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Zhu C.-M.,Tongji University | Ye W.-M.,Tongji University | Ye W.-M.,United Research Center for Urban Environment and Sustainable Development | Chen Y.-G.,Tongji University | And 3 more authors.
Engineering Geology | Year: 2013

During the long-term operation of a deep geological repository, infiltration of groundwater with different chemical compositions can affect the buffer/backfill properties of compacted bentonite. Using a newly developed apparatus, swelling pressure and permeability tests were carried out on densely compacted GMZ01 bentonite samples, which has an initial dry density of 1.70Mg/m3, with de-ionized water as well as NaCl and CaCl2 solutions at different concentrations. Salinity effects of infiltrating solutions on swelling pressure and hydraulic conductivity of tested samples were investigated. Results obtained show that the swelling pressure of GMZ01 bentonite decreases with increasing concentration of infiltrating solutions, while the degree of the impact decreases with the increase of concentrations. Moreover, swelling pressure reaches stability more rapidly in case of high concentrations. The hydraulic conductivity of GMZ01 bentonite increases with the increase of solution concentrations. Comparison shows that the impact of NaCl solutions on the swelling pressure and hydraulic conductivity is higher than that of CaCl2 solutions at same concentrations. This may be explained by the impact of cation types on the microstructure of bentonite. © 2013 Elsevier B.V. Source


Ye W.M.,Tongji University | Ye W.M.,United Research Center for Urban Environment and Sustainable Development | Wan M.,Tongji University | Chen B.,Tongji University | And 4 more authors.
Engineering Geology | Year: 2012

In this study, temperature controlled soil-water retention tests and unsaturated hydraulic conductivity tests for densely compacted Gaomiaozi bentonite - GMZ01 (dry density of 1.70Mg/m 3) were performed under confined conditions. Relevant soil-water retention curves (SWRCs) and unsaturated hydraulic conductivities of GMZ01 at temperatures of 40°C and 60°C were obtained. Based on these results as well as the previously obtained results at 20°C, the influence of temperature on water-retention properties and unsaturated hydraulic conductivity of the densely compacted Gaomiaozi bentonite were investigated. It was observed that: (i) water retention capacity decreases as temperature increases, and the influence of temperature depends on suction; (ii) for all the temperatures tested, the unsaturated hydraulic conductivity decreases slightly in the initial stage of hydration; the value of the hydraulic conductivity becomes constant as hydration progresses and finally, the permeability increases rapidly with suction decreases as saturation is approached; (iii) under confined conditions, the hydraulic conductivity increases as temperature increases, at a decreasing rate with temperature rise. It was also observed that the influence of temperature on the hydraulic conductivity is quite suction-dependent. At high suctions (s>60MPa), the temperature effect is mainly due to its influence on water viscosity; by contrast, in the range of low suctions (s<60MPa), the temperature effect is related to both the water viscosity and the macro-pores closing phenomenon that is supposed to be temperature dependent. © 2011 Elsevier B.V. Source


Ye W.-M.,Tongji University | Ye W.-M.,United Research Center for Urban Environment and Sustainable Development | Min W.,Tongji University | Bao C.,Tongji University | And 2 more authors.
Unsaturated Soils - Proceedings of the 5th International Conference on Unsaturated Soils | Year: 2011

Gaomiaozi (GMZ01) bentonite, which originated from a deposit locates in Xinghe County, Inner Mongolia Autonomous Region, China, has been considered as the first choice for using as buffer/ backfill materials for deep disposal of high radioactive waste in China. For this purpose, densely compacted GMZ01 bentonite should present satisfactory thermal stability. In this paper, laboratory tests were conducted to obtain hydro-thermal behaviour of densely-compacted GMZ01 and its water retention properties were determined under both confined and unconfined conditions at temperature: 20, 40, 60 and 80°C. The vapour equilibrium technique was employed for suction control. Results show that, i) the water retention capacity of the highly compacted bentonite under confined/unconfined conditions falls with heating; ii) for high suction (greater than 4 M Pa), no difference was observed between the water retention curves of confined and unconfined specimen, while for low suction, the water-retention capacity of confined sample was found to be significantly lower than that of unconfined sample at same temperature; iii) the hysteresis of the unconfined compacted bentonite decreases as temperature increases; for high suctions (greater than 4 MPa), there is an increasing trend in hysteresis of unconfined bentonite when suction decreases. © 2011 Taylor & Francis Group, London. Source


Ye W.-M.,Tongji University | Ye W.-M.,United Research Center for Urban Environment and Sustainable Development | Wan M.,Tongji University | Chen B.,Tongji University | And 2 more authors.
Yantu Gongcheng Xuebao/Chinese Journal of Geotechnical Engineering | Year: 2011

With temperature and suction control, the evaluation of microstructure of densely compacted GMZ01 bentonite under different constraint conditions is conducted under wetting/drying cycles. The results indicate that for high suction (>5 MPa), there is a significant difference between the effects of the drying/wetting cycles on the inter-aggregate pores and the intra-aggregate pores. Following the wetting path, all the pores with different sizes expand at different levels. While following the drying process, some inter-aggregate pores with larger size (>3000 nm) shrink, but the intra-aggregate pores almost do not change with the increase of suction. For confined GMZ01 bentonite specimen, when it hydrates to 5.1 MPa, the swelling of aggregates squeezes into the large pores, which causes the volume of inter-aggregate pores to reduce rapidly. However, the volume of the intra-aggregate pores almost unchanges. After that, when the specimen dehydrates to 103 MPa, the volume of intra-aggregate pores still unchanges. While the inter-aggregates pores differentiate, some of the lager pores (>2000 nm) shrink into smaller pores (300 nm < D < 700 nm), which results in the decrease of the lager pores and the increase of the latter ones, but the increment is limited. For the same confining conditions, temperature significantly enhances the influence of wetting/drying circles on the microstructure of densely compacted bentonite, that is to say, the higher the temperature, the more obvious of the influence of the wetting/drying circles on the microstructure of the bentonite. Source


Ye W.M.,Tongji University | Ye W.M.,United Research Center for Urban Environment and Sustainable Development | Zhang F.,Tongji University | Chen B.,Tongji University | And 3 more authors.
Environmental Earth Sciences | Year: 2014

In this study, the effects of salinity of infiltrating solutions on the swelling strain, compressibility, and hydraulic conductivity of compacted GMZ01 Bentonite were investigated. After swelling under vertical load using either distilled water or NaCl solutions with concentrations of 0.1, 0.5 M, and 1 M, laboratory oedometer tests were conducted on the compacted GMZ01 Bentonite. Based on the oedometer test results, hydraulic conductivity was determined using the Casagrande’s method. Results show that the swelling strain of highly compacted GMZ01 Bentonite decreases as the concentration of NaCl solution increases. The compression index Cc * increases and then turns to decrease with an increase in the vertical stress or a decrease in the void ratio for different solutions, and the Cc * decreases as the concentration of NaCl solution increases. The secondary consolidation coefficient Cα increases linearly with the increase of the compression index Cc *. Furthermore, a bi-linear relationship between the swelling index Cs * and the secondary consolidation coefficient Cα can be characterized clearly. The hydraulic conductivity increases as the concentration of NaCl solution increases, however, this increase can be prevented if a high confining stress is applied. © 2014, Springer-Verlag Berlin Heidelberg. Source

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