Spatial variability and source analysis of typical soil trace elements at permafrost section along national highway 214 in the eastern Qinghai-Tibet Plateau.

This paper attempts to reveal the enrichment status, spatial characteristics and material sources of typical soil trace elements at permafrost section along National Highway 214 on the Qinghai-Tibet Plateau. Therefore, the samples of typical trace elements in surface soil, being located at the northern slope of Bayan Kara Mountains, were collected and tested. The concentrations of typical trace elements in soil were analysed by mathematical statistics, spatial analysis and ecological assessment. The results show that: (1) the concentrations of As, Cd and Hg in the soil are higher than the local background values, and their degrees of variation were high. There was a certain degree of accumulation. Soil As and Hg elements constitute "slight pollution", indicating there is a none-to-slight ecological hazard. (2) The distributions of soil As, Cd, Pb and Zn concentrations are lower near the highway and increase with distance from it and then become relatively low further away. The distributions of Cr, Cu, Hg and Ni concentrations show no obvious trends in any direction. (3) The spatial heterogeneity of typical trace elements in soil is affected by soil organic matter (SOM), cation exchange capacity (CEC), pH, slope curvature and aspect. At the local scale, soil texture and topography were the main affecting factors. Concentrations of Cd, Cr, Cu, Ni, Pb and Zn were mainly affected by natural factors, while those of As and Hg were affected by both natural and human factors.

Mitigation of sand liquefaction under static loading condition using biogas bubbles generated by denitrifying bacteria.

Nitrogen bubbles that is generated by microbial denitrification process is a new pollution-free clean material for mitigation of sand liquefaction. The current study aims to assess sustainability as well as distribution character of biogas bubbles in sand column under the condition of hydrostatics along with its performance of mitigating sand liquefaction under static loading. A series of laboratory experiments were conducted, and test results indicate that biogas bubbles has excellent sustainability in sand pores, and after 92 weeks, an increase of saturation from 84.5 to 85.1% marking only 0.6% rise. The volume of biogas generated by bacteria increases linearly with decrease of depth. Under undrained condition, if saturation of sample decreased from 100% to around 92.4%, strain softening behavior will transfer to strain hardening, and undrained shear strength can be increased by approximately two times in both of compression and extension tests. The excess pore water pressure ratio and liquefaction potential index have significant reduction with the decrease of saturation, and the magnitude of impact on compression is comparatively bigger than the extension tests. This study validates that as a new material, biogas bubbles are very stable in soil, desaturation using nitrogen bubbles is an effective method for mitigating the liquefaction of sand under static loading conditions. Moreover, the study provides support for the desaturation mitigating static liquefaction of sand to prevent geological disasters and reveals its potential engineering practical value.

Long-term sustainability of biogas bubbles in sand.

Desaturation is a new method to mitigate liquefaction of sand. It aims to prevent liquefaction by generating gas/air in the pores of fully saturated sands, and biogas is one of the most suitable gas. In order to evaluate the long-term sustainability of biogas bubbles, a series sustainability test on biogas bubbles in pores of sand was conducted with a one-dimensional device under hydrostatic condition, hydraulic gradient flow condition and horizontal excitation condition. The variation trend of the retention of biogas bubbles in the pores of soil under the aforementioned conditions was analyzed. Test results indicated that after 72 weeks of monitoring sand samples, biogas bubbles existed stably in the pores of soil under hydrostatic conditions. In hydraulic gradient flow, the stability under upward seepage flow showed a similar trend to that of downward seepage flow. When the hydraulic gradient was constant, the degree of saturation increased in a certain period and finally remained constant. When the hydraulic gradient increased by 0.1, 0.2, 0.3, 0.4, and 0.5, the degrees of saturation increase were 0.8%, 11.5%, 0.5%, 0.1%, and 0%, respectively. After 41,200 cycles with different accelerations, the degree of saturation of the sample increased slightly, and the biogas bubbles basically remained stable.