Understanding the failure mechanism of a large underground cavern in steeply dipping layered rock mass using an enhanced ubiquitous-joint model

To understand the mechanical behavior and failure mechanism of the surrounding layered rock masses during the excavation of underground caverns for a hydropower station, an enhanced equivalent continuum model based on the ubiquitous-joint concept is developed and compiled as a plugin DLL file in FLAC^3D. This model is then applied to analyze two engineering geological issues arisen during the excavations of a large underground powerhouse. Both cases are the typical responses controlled primarily by the internal structure of layered rock mass. The first case is mainly concerned with the continuous increase in the displacement of the upstream sidewall after removal of an auxiliary tunnel crown. Numerical simulation reveals the mixed shear-tensile fractures developed along bedding planes. The preserved crown thickness can affect the maximum displacement of sidewall. Failure region will progressively deepen into the interior if no effective reinforcements are adopted. The second case exhibits as the gradual cracking or slabbing of the shotcrete at downstream roof. Stress concentration is the main cause of shotcrete cracking as shown by numerical simulation. The degree of stress concentration can be influenced largely by the angle between rock strata and cavern axis. This degree also varies with the initial stress level and with excavation process, which is confirmed by field monitoring data. The validity and capability of the newly developed model are thus verified by the actual engineering issues.

     

Characterization of Vibrio cholerae O139 isolated in Taiwan

Vibrio cholerae O139 Bengal is the second aetiologic agent of the cholera epidemic that emerged in late 1992 in India which has subsequently spread to other nations. A few sporadic cases of V. cholerae O139 infection have been reported in Taiwan since 1997. The V. cholerae O139 strains isolated from clinical and environmental sources in Taiwan were characterized based on subspecies molecular typing, toxin production, and susceptibility to antibiotics and environmental stresses, and were compared to several O1 and other non-O1/non-O139 strains. Typing by pulsed-field gel electrophoresis revealed that the domestic O139 strains were identical to three of the four Japanese strains. Most of these domestic O139 strains were alpha-haemolytic and produced highly variable amounts of cholera toxin. The presence of cholera toxin gene was confirmed in all of these domestic strains by polymerase chain reaction. Meanwhile, most of the domestic O139 strains were resistant to nitrofurantoin, streptomycin, furazolidone and trimethoprim–sulfamethoxazole, while the O1 strains were not. Susceptibility of selected O139 strains to temperature stresses at 55°C and −20°C, acid treatment at pH 3·0, and N-alkyl dimethyl benzyl ammonium treatment did not differ significantly from other strains examined. Accordingly, these Taiwanese O139 strains were genetically close to the Japanese strains and also shared some common biological traits.     

Anesthetic Propofol Overdose Causes Vascular Hyperpermeability by Reducing Endothelial Glycocalyx and ATP Production

Prolonged treatment with a large dose of propofol may cause diffuse cellular cytotoxicity; however, the detailed underlying mechanism remains unclear, particularly in vascular endothelial cells. Previous studies showed that a propofol overdose induces endothelial injury and vascular barrier dysfunction. Regarding the important role of endothelial glycocalyx on the maintenance of vascular barrier integrity, we therefore hypothesized that a propofol overdose-induced endothelial barrier dysfunction is caused by impaired endothelial glycocalyx. In vivo, we intraperitoneally injected ICR mice with overdosed propofol, and the results showed that a propofol overdose significantly induced systemic vascular hyperpermeability and reduced the expression of endothelial glycocalyx, syndecan-1, syndecan-4, perlecan mRNA and heparan sulfate (HS) in the vessels of multiple organs. In vitro, a propofol overdose reduced the expression of syndecan-1, syndecan-4, perlecan, glypican-1 mRNA and HS and induced significant decreases in the nicotinamide adenine dinucleotide (NAD+)/NADH ratio and ATP concentrations in human microvascular endothelial cells (HMEC-1). Oligomycin treatment also induced significant decreases in the NAD+/NADH ratio, in ATP concentrations and in syndecan-4, perlecan and glypican-1 mRNA expression in HMEC-1 cells. These results demonstrate that a propofol overdose induces a partially ATP-dependent reduction of endothelial glycocalyx expression and consequently leads to vascular hyperpermeability due to the loss of endothelial barrier functions.     

An experimental study on the shear strength behaviour of an interlayered shear weakness zone

In order to evaluate the overall stability of the underground powerhouse at the proposed Baihetan hydropower station in China, the shear behaviour of a weak interlayered soil in the host rock and the soil/host rock interface was investigated in the laboratory by direct shear testing under different degrees of saturation. Shear creep tests were also undertaken on the interlayered soil. Typical results obtained from the multi-sample method (MSM) and single-sample method (SSM) are presented, which show that the shear behaviour of both the interlayered soil and the soil/rock interface is elastic perfect-plastic without dilatancy. It was not possible to determine the shear strength parameters using the multi-sample method due to the heterogeneity of the interlayered soil in terms of mineralogical composition and physical properties. Creep of the interlayered soil was found to be insignificant. The shear behaviour of the soil/rock interface depends strongly on the interface morphology: the rougher the interface the higher the shear strength.