Showing papers by "Anh Minh Tang published in 2010"

Clays in radioactive waste disposal

Abstract: Clays and argillites are considered in some countries as possible host rocks for nuclear waste disposal at great depth. The use of compacted swelling clays as engineered barriers is also considered within the framework of the multi-barrier concept. In relation to these concepts, various research programs have been conducted to assess the thermo-hydro-mechanical properties of radioactive waste disposal at great depth. After introducing the concepts of waste isolation developed in Belgium, France and Switzerland, the paper describes the retention and transfer properties of engineered barriers made up of compacted swelling clays in relation to microstructure features. Some features of the thermo-mechanical behaviors of three possible geological barriers, namely Boom clay (Belgium), Callovo-Oxfordian clay (France) and Opalinus clay (Switzerland), are then described, including the retention and transfer properties, volume change behavior, shear strength and thermal aspects.

Effects of mineralogy on thermo-hydro-mechanical parameters of MX80 bentonite

Abstract: Thermal conductivity, water retention curve and swelling behavior of two MX80 bentonite samples were studied in the present work. The difference obtained from these two MX80 bentonite samples was then analyzed in terms of mineralogical effects (effects of the proportion of quartz and montmorillonite). It was concluded that the mineralogical effect was significant on the thermal conductivity and the swelling capacity; on the contrary, it was negligible on the water retention property.

Calibration of the osmotic technique of controlling suction with respect to temperature using a miniature tensiometer

Abstract: The osmotic technique was calibrated at various temperatures (20–40 °C) using a high-capacity tensiometer. The effect of temperature on the calibration curve of the high-capacity tensiometer in the positive range has been found to be insignificant, i.e., about 0.03%/°C. The measurement at ambient temperature shows that the suction value is not significantly dependent either on the molar mass of polyethylene glycol (PEG) or on the molecular weight cutoff (MWCO) of the semipermeable cellulose membrane. On the other hand, the matric suction measured in the present work by tensiometer was lower than the total suction found in the literature measured by psychrometer. This shows that the so-called membrane effects must be characterized by not only the crossing of PEG molecules but also other complex phenomena. The calibration at controlled temperatures showed a slight suction decrease of 1%/°C. One possible explanation for this decrease is that increasing the temperature decreases the viscosity of PEG solutions...

Experimental study on hydro-mechanical coupling behaviours of highly compacted expansive clay

Abstract: Highly compacted expansive clays have been usually considered as a possible material for sealing and backfill in deep geological disposal of radioactive waste. In this condition, the material is simultaneously subjected to water infiltration from the geological barrier and stresses generated by the swelling of engineered barriers in confined conditions. Its behaviour under hydro-mechanical loading is essential to the safe design of the whole storage system. In the present work, MX80 bentonite, a kind of expansive clay from Wyoming, USA, was studied. After compaction, its dry density was 1.8 Mg/m3 and its initial suction was 110 MPa. Firstly, the soil was humidified under controlled suction and free-swelling conditions. Significant swelling was observed. Secondly, four values of suction of 110, 39, 9 and 0 MPa were employed to perform isotropic compressive tests at constant suction conditions. That allowed studying the effect of suction on the yield pressure, elastic and plastic compressibility parameters. The results show that the elastic and plastic compressibility parameters increase when the suction decreases. The relationship between these parameters and the logarithm of suction can be linearly correlated. The yield stress drastically decreases upon wetting under free-swelling conditions, from 12 – 18 MPa (at an initial suction of 110 MPa) to 0.2 MPa at saturated state.

The coupled hydro-mechanical behaviours of compacted crushed Callovo-Oxfordian argillite

Abstract: Callovo-Oxfordian (COx) argillite obtained from the excavation of high-level radioactive waste geological disposal has been evaluated as an alternative sealing/backfill material in France. This paper presents an experimental investigation into the hydro-mechanical behaviour of compacted crushed COx argillite. A series of oedometer compressive tests including various loading-unloading cycles were conducted on COx argillite powders at different initial water contents. After reaching the desired dry density (2.0 Mg/m 3 ), the vertical stress was reduced to different levels (7.0 and 0.5 MPa) and the compacted sample was then flooded under constant volume conditions while measuring the changes in the vertical stress. It was found that the initial water content significantly affects the compressive behaviour. The measured saturated hydraulic conductivity is less than 1×10 −10 m/s.

