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

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/m^3 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

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 oedorneter 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^-10m/s.

Revisiting the methods of determining hydraulic conductivity of saturated expansive clays in low-compressibility zone

The hydraulic conductivity of saturated clays is commonly determined either directly by monitoring water flux or indirectly based on Terzaghi’s consolidation equation.Similar results are generally obtained from the two methods,but sometimes a significant difference can be observed,in particular for expansive soils.In this study,the hydraulic conductivities determined by the two methods are first compared based on existing data in the literature.The indirect method is then revisited attempting to explain the difference identified.A modified effective stress,considering physico-chemical interaction between face-to-face oriented particles,is finally introduced to better describe the compressibility of expansive clays and to further improve the indirect method in determining hydraulic conductivity of such soils in the low-compressibility zone.Extra tests were performed on Gaomiaozi(GMZ)bentonite slurry and the results obtained allowed the modified indirect method to be verified.

On the thermo-mechanical properties of unsaturated soils

The establishment of energy balance equation is necessary to study the thermo-mechanical properties of unsaturated soils.To solve this equation,the determination of two fundamental parameters as volumetric specific parameter and thermal conductivity coefficient is essential.In this paper,the effective thermal conductivity coefficient of dry soil grain is analyzed for soils with different compositions,and the thermo-mechanical properties of porous media with water and gas are studied by considering the soil water retention curve（SWRC）.Different methods,i.e.volumetric average method,self-consistent method,Hashin-Strikman method,are employed to calculate thermal conductivity coefficients,and a new method is proposed to determine the thermo-mechanical parameters.Comparison of the results obtained by different methods shows that the proposed method is in a good agreement with the experimental results and is suitable for describing the main properties of the thermo-mechanical behaviors of soils.The relationship between the SWRC and the seepage curve is further studied by the natural proportional rule.The characteristics of the SWRC,its differential coefficient and the seepage curve,are investigated by considering the physico-mechanical mechanism;the limit scopes of the indices of the SWRC and the seepage curve are also given.

Clays in radioactive waste disposal

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

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.