Experimental Investigation on the Influence of Depth on Rockburst Characteristics in Circular Tunnels.

To investigate the influence of depth on the rockburst of surrounding rock in a circular tunnel, true-triaxial tests at different depths were carried out on cubic granite specimens with a circular through-going hole. A micro camera was used to monitor the rockburst process of the circular hole sidewall in real time. The test results show that the failure process at different depths can be divided into four periods: the calm period, the particle ejection period, the rock fragment exfoliation period, and the rock bursting period. With an increase in depth, the three-dimensional unequal stress state gradually increased; the failure range and the size of rock fragments increased, the initial failure vertical stress linearly increased, and the strength and stability of the surrounding rock were enhanced. Therefore, the support range of surrounding rock should be increased as the depth increased to improve the overall stability of surrounding rock and reduce the failure range.

A Review of Mechanical Properties and Rockburst Investigation of Transversely Isotropic Rocks by Experimental Technique.

Under complex geostress caused by long-term geological evolution, approximately parallel bedding structures are normally created in rocks due to sedimentation or metamorphism. This type of rock is known as transversely isotropic rock (TIR). Due to the existence of bedding planes, the mechanical properties of TIR are quite different from those of relatively homogeneous rocks. The purpose of this review is to discuss the research progress into the mechanical properties and failure characteristics of TIR and to explore the influence of the bedding structure on the rockburst characteristics of the surrounding rocks. First, the P-wave velocity characteristics of the TIR is summarized, followed by the mechanical properties (e.g., the uniaxial compressive strength, the triaxial compressive strength, and tensile strength) and the related failure characteristics of the TIR. The strength criteria of the TIR under triaxial compression are also summarized in this section. Second, the research progress of the rockburst tests on the TIR is reviewed. Finally, six prospects for the study of the transversely isotropic rock are presented: (1) measuring the Brazilian tensile strength of the TIR; (2) establishing the strength criteria for the TIR; (3) revealing the influence mechanism of the mineral particles between the bedding planes on rock failure from the microscopic point of view; (4) investigating the mechanical properties of the TIR in complex environments; (5) experimentally investigating the rockburst of the TIR under the stress path of "the three-dimensional high stress + internal unloading + dynamic disturbance"; and (6) studying the influence of the bedding angle, thickness, and number on the rockburst proneness of the TIR. Finally, some conclusions are summarized.

[Sequence analysis of gag-pol gene of HIV-1 B/C recombinant viruses in China].

OBJECTIVE: To characterize full length gag gene and partial pol gene of Chinese prevalent HIV-1 B/C recombinant strains, to explore the genetic difference between parent strains and B/C recombinant strains, and to investigate the mechanism of different biologic phenotype among them. METHODS: The peripheral blood samples were collected from 138 HIV-positive persons from 12 regions of China respectively. Samples of total DNA were extracted from the peripheral blood mononuclear cells to undergo nested PCR and sequencing. The 117 HIV-1 CRF07-BC strains and 21 HIV-1 CRF08-BC strains were screened with the resulted that 5 CRF07-BC samples from Xinjiang and 1 CRF08-BC sample from Chongqing were regarded as the most potential new type recombinant viruses. The sequences thus obtained underwent phylogenetic tree analysis and amino acid variation analysis. Simplot software was used to analyze the sequence recombination and identify the breakpoints of B/C recombinant strains. To confirm the breakpoints, separate phylogenetic analysis according to the breakpoints was performed with MEGA software. The genetic distances of different gene fragments were calculated by DISTANCE program in GCG software package. The gene dispersion in the gene fragment of a length of 2550 bp of the recombinant HIV-1-B/C and the potential influence of gene recombination on its function were further analyzed. RESULTS: No change of breakpoint was found in the 5 samples from Xinjiang. But a breakpoint shift of 160 nucleotides occurred in RT region of a sample from Congqing city. CONCLUSION: CRF07-BC and CRF08-BC remain the main prevalent HIV-1 B/C recombinant strains in China. No epidemic of new mosaic recombinant strain is found. The variation of amino acids at the sites 286 and 799 may be the reasons of the transmission dominance of the B/C recombinant strain.

The prevalence of transmitted antiretroviral drug resistance in treatment-naive HIV-infected individuals in China.

