Influence of coplanar double fissures on failure characteristics of sandstone and fracture mechanics analysis

The influence of fissure angles and bridge lengths on rock mechanical properties and failure was analyzed using the uniaxial compression test and the Digital Image Correlation (DIC) technique. The research findings are as follows: 1) Peak stress and elastic modulus of the samples exhibited an obvious change trend with the change in fissure angle. The fissure angle has a more significant effect on a rock’s mechanical properties than the length of the rock bridge. 2) With an increase in the fissure angle, the number of surface cracks, main failure cracks and surface spalling decreased, whereas the area of the falling blocks significantly increased. However, with an increase in bridge length, the characteristics of crack propagation and spalling are essentially the same. During crack propagation, the connection of the rock bridge is related to its fissure angle and length. 3) At a low fissure angle, the failure mode of rock samples is dominated by tensile-failure cracks; with an increase in fissure angle, the tension-damage to shear-damage crack transformation will form a mixed tensile-shear damage mode; at the same time, with an increase in bridge length, the rock bridge becomes more difficult to connect, and the local crack expansion failure changes from tensile-shear cracks to tensile cracks. 4) Stress on the coplanar double-fissured rock sample was simplified and analyzed to explain the behavior of fractures on the sample. These research results have an important guiding value for engineering optimal designs

The impact of the COVID-19 pandemic lockdown on rhegmatogenous retinal detachment services—Experiences from the Tongren eye center in Beijing

Purpose- To investigate the impact on services for rhegmatogenous retinal detachment (RRD) patients during the COVID-19 (2019coronal virus disease) pandemic in one tertiary center in Beijing. Methods- A retrospective cohort study. Two reviewed consecutive RRD patients cohorts of the same length were treated during two different periods: the COVID-19 pandemic and the pre-COVID-19 group. The characteristics of patients, surgery, anesthesia methods, length of hospital stay, and the latest follow-up were recorded and analyzed. Results- There were 79 patients in the COVID-19 pandemic group with a 55.9% reduction (179). Compared to patients in the pre-COVID-19, patients in the COVID-19 pandemic had a longer presurgical waiting times (28days, 3days, p<0.001), a higher percentage of patients with presurgical poor (less than 0.02) visual acuity (55.7%, 32.4%, p = 0.009), and a higher percentage of patients with presurgical choroidal detachment (34.2%, 19.6%, p = 0.01). There was no significant difference in the severity of presurgical proliferative vitreoretinopathy between the two groups (p = 0.64). Surgeries on pathological myopia patients with macular hole retinal detachment were postponed in the COVID-19 pandemic. There was a lower percentage of scleral buckling (27.8%, 41.3%, p = 0.02) and a lower rate of subretinal fluid drainage (45.4%, 75.7%, p = 0.01) in the COVID-19 pandemic. There was no significant difference in either postoperative visual acuity (p = 0.73) or the rate of single-surgery retinal attachment (p = 1) between the two groups. Patients in the COVID-19 pandemic had a shorter length of hospital stay (3hours, 35 hours, p<0.001), and a lower percentage of patients received general anesthesia (48.1%, 83.2%, p<0.001). None was infected with COVID-19 disease during the pandemic. Conclusion- The COVID-19 pandemic lockdown caused prolonged presurgical waiting times, shorter hospital stays, less general anesthesia, and a significant reduction of RRD surgeries. The RD were more complicated, the surgeons were more conservative on procedures and patients selection, while the surgery outcomes were comparable

Mechanical properties and failure law of composite rock containing two coplanar fractures

Composite rocks comprise the rock structures that are commonly used in geotechnical engineering. The fracture configuration has a substantial influence on the mechanical behavior, failure mode, and crack propagation of composite rocks. In this study, we considered a composite rock with two prefabricated coplanar fractures. Through laboratory uniaxial compression tests and using a digital image acquisition system, we systematically studied the effects of different fracture lengths and inclination angles on the mechanical properties and failure characteristics of the rocks. We obtained the following results: 1) during the loading deformation of the rock sample, the peak stress and elastic modulus increased with an increase in the fracture inclination angle and decreased with an increase in the fracture length. The deterioration coefficient k (the ratio of the difference between the peak strength of intact and fractured rock sample to that of intact rock sample) decreased with an increase in the fracture inclination angle and increased with an increase in the fracture length. 2) The failure type of the rock samples was primarily controlled by the fracture inclination angle and material of the two rock types, and the fragmentation degree was primarily controlled by the fracture length. With an increase in the fracture inclination angle, the failure mode of rock sample exhibited the following order of changes leading to failure: a double-Y type (trwo wing and one antiwing cracks appeared on each prefabricated fracture) → double-Z type (two wing cracks appeared on each prefabricated fracture) → Z type (one wing crack appeared on each prefabricated fracture). 3) The type of coalescence of the rock bridge was controlled by the fracture inclination angle and structural plane. The crack positions were primarily affected by the fracture length. 4) At a low fracture inclination angle (α ≤ 30°), the propagation of the microcracks showed aggregated band formation. Above moderate fracture inclination angles (α &gt; 30°), the microcrack aggregation band gradually weakened and expanded in the direction of dispersion

