Last data update: Sep 16, 2024. (Total: 47680 publications since 2009)
Records 1-10 (of 10 Records) |
Query Trace: Rashed G[original query] |
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Exploration of limestone pillar stability in multiple-level mining conditions using numerical models
Rashed G , Slaker B , Murphy M . Min Metall Explor 2022 39 (5) [Epub ahead of print] Pillar stability continues to be a significant concern in multiple-level mining conditions, particularly for deep mines when pillars are not stacked or the thickness of interburden between mining levels is thin. The National Institute for Occupational Safety and Health (NIOSH) is currently conducting research to investigate the stability of pillars in multiple-level limestone mines. In this study, FLAC3D models were created to investigate the effect of interburden thickness, the degree of pillar offset between mining levels, and in situ stress conditions on pillar stability at various depths of cover. The FLAC3D models were validated through in situ monitoring that was conducted at a multiple-level stone mine. The critical interburden thickness required to minimize the interaction between the mining levels on top-level pillar stability was explored, where the top level mine was developed first followed by the bottom level mine. The model results showed that there is an interaction between numerous factors that control the stability of pillars in multiple-level conditions. A combination of these factors may lead to various degrees of pillar instabilities. The highest degree of local pillar instability occurred when pillar overlap ranges between 10 and 70%. On the contrary, the highest degree of stability occurs when the pillars are stacked, the underlying assumption is that the interburden between mining levels is elastic (never fails). Generally, for depths of cover investigated in this study, the stability of top-level pillars shallower than 100 m (328 ft) or with interburden thicknesses greater than 1.33 times the roof span - 16 m (52.4 ft) in this study - does not appear significantly impacted by pillar offset. The results of this study improve understanding of multiple-level interactions and advances the ultimate goal of reducing the risk of pillar instability in underground stone mines. |
A parametric study for the effect of dip on stone mine pillar stability using a simplified model geometry
Rashed G , Slaker B , Sears MM , Murphy MM . Min Metall Explor 2021 38 (2) 967-977 In this study, a parametric study was conducted using FLAC3D numerical models to examine the impact of oblique loading, generated from seam dip, on the strength and the failure propagation pattern of a stone pillar using two simplified geometry types. In type 1, the sidewalls of the pillars were assumed to be perpendicular to the roof and the floor, while in type 2, the sidewalls of pillars were assumed to be vertical. The complex pillar geometry in dipping mines was frequently modeled using these two geometries. To capture a complete picture of the effect of seam dip on pillar stability, the modeled width-to-height (W/H) ratio of the pillars, in situ stress field, and pillars roof/floor interfaces were systematically varied to account for the potential distribution of values for these parameters across the underground stone mines in USA. Results from the numerical modeling indicate that dipping pillars have reduced strength compared with horizontal pillars. Also, an asymmetric failure propagation pattern could be obtained depending on an interaction between the W/H ratio, seam dip, in situ stresses, and pillar geometry. |
Analysis of the impacts of mining sequence and overburden depth on stability at a dipping limestone mine
Sears MM , Slaker B , Rashed G , Miller J . Min Metall Explor 2021 38 (2) 959-965 Ground falls represent a significant hazard at underground mines in the stone, sand, and gravel (SSG) sector in the USA. Researchers from the National Institute for Occupational Safety and Health (NIOSH) are currently conducting detailed investigations into the complex loading conditions at underground stone mines operating in challenging conditions. This paper presents the application of numerical modeling to analyze pillar and roof stability at a dipping underground limestone mine. A validated numerical model was used to explore the potential behavior of the pillars and roof as loading conditions change. The validated model was used to compare changes in mining sequence, overburden depth, and the in situ stress field. This will allow mine operators and engineers to have a better idea of the conditions that could be encountered as mining progresses. Results from the numerical modeling indicate that roof displacement more than doubles as the vertical stress increases from 10 MPa (1450 psi) to 19 MPa (2750 psi) when the maximum and minimum horizontal stresses were 41 MPa (5950 psi) and 22 MPa (3190 psi), respectively. Consequently, as the pillar load increases, the safety factor of the pillars is projected to decrease by about 25%. The impact of the practical application of numerical models can result in a reduction of ground-fall accidents and injuries as well as generally safer working conditions. |
