Engineering Geological Effects of Burnt Rocks in the Northeastern Margin of Ordos Basin

Burnt rocks is formed by the spontaneous combustion of coal seams on the surrounding rock mass at high temperature, and is mostly distributed in coalfields in northwest China (Xinjiang, Ningxia, Shaanxi, Gansu, Inner Mongolia, etc.). In this paper, the formation mode and engineering geological effects of burnt rocks along the Yellow River and Junger Banner in Shenmu, Fugu, Baode-Hequ Yellow River and Junger Banner were studied through field geological survey, actual measurement and data collection. The results show that burnt rock can be divided into four types: baked rock, baked metamorphic rock, sintered rock and burnt lava. After baking at high temperatures from spontaneous combustion in the coal seams, compared with the original rock, the microstructure and mineralogical composition of the burnt rock have changed significantly, with the development of pores and cracks, decreased strength, and increased permeability and hydraulic conductivity. The structure of the burn rock changes from laminated to fractured, the rock mass decreases, and it becomes an important groundwater storage space in the mining area. Affected by spontaneous combustion of coal seam, the bearing capacity of rock mass decreases, and the burned-out zone above the burned rock produces the unloading effect of slope, forming through joint surface and weak contact surface in the rock mass, and inducing slope instability under rainfall, freeze-thaw and surface water infiltration, resulting in geological disasters. The low water table in the burnt rock area leads to sparse vegetation and fragile ecology. This study describes the engineering geological and environmental effects induced by burnt rocks from the aspects of formation mechanism and engineering geological conditions, which are mainly manifested in the aspects of slope failure and instability, ground subsidence and ground cracks, water damage in mines, and degradation of vegetation in burnt rock areas, and provides theoretical and practical references to engineering activities and disaster prevention and control in burnt rock areas.