The Impact of the Covid-19 Pandemic on Rhegmatogenous Retinal Detachment Treatment Patterns

Abstract Background: To describe changes in rhegmatogenous retinal detachment (RRD) surgical procedures in Beijing during the COVID-19 pandemic. Material and Methods:A retrospective cohort of RRD patients was analyzed. Patients were divided into the COVID-19 pandemic group and pre-COVID-19 group according to their presentation. The presurgery characteristics, surgical procedures, and surgery outcomes were collected. The potential factors related to the choice of pars plana vitrectomy (PPV) or scleral buckling (SB) were analyzed using logistic regression. The differences in the procedure choice under specific conditions were compared. Surgery outcomes were compared between the two groups.Results:In the COVID-19 pandemic group, less patients received SB (27.8%, 41.3%, p=0.02) while more patients received PPV (72.2%, 58.6%, p=0.02); in patients who received SB, fewer patients received subretinal fluid drainage (45.4%,75.7%, p=0.01); in patients who received PPV, fewer patients received phacovitrectomy (7.0%, 21.0%, p=0.02). The choice of PPV was related to older age (1.03, p=0.005), the presence of RRD with choroidal detachment (RRD-CD) (2.92, p=0.03), pseudophakia (5.0, p=0.002), retinal breaks located posterior to the equator(4.87, p&lt;0.001), macular holes (9.76, p=0.005), and a presurgery visual acuity (VA) less than 0.02 (0.44 , p=0.03). Fewer phakia patients with retinal breaks located posterior to the equator (1/28, 11/30, p=0.01) and fewer patients with chronic RRD and subretinal strand (1/9, 9/16, p=0.03) received SB in the COVID-19 pandemic group. More patients with improved VA (55.7%, 40.2%, p=0.03) in the COVID-19 pandemic group. The overall single-surgery retinal attachment rate was similar in the two groups (94.9%, 94.5%, p=0.99).Conclusions:During the COVID-19 pandemic, the main reason for the increased number of PPV in RRD treatment was that more complicated cases were presented. However, the surgeons were conservative in procedure choice in specific cases. The adjustments on RRD treatments leads to comparable surgery outcomes.</jats:p

The Impact of the Covid-19 Pandemic on Rhegmatogenous Retinal Detachment Treatment Patterns

Abstract Background: To describe changes in rhegmatogenous retinal detachment (RRD) surgical procedures in Beijing during the COVID-19 pandemic. Material and Methods:A retrospective cohort of RRD patients was analyzed. Patients were divided into the COVID-19 pandemic group and pre-COVID-19 group according to their presentation. The presurgery characteristics, surgical procedures, and surgery outcomes were collected. The potential factors related to the choice of pars plana vitrectomy (PPV) or scleral buckling (SB) were analyzed using logistic regression. The differences in the procedure choice under specific conditions were compared. Surgery outcomes were compared between the two groups.Results:In the COVID-19 pandemic group, less patients received SB (27.8%, 41.3%, p=0.02) while more patients received PPV (72.2%, 58.6%, p=0.02); in patients who received SB, fewer patients received subretinal fluid drainage (45.4%,75.7%, p=0.01); in patients who received PPV, fewer patients received phacovitrectomy (7.0%, 21.0%, p=0.02). The choice of PPV was related to older age (1.03, p=0.005), the presence of RRD with choroidal detachment (RRD-CD) (2.92, p=0.03), pseudophakia (5.0, p=0.002), retinal breaks located posterior to the equator(4.87, p&lt;0.001), macular holes (9.76, p=0.005), and a presurgery visual acuity (VA) less than 0.02 (0.44 , p=0.03). Fewer phakia patients with retinal breaks located posterior to the equator (1/28, 11/30, p=0.01) and fewer patients with chronic RRD and subretinal strand (1/9, 9/16, p=0.03) received SB in the COVID-19 pandemic group. More patients with improved VA (55.7%, 40.2%, p=0.03) in the COVID-19 pandemic group. The overall single-surgery retinal attachment rate was similar in the two groups (94.9%, 94.5%, p=0.99).Conclusions:During the COVID-19 pandemic, the main reason for the increased number of PPV in RRD treatment was that more complicated cases were presented. However, the surgeons were conservative in procedure choice in specific cases. The adjustments on RRD treatments leads to comparable surgery outcomes.</jats:p