Investigating the Microstructure of Compacted Crushed Callovo-Oxfordian Argillite

Abstract: Some aspects of the macroscopic geotechnical behavior of compacted soils are highly related to their microstructure. This paper presents a qualitative and quantitative microstructure investigation of compacted samples made up of crushed Callovo-Oxfordian argillite. This material is considered as being possibly used as backfill material in radioactive waste disposal at great depth. Scanning electron microscopy and mercury intrusion porosimetry technique were employed to determine the fabric and pore size distribution properties of the samples. The results show that an increase of the compaction effort only results in the reduction of the inter-aggregates porosity. After wetting, the sample displays a relative homogenous microstructure with a decrease in the macropores population.

Investigation of the hydro-mechanical behaviour of compacted expansive clay

Abstract: The hydro-mechanical behaviour of compacted expansive Romainville clay was investigated. The soil was air-dried, crushed, and passed through a 2 mm sieve before being statically compacted to a dry density of 1.35 Mg/m3. The mechanical behaviour was investigated by tests in oedometer with controlled suction using the vapor equilibrium technique (suction s = 0, 9, 39, and 110 MPa). The vertical stress was applied in the range of 0–800 kPa. The experimental results are shown as follows: 1) wetting-induced swelling was higher at lower vertical stresses; 2) the vertical stress under which no swelling occurred during water flooding was estimated at 60 kPa, which can be considered as the swelling pressure of the soil tested; 3) the soil compressibility (changes of volume upon stress increases) was strongly influenced by the soil suction: the lower the suction, the higher the compressibility. The hydraulic behaviour was investigated using a large-scale inltration chamber (800 mm × 1000 mm in section and 1000 mm high). The large size of the soil column allowed burying the volumetric water content sensors (ThetaProbe) without signicantly affecting the water transfer and the soil swelling during inltration. The soil suction was monitored along the soil height (every 100 mm) using various relative humidity sensors and psychrometers. In the inltration test, water was kept on the soil surface and changes in suction and volumetric water content were monitored for 338 d. The wetting front has reached the bottom of the soil column at the end of the test. The data from the simultaneous monitoring of suction and water content were used to determine the water retention curve and the unsaturated hydraulic conductivity using the instantaneous prole method. It has been observed that the soil water retention curve depends on the soil depth; that is to be related to the soil depth-dependent swelling. The unsaturated hydraulic conductivity was found to be quite low, comprised between 3 × 10−11 m/s (at saturated state) and 10−14 m/s (at about 100 MPa suction).

Stress and strain evolution of remoulded COx argillite during saturation

Abstract: The crushed Callovo-Oxfordian(COx) argillite powder is proposed as an alternative backfilling material in France,which will be used in the engineering barrier of high-level radioactive waste(HLW) repository.In this investigation,one-dimensional oedometer tests are carried out on highly compacted(dry density of 2.0 g/cm3) crushed COx argillite powder.The evolution of axial stress and the volumetric deformation characteristics of the samples are studied during saturation process.The effects of the initial axial stress level and the grain size distribution(GSD) on the hydro-mechanical behaviors are discussed.The results show that the volumetric deformation characteristics of samples greatly depend on the applied initial axial stress during saturation.Under the initial condition of higher axial stress,the monotone collapse behaviors are observed for the coarser powder samples.Whereas the finer powder samples present collapse-swelling-collapse behaviors,and the axial stress decreases firstly,then increases and finally decreases again.The initial lower axial stress causes obvious swelling behaviors for both the coarser and finer powder samples and the corresponding axial stress increases gradually.After saturation,the equilibrium axial stresses of finer powder samples are higher than those of coarser powder samples.The hydraulic conductivity is less than 10-11 m/s for all the studied samples after the dry density of 2.0 g/cm3 is reached.