BACKGROUND: Transmission of HIV drug resistance (TDR) gives rise to reduced efficacy of initial antiretroviral treatment and has become a public health concern. METHODS: A nationwide survey on TDR was conducted in antiretroviral therapy-naive HIV-1-infected individuals from September 2004 to October 2005 in China. Drug resistance genotyping was performed on subjects' plasma samples. Drug resistance mutations were determined and scored by Stanford HIV Drug Resistance algorithm. RESULTS: Sequences were obtained from 676 individuals, of whom 61.2% were former plasma and blood donors, 17.3% were infected sexually, and 17.2% were intravenous drug users. Subtype B' HIV-1 strains were found in 73.5%, CRF01_AE in 13.9%, CRF07_BC in 6.2%, CRF08_BC in 2.7%, subtype C in 1.04%, subtype B in 0.9%, CRF02_AG in 0.4%, and B'/C intersubtype recombinant strains in 1.3% of the subjects. Twenty-six (3.8%) were found to harbor drug resistance strains. The rates of resistance to protease inhibitors, nucleoside reverse transcriptase inhibitors, and nonnucleoside reverse transcriptase inhibitors were 0.4%, 1.6%, and 2.1%, respectively. Though there was no significant difference in TDR rates between 2004 and 2005 (2.9% vs. 4.4%), an increased trend was observed in the rate of high-level drug resistance (0.8% in 2004 vs. 3.0% in 2005, P = 0.0634). CONCLUSIONS: The rate of TDR was relatively low in China, as compared with those in developed countries. Surveys among recently HIV-infected subjects should be performed continually to ensure the success of the scale-up antiretroviral treatment.

Genotype and phenotype patterns of drug-resistant HIV-1 subtype B' (Thai B) isolated from patients failing antiretroviral therapy in China.

Many AIDS patients in China who received free-of-charge antiretroviral therapeutics, including nucleoside reverse transcriptase inhibitors (NRTIs) and nonnucleoside reverse transcriptase inhibitors (NNRTIs), showed significant life improvement. Increasing numbers of patients, however, are experiencing antiretroviral therapy failure due to the emergence of drug-resistant viruses. The aim of this study was to investigate the genotypic and phenotypic drug resistance patterns of HIV-1 subtype B' variants, which are prevalent in China, in order to rationally design more efficient anti-HIV-1 regimens for future treatment of AIDS patients. 13 out of 16 patients (81%) who were treated with two NRTIs (ddI, and d4T or AZT) and one NNRTI (NVP) exhibited high resistance to NVP, 8 of them with a >1,000 IC50 fold increase. Five codons (101, 103, 108, 181, and 190) were involved in the NVP-resistant mutations, and K103N and Y181C mutations were predominant in these isolates. Fifteen isolates were resistant to at least one of the NRTIs, with high resistance to AZT (>10 IC50 fold increase) and intermediate resistance to d4T or ddI (approximately 4 IC50 fold increase). More than 10 codons were involved in the NRTI-resistant mutation and located in two regions (M41-V75 and T215-K219) of the reverse transcriptase. Concordance between the genotype and phenotype patterns for both NRTIs and NNRTI were detected in a majority of the isolates, suggesting that phenotypic resistance is predictable from genotyping assays, which are faster and less expensive than phenotypic assay. Because NVP and AZT can induce high resistances in these patients, these two drugs should be replaced with others more effective NNRTIs and NRTIs.

[Prevalence of drug resistance mutations among antiretroviral drug-naive HIV-1-infected patients in China].

OBJECTIVE: To collect background information on drug-resistant HIV-1 strains in various regions before the start of nation-wide antiretroviral therapy in China. METHODS: Twenty percent of the 2,000 blood samples from antiretroviral therapy naive patients collected for the 2nd national HIV molecular epidemiology survey (NHMES) in 2002 were randomly sampled for this study. The entire protease gene and 20-230 amino acids of the reverse transcriptase gene were amplified by PCR from provirus DNA and sequenced. The results were analyzed with HIV db-Drug Resistance Algorithm and genotypic resistance mutations were determined to particular anti-HIV drugs. RESULTS: Totally 164 protease gene sequences and 138 reverse transcriptase gene sequences were obtained from patients; 0.61% of 164 sequences displayed primary resistance mutations in the protease gene, whereas 99.39% carried 1 or more secondary mutations. Genotypic resistance to at least one nucleoside reverse transcriptase inhibitors (NRTI) was present in 5.80%,and resistance to at least one non-nucleo side reverse transcriptase inhibitors (NNRTI) was present in 1.45% of samples. CONCLUSION: The prevalence of genotypic drug resistance is very low in drug-naive HIV infected patients from 21 provinces of China tested in this study. Laboratories participated in the NHMES have organized a network to provide drug resistance monitoring service in the current nation-wide antiviral treatment program in China.