The impact of the COVID-19 Pandemic on rhegmatogenous retinal detachment treatment patterns

Abstract Backgrounds To describe changes in rhegmatogenous retinal detachment (RRD) surgical procedures in Beijing during the COVID-19 Pandemic. Methods A retrospective cohort of RRD patients was analyzed. Patients were divided into the COVID-19 pandemic group and pre-COVID-19 group according to their presentation. The presurgery characteristics, surgical procedures, and surgery outcomes were collected. The potential factors related to the choice of pars plana vitrectomy (PPV) or scleral buckling (SB) were analyzed using logistic regression. The differences in the procedure choice under specific conditions were compared. Surgery outcomes were compared between the two groups. Results In the COVID-19 pandemic group, less patients received SB (27.8, 41.3%, p = 0.02) while more patients received PPV (72.2, 58.6%, p = 0.02); in patients who received SB, fewer patients received subretinal fluid drainage (45.4,75.7%, p = 0.01); in patients who received PPV, fewer patients received phacovitrectomy (7.0, 21.0%, p = 0.02). The choice of PPV was related to older age (1.03, p = 0.005), the presence of RRD with choroidal detachment (RRD-CD) (2.92, p = 0.03), pseudophakia (5.0, p = 0.002), retinal breaks located posterior to the equator (4.87, p &lt; 0.001), macular holes (9.76, p = 0.005), and a presurgery visual acuity (VA) less than 0.02 (0.44, p = 0.03). Fewer phakia patients with retinal breaks located posterior to the equator (1/28, 11/30, p = 0.01) and fewer patients with chronic RRD and subretinal strand (1/9, 9/16, p = 0.03) received SB in the COVID-19 pandemic group. There were more patients with improved VA (55.7, 40.2%, p = 0.03) in the COVID-19 pandemic group. The overall single-surgery retinal attachment rate was similar in the two groups (94.9, 94.5%, p = 0.99). Conclusions During the COVID-19 Pandemic, the main reason for the increased number of PPV in RRD treatment was that more complicated cases were presented. However, the surgeons were conservative in procedure choice in specific cases. The adjustments on RRD treatments lead to comparable surgery outcomes. </jats:sec

The risk factors of the progression of rhegmatogenous retinal detachment on patients with the fourteen-day quarantine in the early period of COVID-19 outbreak

Abstract Backgrounds The COVID-19 Pandemic has a great impact on hospitals and patients. The 14-day quarantine caused surgery of rhegmatogenous retinal detachment (RRD) postponed. We aimed to explore the risk factors of RRD progression in a group of patients whose surgery was postponed during the top-level emergency response of COVID-19. Methods A retrospective case series. Medical records of all consecutive patients with a diagnosis of RRD who underwent a surgical treatment at Beijing Tongren Hospital’s retina service from February 16, 2020, to April 30, 2020 have been reviewed retrospectively. Medical history, symptoms, and clinical signs of progression of RRD were recorded. RRD progression was defined as the presence of either choroidal detachment or proliferative vitreoretinopathy (PVR) progression during the quarantine period. Risk factors were analyzed using the Cox proportional hazards model, survival analysis, and logistic regression. Results Seventy-nine eyes of 79 patients met the inclusion criteria and were included in the study. The median time from the patients’ presentation at the clinic to admission for surgery was 14 days (3–61 days). There were 70 cases (88.6%) who did not present to the hospital within 1 week of the onset of visual symptoms. There were 69 (87.3%) macular-off cases at the presentation and 27 (34.2%) cases combined with choroidal detachment. There were 49 (62.0%) cases with PVR B, 22 (27.8%) cases with PVR C, 4 (5.1%) cases with PVR D, and 4 (5.1%) cases with anterior PVR. After the 14-day quarantine, 21 (26.6%) cases showed RRD progression, and 9 cases showed RRD regression at the time of surgery. Neither the time of onset of the visual symptom (p = 0.46) nor the time between presentation and admission (p = 0.31) was significantly different between the patients with RRD progression and patients without RRD progression. The combination of choroidal detachment (3.07, 1.68-5.60, p&lt;0.001) and retinal breaks located posterior to the equator (3.79, 1.21-11.80, p=0.02) were factors related to the progression of RRD. Conclusions In our study during the COVID-19 outbreak, the RRD progression risk factors included a combination of choroidal detachment and retinal breaks posterior to the equator. Ophthalmologists should schedule the surgeries for RRD patients with these signs as soon as possible. </jats:sec