Preliminary rib support requirements for solid coal ribs using a coal pillar rib rating (CPRR)
Mohamed K , Van Dyke M , Rashed G , Sears MM , Kimutis R . Int J Min Sci Technol 2020 31 (1) 15-22 Researchers from the National Institute for Occupational Safety and Health (NIOSH) are developing a coal pillar rib rating (CPRR) technique to measure the integrity of coal ribs. The CPRR characterizes the rib composition and evaluates its impact on the inherent stability of the coal ribs. The CPRR utilizes four parameters: rib homogeneity, bedding condition, face cleat orientation with respect to entry direction, and rib height. All these parameters are measurable in the field. A rib data collecting procedure and a simple sheet to calculate the CPRR were developed. The developed CPRR can be used as a rib quality mapping tool in underground coal mines and to determine the potential of local rib instabilities and support requirements associated with overburden depth. CPRR calculations were conducted for 22 surveyed solid coal ribs, mainly composed of coal units. Based on this study, the rib performance was classified into four categories. A preliminary minimum primary rib support density (PRSD) line was obtained from these surveyed cases. Two sample cases are presented that illustrate the data collection form and CPRR calculations. |
A coal rib monitoring study in a room-and-pillar retreat mine
Rashed G , Mohamed K , Kimutis R . Int J Min Sci Technol 2020 31 (1) 127-135 The National Institute for Occupational Safety and Health (NIOSH) conducted a comprehensive monitoring program in a room-and-pillar mine located in Southern Virginia. The deformation and the stress change in an instrumented pillar were monitored during the progress of pillar retreat mining at two sites of different geological conditions and depths of cover. The main objectives of the monitoring program were to better understand the stress transfer and load shedding on coal pillars and to quantify the rib deformation due to pillar retreat mining; and to examine the effect of rib geology and overburden depth on coal rib performance. The instrumentation at both sites included pull-out tests to measure the anchorage capacity of rib bolts, load cells mounted on rib bolts to monitor the induced loads in the bolts, borehole pressure cells (BPCs) installed at various depths in the study pillar to measure the change in vertical pressure within the pillar, and roof and rib extensometers installed to quantify the vertical displacement of the roof and the horizontal displacement of the rib that would occur during the retreat mining process. The outcome from the monitoring program provides insight into coal pillar rib support optimization at various depths and geological conditions. Also, this study contributes to the NIOSH rib support database in U.S coal mines and provides essential data for rib support design. |
Monitoring of multiple-level stress interaction at two underground limestone mines
Slaker B , Murphy M , Rashed G , Gangrade V , Van Dyke M , Minoski T , Floyd K . Min Metall Explor 2020 38 (1) 623-633 The National Institute for Occupational Safety and Health (NIOSH) has previously established pillar design guidelines for shallow, flat-lying mines and single-level operations. Little guidance exists for ground control design in multiple-level stone mines, and understanding the interactions between levels would allow engineers to better select interburden thicknesses and the necessary amount of pillar columnization. To investigate these loading conditions in multiple-level environments, NIOSH has partnered with two separately operated multiple-level mines to study the stress interaction between the levels as undermining occurs. The first mine is located in Tennessee with up to a 243-m overburden and 7-m interburden thickness between levels. The second mine is located in Kentucky with a 304-m overburden and 26-m interburden thickness between levels. The monitoring program at these sites includes stressmeters and LiDAR for tracking stress redistributions and rock displacement in response to undermining. Monitoring is ongoing, but numerical modeling results show the expected interaction between levels. |
Molecular characterization and clinical outcomes of Candida auris infection: Single-center experience in Saudi Arabia.
