ABSTRACT
The longwall mining method with gob-side entry retaining via roof cutting is a new underground coal mining method which has the characteristics of a high resource recovery ratio and environmental friendliness. Due to the complexity of this method, the research method of case-based dynamic on-site monitoring, analysis, adjustment, and optimization is usually adopted. Based on a roadway retaining via roof cutting project, in addition to the traditional indirect monitoring method of hydraulic support pressure, this study innovatively establishes a direct monitoring method for roof caving by monitoring the gangue pressure in the goaf, which provides data for the roof cutting effect and offers a new method for studying the overlying strata movement. In the project, a comprehensive monitoring and analysis system was established, including gangue pressure, cable bolt stress, bracket pressure, roadway deformation, and roof separation, which was used to dynamically analyze the effect of roof cutting and optimize the support design. The results show that the pressure of the hydraulic support close to the roof cutting is low, indicating that roof cutting is favorable in the roadway retaining mining method. The roadway deformation in the advanced abutment pressure area of the working face is small. The mining-induced stress caused by the collapse and compaction of the overlying strata in the goaf is the dominant factor affecting the effect of roadway retaining, especially in the 50-100 m range behind the working face, where the dynamic load causes high bearing capacity of the support elements, large roadway convergence, and roof separation. Temporary support and supplementary reinforcement should be added when necessary. The monitoring system presented in this study is highly comprehensive, simple, reliable, and low in cost, providing a reference for roof cutting roadway retaining projects and roof caving-related studies.
ABSTRACT
Background: Elderly people are susceptible to atrial fibrillation (AF) and ischemic stroke (IS); however, less information is known about the association between AF and the risk of cardiovascular disease (CVD) mortality in elderly population with IS. We aimed to investigate the features of AF among aged people with IS and to illustrate whether AF accounted for CVD mortality. Methods: At baseline, 790 patients with IS were enrolled from the general northeast Chinese elderly population (>60 years) between September 2017 to March 2019. The prevalence, awareness, and treatment of AF in each age group were analyzed, as well as major-related cardiovascular risk factors. The population was followed until July 31, 2021, and information on CVD death was obtained. Results: A total of 25 people had AF, and the prevalence of AF in the elderly population with IS was 3.2%. The AF prevalence grew along with age from 1% (60-64 years) to 4.3% (70-74 years) and 4.2% (≥75 years), which was higher in the urban residents than in the rural residents (5.7 vs. 2.2%, P = 0.014). The awareness and treatment rates of patients with AF were 80 and 8%. After a median follow-up period of 3.3 years, 58 subjects died due to CVD and 5 subjects were accompanied with AF (rate 70.6/1,000 person-years). Elderly IS patients with AF had a 3.65-fold increased risk of CVD death in the fully adjusted model when compared with non-AF participants. Conclusion: The AF prevalence increased with age among the elderly population with IS. Moreover, elderly patients with IS in northeast China with AF had a higher CVD mortality. Therefore, early screening and prompt management of AF in elderly population with IS in northeast China are required.