Crack propagation law of rock with single fissure based on PFC2D

In the study of pre-fabricated single-fissured rock, the failure mode, mechanical properties and propagation law of rock are affected by the length and inclination angle of fissure. In this study, artificially prepared marble-like rocks with different fissure characteristics were used as rock representatives, using PFC2D software to establish a uniaxial compression model tests for rocks with fissures. The effects of different fissure lengths and inclination angles on the failure mode and crack initiation and propagation of rock-like structures were systematically studied, revealing the mechanism of macroscopic mechanical behavior of rocks containing single fissures in the process of crack initiation, propagation, and failure at the micro-level. The results show that: 1) Failure mode of the rock sample is mainly controlled by fissure inclination angle and fragmentation degree is mainly controlled by fissure length. 2) The initiation stress, damage stress, and peak stress of rock samples during loading deformation increase with the increase of fissure inclination angle, and decrease with the increase of fissure length. The crack initiation angle decreases with the increase of the fissure inclination angle. 3) Crack initiation characteristics: at a low fissure inclination angle (α&amp;lt; 45°), the crack initiation position has a certain offset to the center of the prefabricated fissure, or starts from the tip of the prefabricated fissure. At high fissure inclination angles (α≥45°), the crack initiates from the tip of the prefabricated fissure and forms a “dispersed” distribution. 4) The crack propagation law is mainly shear cracks, at a low fissure inclination angle (α&amp;lt; 45°), the crack propagation has obvious aggregation band formation. At high fissure inclination angles (α≥45°), the crack aggregation zone gradually weakened and expanded in the direction of dispersion.</jats:p

The impact of the COVID-19 pandemic lockdown on rhegmatogenous retinal detachment services—Experiences from the Tongren eye center in Beijing

Purpose To investigate the impact on services for rhegmatogenous retinal detachment (RRD) patients during the COVID-19 (2019coronal virus disease) pandemic in one tertiary center in Beijing. Methods A retrospective cohort study. Two reviewed consecutive RRD patients cohorts of the same length were treated during two different periods: the COVID-19 pandemic and the pre-COVID-19 group. The characteristics of patients, surgery, anesthesia methods, length of hospital stay, and the latest follow-up were recorded and analyzed. Results There were 79 patients in the COVID-19 pandemic group with a 55.9% reduction (179). Compared to patients in the pre-COVID-19, patients in the COVID-19 pandemic had a longer presurgical waiting times (28days, 3days, p&lt;0.001), a higher percentage of patients with presurgical poor (less than 0.02) visual acuity (55.7%, 32.4%, p = 0.009), and a higher percentage of patients with presurgical choroidal detachment (34.2%, 19.6%, p = 0.01). There was no significant difference in the severity of presurgical proliferative vitreoretinopathy between the two groups (p = 0.64). Surgeries on pathological myopia patients with macular hole retinal detachment were postponed in the COVID-19 pandemic. There was a lower percentage of scleral buckling (27.8%, 41.3%, p = 0.02) and a lower rate of subretinal fluid drainage (45.4%, 75.7%, p = 0.01) in the COVID-19 pandemic. There was no significant difference in either postoperative visual acuity (p = 0.73) or the rate of single-surgery retinal attachment (p = 1) between the two groups. Patients in the COVID-19 pandemic had a shorter length of hospital stay (3hours, 35 hours, p&lt;0.001), and a lower percentage of patients received general anesthesia (48.1%, 83.2%, p&lt;0.001). None was infected with COVID-19 disease during the pandemic. Conclusion The COVID-19 pandemic lockdown caused prolonged presurgical waiting times, shorter hospital stays, less general anesthesia, and a significant reduction of RRD surgeries. The RD were more complicated, the surgeons were more conservative on procedures and patients selection, while the surgery outcomes were comparable. </jats:sec