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The use of digital images and the correlation of them in time is commonly used in geotechnical laboratory experiments to measure surface variables in a non-invasive and massive way. Digital image correlation-based methods provide a measure of the motion of soil. More recently, measurements of the degree of saturation on soil surface using digital images have also been developed, although its application and validation are still limited. This methodology is based on the fact that almost all soils become darker (less light reflective) due to wetting. This article uses image-based analysis techniques to evaluate the behaviour during wetting of a small-scale dam made of compacted fine sand. With the aim of validating the measurements from images, soil moisture sensors were installed in the dam. The tested soil was previously calibrated to determine the correlation of light reflectance and the degree of saturation covering from dry to fully saturated range. Measurements of velocity, displacements, deformations and the degree of saturation in time are evaluated during the experiment based on images recorded in the visual and shortwave infrared (SWIR) light range. The methodology is described step-by-step including camera and lighting system, sample preparation, image acquisition and image analysis. The techniques used allow the changes in the degree of saturation in soil in motion and the failure mechanisms associated with the wetting process to be observed. The results obtained through image analyses show good agreement with the parameters obtained in point measurements with conventional sensors. The results also show some limitations of the method, mainly associated with the accumulation of errors in the tracking of soil particles during large displacements.

An example of upscaling phenomena with experimental techniques is presented and discussed within an engineered seal concept for a future deep geological repository for low and intermediate radioactive waste mainly produced by the energy/medicine/industry sectors. The seal will undergo a long-term saturation stage and subsequent gas transport through such a barrier. Consequently, gas entrapment and generation processes (dominated by the degradation of organic substances and metals) are expected to occur in the emplacement caverns along the pollutant waste storage. The case concerns a compacted 80/20 (dry mass) sand/bentonite S/B mixture with a complex microstructure that significantly evolves on hydration affecting the gas transport properties. This type of seal limits the gas pressure by increasing its gas transport capacity (gas-permeable seal). The large-scale and demonstration experiment GAST (Nagra’s GTS, Switzerland) focuses on the S/B response to saturation and the gas transport capacity in the later gas invasion phase. A series of laboratory experiments running parallel to the in situ test and bridging different scales (from bentonite inter-sand filling microstructural tests and point tests to dm-scale mock-ups) are discussed within the saturation, gas breakthrough pressure and gas dissipation process of the gas-permeable seal context.

This paper presents results on the deformation response of an artificially prepared sand-silt mixture in a hollow cylinder apparatus. The wetting stage was performed under constant mean net stress (p″=200 kPa) and deviatoric stress (q=200 kPa) but at different intermediate principal stresses (controlled through the principal stress parameter b=(σ2-σ3)/(σ1-σ3) and with values b=0, 0.5 and 0.8). Shear strength tests were first performed at constant mean net stress, different Lode angles and water contents (as-compacted and saturated) to ensure that the aforementioned stress state could be applied at the as-compacted water content. Consistent shear strength results were obtained when compared to triaxial compression and extension results at different water contents, which allowed defining the variation of the critical state line with Lode angle and suction. The soaking results indicated that collapse under constant mean and deviatoric stresses was larger when the intermediate stress coincided with the minor one, i.e. under conventional axi-symetric triaxial compression state (b=0). This is a consequence of the dominant shear strains that occurred during saturation when the stress point reached the critical state line at b=0.5 and 0.8.

Compaction induces anisotropy on soil deformational response due to preferential straining associated with the fabrication process. An experimental insight into the stress-strain response of Barcelona clayey silt is provided here. The material was statically compacted at low dry density to induce high collapsibility upon wetting (dry density 1.48 Mg/m3, water content 12%). Two types of controlled-suction tests were carried out: a) constant suction radial paths on as-compacted samples following different stress ratios (deviator stress / mean net stress: q/p″, including isotropic and K0 stress paths; and b) the same type of radial stress paths but performed after saturation under low confining stress. These paths were intended to define the as-compacted and saturated yield surfaces. The experimental data on as-compacted state is satisfactorily interpreted adopting an anisotropic yield surface with an inclination linked to the compaction under oedometer conditions. Development of shear strain was observed during isotropic loading of the compacted samples. This anisotropy that the soil exhibits at the end of the compaction vanishes after collapse by saturation and as plastic volumetric straining develops.