the precipitation was concentrated in the rainy season from May to September, and there were many heavy rains during this period. The precipitation permeated through the soil and penetrated into the rock and soil mass along the pores and fissures in a large amount in a short time, which made the rock and soil mass unstable and lead to a large number of geological disasters. At the same time, mining engineering activities were also the main inducing factors of landslides in the mining area. With the continuous expansion of the construction scale of the mining area, the possibility of geological disasters in the mining area was also increasing.
2.2. Formation Conditions of Geological Disaster Secondary Coal Mine Accidents
The occurrence of geological disasters is usually the result of comprehensive factors. Generally, it includes natural factors and human factors. The natural factors include rock and soil mass type, landform, geological structure, rainfall, etc. Human factors mainly include slope cutting, building houses, road construction, mining, and other human engineering activities. The scale and number of geological disasters around the coal mining area are related to these factors. The following details the formation conditions and influencing factors of geological disasters around the coal mining area and the coal mine accidents induced by them.
2.2.1. Topography
The development of geological disasters was strongly affected by the landform, and the undulating mountains provide a prerequisite for the occurrence of geological disasters. At the same time, the slope structure and shape played a decisive role in the distribution of the internal stress of the slope, and directly affected the stability and sliding deformation mode of the slope. The following will analyze the control effect of landform on geological disasters from three aspects: macro landform, slope angle, and slope height.
- (1)
Landform
The landform around the mining area is restricted by geological structure, neotectonic movement, and stratum lithology.
- (2)
Slope angle
The slope angle has a key influence on the stress distribution and magnitude of the slope. The steeper the slope, the greater the tensile stress near the slope face, and the larger the influence range of tensile stress. The larger the slope is, the more concentrated the stress near the free face will be. The corresponding shear stress will also be concentrated and increased, thus causing the slope to lose stability and produce various disasters.
- (3)
Slope height
The occurrence of geological disasters is obviously affected by the slope height. The slope height directly increases the free face of the slope, making the stress value inside the slope increase with the increase of the slope height. At the same time, the higher the slope, the more concentrated the stress at the free face of the slope, which reduces the safety factor of the slope. When an area is at a high altitude, it will be exposed to solar radiation for a long time. The temperature will rise quickly in the daytime and cool down quickly at night. This larger scale of cold and hot alternation makes the rock and soil on the surface easier to break and is more likely to cause rock avalanches and landslides.
2.2.2. Geological Structure
Geological disasters such as collapses and landslides are affected by faults and special tectonic systems. The geological structure has a direct impact on the structure and fragmentation of rock and soil mass, thus indirectly affecting the development of collapse, landslide, and unstable slope. Generally, under the influence of structure, the joints and fissures of regional rocks with strong faults and special structural systems are developed, the rock mass is relatively broken, and geological disasters are prone to occur.
2.2.3. Types of Rock and Soil Mass and Geological Disasters
The type of rock and soil mass directly affects the basic development of geological disasters. Generally, different rock and soil mass may form geological disasters internally. If the lithology and sedimentary types of the strata around the mining area are complex, the lithology changes greatly, and the sedimentary age span is large, providing conditions for the development of geological disasters.
Under the stability environmental conditions, the aeolian sediments, alluvial proluvial sediments, and residual slope sediments deposited at the foot of the slope are relatively stable. However, in case of rain or when the free face is formed at the foot of the slope due to manual excavation, deformation and damage are likely to occur, resulting in landslide or collapse. Collapse is mainly located on the bedrock. When the rock mass structure is broken, highly weathered, and joints are developed, collapse is easy to occur on the slope with free face.
The pores of rock and soil around the mining area are developed, and some of them produce vertical joints. In addition, the impact of precipitation is very easy to induce geological disasters such as collapse and landslides. Due to the poor engineering properties of rock and soil mass, and a large number of artificial mining activities in this area, a large number of coal seam goafs have been formed, resulting in a large area of ground collapse.
2.2.4. Rainfall
The formation of rock avalanches, landslides, and debris flow are closely related to rainfall. Generally, precipitation will not directly cause landslides and collapse geological disasters. However, rainwater will infiltrate along loess structural joints to form sinkholes and other areas, and long-term rainwater infiltration will greatly reduce the strength of rock and soil mass, thus causing geological disasters. The former study found that the hidden danger of geological disasters is basically positively related to precipitation.
2.2.5. Water System
Most of the rivers near the mining area originate from the surrounding mountains and belong to seasonal rivers. Most of the river water comes from atmospheric precipitation. The precipitation is mostly concentrated in summer and autumn, especially in the case of heavy rain and rainstorm, which makes the water flow gather in the valley in a short time, causing different degrees of downward erosion and lateral erosion to the surface rock and soil mass, creating unique disaster conditions.
2.2.6. Human engineering activities
In recent years, with the substantial increase of human activities, such as slope cutting, building houses and roads, reducing vegetation, and a large number of mining activities, the original geological environmental conditions have been changed, creating conditions for the occurrence of geological disasters.
- (1)
Geological disasters caused by mining
Large scale mining has destroyed the original geological structure, reduced the stability of the mountain, and is prone to collapse and ground collapse. At the same time, a large number of solid wastes formed by mining are piled up randomly, providing conditions for the development of landslides and debris flows.
- (2)
Geological disasters caused by building
Due to the vertical and horizontal gullies, there is less flat land available for building houses. Most of the houses and surface structures in the mining area are built at the foot of the slope. Slope cutting changes the natural shape of the natural slope, affects its stability, and causes a large number of hidden soil slopes or adverse geological phenomena.
- (3)
Geological disasters caused by road construction
For coal mining, roads need to be built around the mining area. Slope cutting, road construction, and slope toe excavation are directly related to the induction of geological disasters. If the hidden danger points are not protected, rock avalanches, landslides, collapses, and rockfalls will become serious.
2.2.7. Vegetation Coverage
Vegetation can play a role in slope protection to some extent, and has a positive role in preventing water and soil loss, and generally has a certain degree of impact on the development and stability of slope morphology. The higher the vegetation coverage is, the less geological disasters are developed. The lower the vegetation coverage, the more developed the geological disasters. Generally, the vegetation coverage and the distribution of geological disasters have a certain degree of correlation, but it does not play a decisive role in the development of geological disasters.