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Focusing on the patterns and characteristics of extraordinarily severe gas explosion accidents in Chinese coal mines.Process Safety and Environment Protection

Changzhou City, underlain by a multi layered aquifer system in Quaternary sediments in the Great Yangtze River Delta region, experienced a maximum land subsidence rate of 147 mm/year in the early 1980s due to excessive groundwater extraction. A large scale monitoring station of 11 borehole extensometers was established in the city in 1983 to investigate land subsidence. Nine strata were predetermined in the Quaternary depth interval and monitored by borehole extensometers. Presented in this paper are the long term observations of land subsidence, strata compression and groundwater level in four aquifers from 1984 to 2002, and discussion on strata compression based on the measured data with reference to the stratigraphy, soil properties, groundwater withdrawn and literature of similar situations. The compression of strata varies significantly and is strongly influenced by groundwater drawdown in the second confined aquifer, or CA2. The groundwater level in CA2 declined from −55 m in 1981 to −76 m in 1994 and the land subsidence rate remained high. After the city government enforced restrictions on groundwater extraction in 1995, the extraction rate sharply reduced, the groundwater level in CA2 steadily recovered to −56 m until 2003, and the land subsidence rate declined to 10 mm/year in 2002. From 1984 to 2002, the land subsidence was mainly attributed to the consolidation of the thick aquitard overlying CA2. In the future, to prevent the subsidence rate from rising, it is vital to stop groundwater drawdown in aquifers.

Earth fissures in Jiangsu Province, China have caused serious damages to properties, farmlands and infrastructures, and adversely affected the local or regional economic development. Under the geological and environmental background in Jiangsu Province, this paper presents the earth fissures caused by excessive groundwater withdrawal and coupled by distinctive geological structures, such as Ancient Yellow River Fault in Xuzhou karst area, and Ancient Yangtze River Course and bedrock hills in Suzhou, Wuxi and Changzhou area. Although all the earth fissures are triggered by groundwater exploitation, the characteristics are strongly affected by the specific geological and hydrogeological settings. In particular, in the water-thirsty Xuzhou City, the cone of depression caused by groundwater extraction enlarged nearly 20 times and the piezometric head of groundwater declined 17 m over a decade. As groundwater is extracted from the shallowly buried karst strata in the Ancient Yellow River Fault zone, the development of earth fissures is highly associated with the development of karstic cavities and sinkholes and their distribution is controlled by the Ancient Yellow River Fault with all the 17 sinkholes on the fault. On the other hand, in the rapidly developing Southern Jiangsu Province, groundwater is mainly pumped from the second confined aquifer in the Quaternary, which is distributed neither homogeneously nor isotropically. The second confined aquifer comprises more than 50 m thick sand over the Ancient Yangtze River Course, but this layer may completely miss on the riverbank and bedrock hills. With a typical drawdown rate of 4 m to 6 m per annum, the piezometric head of groundwater in the second confined aquifer has declined 76 m at Maocunyuan since 1970s and 40 m at Changjing since mid-1980s, and a large land subsidence, e.g. 1 100 mm at Maocunyuan, is triggered. Coupled with the dramatic change of the bedrock topography that was revealed through traditional geological drilling and modern seismic reflection methods, the geological-structure-controlled differential settlement and earth fissures are phenomenal in this area.

The coal mine production industry is a complex sociotechnical system with interactive relationships among several risk factors. Currently, causation analysis of gas explosion accidents is mainly focused on the aspects of human error and equipment fault, while neglecting the interactive relationships among risk factors. A new method is proposed through risk coupling. First, the meaning of risk coupling of a gas explosion is defined, and types of risk coupling are classified. Next, the coupled relationship and coupled effects among risk factors are explored through combining the interpretative structural modeling (ISM) and the NK model. Twenty-eight representative risk factors and 16 coupled types of risk factors are obtained through analysis of 332 gas explosion accidents in coal mines in China. Through the application of the combined ISM-NK model, an eight-level hierarchical model of risk coupling of a gas explosion accident is established, and the coupled degrees of different types of risk coupling are assessed. The hierarchical model reveals that two of the 28 risk factors, such as state policies, laws, and regulations, are the root risk factors for gas explosions; nine of the 28 risk factors, such as flame from blasting, electric spark, and local gas accumulation, are direct causes of gas explosions; whereas 17 of the risk factors, such as three-violation actions, ventilation system, and safety management, are indirect ones. A quantitative analysis of the NK model shows that the probability of gas explosion increases with the increasing number of risk factors. Compared with subjective risk factors, objective risk factors have a higher probability of causing gas explosion because of risk coupling.