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In order to study the distribution characteristics of the “three zones” of coal spontaneous combustion in the goaf of double cut roof retained roadway, taking the working face 4502 of Shaqu No.1 Mine as the engineering background, a combination of on-site monitoring and numerical simulation was adopted to study the air leakage law and the distribution characteristics of the “three zones” of coal spontaneous combustion in the goaf of the working face 4502 with double cut roof retained roadway. The results show that the main air leakage areas in the goaf of the double cut roof retaining roadway are the corners of the working face and the retaining roadways on both sides. The air flow mainly leaks into the goaf from the corners on both sides, and there is air exchange between the goaf and the retaining roadways on both sides. There is a slender oxidation zone on the side of the double retaining roadway, and the oxidation zone is distributed in a U-shaped throughout the goaf. At different stages of mining, there are significant differences in the distribution of the “three zones” of coal spontaneous combustion in the working face with double cut roof retaining roadway. In the initial stage of mining, it is necessary to strengthen the prevention and control of coal spontaneous combustion within a range of 5 m on the side of the double cut roof retaining roadway and 32-65 m in the middle of the goaf. In the middle and later stages of mining, it is necessary to strengthen the prevention and control of coal spontaneous combustion within a range of 2.5 m on the side of the transportation retaining roadway, within a range of 12 m after 38 m from the track retaining roadway, and within a range of 30-80 m in the middle of the goaf.

Objective Aiming at the problem that the assembly quality of interference fit components is difficult to control, a method for monitoring the assembly quality of such components based on digital twin technology is proposed.Methods Combined with digital twin technology, the framework of an assembly quality monitoring system for interference fit components is constructed; the actual assembly process is comprehensively analyzed and a real-time monitoring process of the assembly process based on the press-fitting force is designed; and a state prediction model of the press-fitting force based on the Markov model is established, and a comprehensive judgment of the assembly quality of the interference fit components is realized by the comprehensive comparison of theoretical press-fitting force, actual press-fitting force and press-fitting force prediction state.Results The interference fit between the output shaft and sun gear of a typical marine planetary reducer is used as an example for verification. The results show that this method can realize the real-time collection and monitoring of the press-fitting force at key points in the assembly process and predict the future quality characteristics of the point. At the same time, the experimental results of the physical assembly platform are compared with the model prediction results, and it is found that the two are consistent, which proves that the proposed method of monitoring the assembly quality of interference fit components based on digital twin technology is effective and feasible.ConclusionThe results of this study have important reference significance for the realization of quality monitoring in the process of the intelligent assembly of mechanical products.

【Objective】As a common mangrove tree species in Hainan, Kandelia obovata is also a tree species widely used in mangrove ecological restoration. In recent years, global climate change and the increase of human disturbance have brought serious threats to mangrove ecosystem. Therefore, establishing fixed plots to monitor the dynamic changes of mangrove communities and populations has important research value for the protection and restoration of mangroves.【Method】The population structure characteristics and the dynamic change of the population in a fixed 1 hm2 plot in the national mangrove reserve of Dongzhai Harbor, Hainan Province were analyzed and studied.【Result】(1) The diameter class structure of the K. obovata population in the area showed an irregular pyramid shape, and the total number of individuals in the Ⅰ-Ⅳ age class accounted for 75.4% of the total number of individuals in the population, and the number of individuals in the Ⅳ age class decreased obviously. (2) The dynamic changes in the number of individuals in various age groups showed a trend of "recession-recession-recession-growth-growth growth-growth". The dynamic indices Vpi and V'pi of the whole population were both greater than 0 and tend to be 0, indicating that the K. obovata population in this area was a growth population with high sensitivity to external interference, and the growth trend was not obvious when subjected to external interference. (3) In the static life table of the K. obovata population, the life expectation of the population showed a trend of decreasing gradually. (4) The survival curve fitting equation was y=2406e-0.505x, R2 = 0.962, F= 194.036, P< 0.001, indicating that the survival curve of the K. obovata population was consistent with the Deevey-Ⅲ curve. (5) Both the mortality rate and disappearance rate showed a trend of increasing first, decreasing then increasing again. The peaks of appeared in Ⅴ and Ⅸ age classes were 52%, 73% and 77%, 92%, respectively. (6) Survival analysis showed that the survival rate (Si) decreased gradually, while the cumulative death rate (Fi) increased gradually. There was a crossover point between the Ⅱ and Ⅲ age classes. The death density (fti) and the risk rate (λti) of the population showed a decreasing trend, and the death density decreased sharply between the I and Ⅱ age classes, and the minimum value of death density was 0.3% in the Ⅷ age class. (7) Time series analysis also reflected that although there were a large number of individuals at the lower age level, relatively few individuals would develop to the older age level in the future several age levels, and the majority of the future population would be individuals at the middle and older age levels.【Conclusion】The results reflect that the K. obovata population of the research area is a growing population, which is more sensitive to external interference. The dynamic change of the numberof the senior population is more stable than that of the younger population, and the population in the future will be dominated by the senior individuals.

After Wenchuan earthquake, many large debris flow have occur in Banzi Gully. Especially in 2019, the "8·20" debris flow caused serious damage to the roads，bridges and villages in the gully mouth, mail river was seriously squeeaed to the other side and there is a big risk of blocking the river in the future. Based on the field investigation and comprehensive analysis of the basic characteristics and formation conditions of the debris flow, the characteristics of river blockage are analyzed, the parameters of river blockage under different debris flow frequency are calculated, and the possible impact of dam-breaking flood under each frequency on Miansi Town is predicted. The results show that debris flow with frequencies of 2%, 5% and 10% are less likely to cause river blockage of Min River, and if there was a river blocking event , the town of Miansi would not be exposed to dam-breaking flood. The debris flow with a frequency of 1% are likely to cause river blockage. The debris flow material with a volume of 57.38×104 m3 can reach the Min River to form a dam with a height of about 51.61 m. Under the action of the main river flood (1890.0 m3/s), the dam will be broken, and the peak flow rate of the flood was 5935.49 m3/s, which dropped to 2312.25 m3/s while arrive at Miansi Town. Since the flood depth (4.00 m) is higher than the height of the protective embankment (3.5 m), the flood is likely to damage the houses near the protective embankment of Miansi Town.This paper provided a reference for the analysis of the characteristics of large-scale debris flow blockage in the future, as well as the possible impact of the collapse flood on downstream towns.