Almaghrabi RS , Albalawi R , Mutabagani M , Atienza E , Aljumaah S , Gade L , Forsberg K , Litvintseva A , Althawadi S . Mycoses 2020 63 (5) 452-460 BACKGROUND: Candida auris is a difficult-to-diagnose multidrug-resistant yeast that can cause invasive infections with high mortality. Since emerging in 2009, this pathogen has been associated with numerous outbreaks around the world. Whole genome sequencing (WGS) is instrumental for understanding the emergence and local transmission of this pathogen. OBJECTIVES: To describe the clinical, molecular characteristics of Candida auris infection and clinical outcome in our center. PATIENTS AND METHODS: Patients with positive cultures for Candida auris were identified in a microbiology database. Clinical characteristics and antifungal susceptibility were obtained. Isolates were sent to the US CDC for whole genome sequencing. RESULTS: Seven unique patients with eight different isolates were identified. Seven isolates were sent to the US CDC for whole genome sequencing. None of the patients had blood stream infection. 30-day mortality was higher in infected patients compared with those who were colonized. Seven of the eight isolates were resistant to both fluconazole and five were resistant to amphotericin B. WGS analysis demonstrated that the seven isolates belonged to the South Asian clade but formed two distinct subclades suggesting two independent introductions and ongoing transmission within the facility. CONCLUSIONS: Candida auris is associated with a high mortality rate in infected patients. Strict infection control measures and surveillance for asymptomatic cases are warranted to halt ongoing transmission. |
Loading characteristics of mechanical rib bolts determined through testing and numerical modeling
Mohamed K , Rashed G , Radakovic-Guzina Z . Int J Min Sci Technol 2020 30 (1) 17-24 Underground coal mines use mechanical bolts in addition to other types of bolts to control the rib deformation and to stabilize the yielded coal ribs. Limited research has been conducted to understand the performance of the mechanical bolts in coal ribs. Researchers from the National Institute for Occupational Safety and Health (NIOSH) conducted this work to understand the loading characteristics of mechanical bolts (stiffness and capacity) installed in coal ribs at five underground coal mines. Standard pull-out tests were performed in this study to define the loading characteristics of mechanical rib bolts. Different installation torques were applied to the tested bolts based on the strength of the coal seam. A typical tri-linear load-deformation response for mechanical bolts was obtained from these tests. It was found that the anchorage capacity depended mainly on the coal strength. Guidelines for modeling mechanical bolts have been developed using the tri-linear load-deformation response. The outcome of this research provides essential data for rib support design. |
Epidemiology of carbapenem-resistant Enterobacteriaceae in Egyptian intensive care units using National Healthcare-associated Infections Surveillance Data, 2011-2017
Kotb S , Lyman M , Ismail G , Abd El Fattah M , Girgis SA , Etman A , Hafez S , El-Kholy J , Zaki MES , Rashed HG , Khalil GM , Sayyouh O , Talaat M . Antimicrob Resist Infect Control 2020 9 (1) 2 Objective: To describe the epidemiology of carbapenem-resistant Enterobacteriaceae (CRE) healthcare-associated infections (HAI) in Egyptian hospitals reporting to the national HAI surveillance system. Methods: Design: Descriptive analysis of CRE HAIs and retrospective observational cohort study using national HAI surveillance data. Setting: Egyptian hospitals participating in the HAI surveillance system. The patient population included patients admitted to the intensive care unit (ICU) in participating hospitals. Enterobacteriaceae HAI cases were Klebsiella, Escherichia coli, and Enterobacter isolates from blood, urine, wound or respiratory specimen collected on or after day 3 of ICU admission. CRE HAI cases were those resistant to at least one carbapenem. For CRE HAI cases reported during 2011-2017, a hospital-level and patient-level analysis were conducted using only the first CRE isolate by pathogen and specimen type for each patient. For facility, microbiology, and clinical characteristics, frequencies and means were calculated among CRE HAI cases and compared with carbapenem-susceptible Enterobacteriaceae HAI cases through univariate and multivariate logistic regression using STATA 13. Results: There were 1598 Enterobacteriaceae HAI cases, of which 871 (54.1%) were carbapenem resistant. The multivariate regression analysis demonstrated that carbapenem resistance was associated with specimen type, pathogen, location prior to admission, and length of ICU stay. Between 2011 and 2017, there was an increase in the proportion of Enterobacteriaceae HAI cases due to CRE (p-value = 0.003) and the incidence of CRE HAIs (p-value = 0.09). Conclusions: This analysis demonstrated a high and increasing burden of CRE in Egyptian hospitals, highlighting the importance of enhancing infection prevention and control (IPC) programs and antimicrobial stewardship activities and guiding the implementation of targeted IPC measures to contain CRE in Egyptian ICU's . |
Coal rib response during bench mining: A case study
Sears MM , Rusnak J , Van Dyke M , Rashed G , Mohamed K , Sloan M . Int J Min Sci Technol 2017 28 (1) 107-113 In 2016, room-and-pillar mining provided nearly 40% of underground coal production in the United States. Over the past decade, rib falls have resulted in 12 fatalities, representing 28% of the ground fall fatalities in U.S. underground coal mines. Nine of these 12 fatalities (75%) have occurred in room-and-pillar mines. The objective of this research is to study the geomechanics of bench room-and-pillar mining and the associated response of high pillar ribs at overburden depths greater than 300 m. This paper provides a definition of the bench technique, the pillar response due to loading, observational data for a case history, a calibrated numerical model of the observed rib response, and application of this calibrated model to a second site. |
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