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Natural collapsible residual soils are a common occurrence in Brazilian unsaturated deposits. The work presented in this paper is part of a larger project designed to identify the most important parameters controlling the mechanical behaviour of this type of material. A laboratory testing programme has been carried out in artificially unsaturated cemented samples reproducing some of the characteristics of natural collapsible soils. A technique for sample preparation has been developed, which has resulted in specimens with high values of void ratio and various degrees of cementation. In particular, it has been decided to use soil cemented mixtures with expanded polystyrene particles which has led to light samples with low density, meta-stable mechanic structure, and good workability. These artificial samples have been subjected to direct shear and oedometer tests, including conventional and suction-controlled oedometer tests. Oedometer tests have been used to characterize the behaviour of this material and to quantify the potential collapse due to loading and wetting. From the observed behaviour it is suggested that the initial void ratio, cement agent and initial suction are all important factors influencing the potential collapse of the soil. For saturated artificial soil samples the critical state lines in the (v, ln p’)-plane and (q, p’)-plane have also been calculated by using an approximated procedure based on the results of direct shear tests.

The main goal of this paper is to present the first results of a study performed to provide insights into the relationship between the hydromechanical stress path experienced by a compacted soil and the modification of its pore space geometry. A new oedometer employing the axis translation technique was used to characterize the hydromechanical behaviour of the tested material. The fabric of the tested samples was determined using the mercury intrusion porosimetry technique under 4 stress levels and two different suctions. From this information, the macro and micropore volume variations were then determined. The results showed that mechanical loading produced a progressive reduction of the macropore volume and a significant increase of the micropore volume beyond a stress of 250 kPa. The obtained results tend to demonstrate that suction strengthens soil fabric, as the initial “double structure” of the tested material was not destroyed in the case of the unsaturated sample by the loading up to 1 000 kPa, whereas, the saturated sample exhibited a more homogenous fabric.

The paper presents an experimental determination of the osmotic component of suction by means of the squeezing technique. Soil was mixed with distilled water at different water contents and squeezed at different pressures. Osmotic suction was estimated from electrical conductivity of water extracted from the soil and two different empirical relationships were used to estimate osmotic suction. Experimental results have shown that osmotic suction depends upon the extraction pressure and the initial water content. The osmotic suction measured using the squeezing technique was then compared with the difference between total suction (measured with transistor psychrometer) and matric suction (measured with axis translation technique).

Since 1974, Belgium investigates the design for disposal of its High Level Radioactive Waste (HLW) in a deep clay formation, the “Boom Clay”. Although the clay formation is the main (natural) barrier against the transport of the radionuclides towards the biosphere, the design also involves several engineered barriers (multi-barrier principle). In the design developed in the late 1980’s, a non-saturated bentonite based material was chosen as part of this barrier system. Prior to demonstrating this design in conditions, a surface mock-up test has been operated between 1997 and 2002. This test served as a preliminary test on the performance of several components of the system, such as bentonite based backfill blocks and instrumentation. With clearly defined heating and hydration conditions, it gave us the opportunity to perform a large scale simulation of the hydration/saturation of the backfill at controlled conditions. After describing the general disposal design and the experimental set-up, this paper will detail the measurements and observations obtained during operating and dismantling the mock-up. To support the interpretation of these measurements and observations, a modelling of the experimental set-up is being performed. We further detail the characterisation programme carried out to obtain the input data for the modelling. Finally, lessons learned for the development of the design for the HLW disposal will be drawn.

This paper is a study of the correlation between the humidity, density and matrix suction of non-plastic soil taken from a region located near the city of Rio de Janeiro. The study was conducted through laboratory tests, using minilysimeters and equipment that indirectly measured the humidity through soil suction: in effect, an automated tensiometer, an equivalence tensiometer (EQ2), a TDR and a specially adapted system, which measures the suction in situ, using filter paper. The electronic instruments were connected to a data logger and placed in three mini-lysimeters. Cycles of wetting and drying of the confined material were simulated. The soil was characterised. This characterisation enabled the determination of physical indexes, density, plasticity, grain size, porosity, and the characteristic curve of water retention, using the Richards pressure pan (cooker) test. The results obtained by the instruments, humidity and matrix suction, were compared with the characteristic curve. Overall, the equipment was satisfactory. The low presence of clay in this soil, around 7%, and its associated high porosity, were factors in limiting water retention. The equilibrium time of the soil humidity and the filter paper, obtained at intervals of fifteen days, presented better results than the weekly intervals (seven days). The equivalence tensiometer EQ2 compared to the filter paper presented quite efficient results for high suction values.

The paper presents suction measurements on pyroclastic soils using vacuum tensiometer and high-suction tensiometers. Laboratory measurements with the vacuum tensiometer aimed at assessing the performance of this instrument, which had to be used for field measurement of matric suction. These measurements were also used to develop and validate a numerical model for predicting air cavity growth in the vacuum tensiometer. The model was intended to be a tool for detecting possible malfunctioning of the tensiometers in the field. Measurement with high-suction tensiometers aimed at corroborating the results of vacuum tensiometer measurements. When using high-suction tensiometers, contact can be better controlled and suctions approaching the limit value of the vacuum tensiometers (about 80 kPa) can be measured with no risk of cavitation.

Soils in arid and semi-arid zones undergoes volume changes due to wetting. Depending upon the type of clay minerals present, degree of saturation, externally applied load and bonding, the fine grained soils either swells or compresses. One of the parameter that affects the volume change behaviour is the primary clay mineral present in their clay size fraction. A simple method of identifying the same has been presented. It has been brought out that in an expansive unsaturated undisturbed soil, the diffuse double layer repulsion, the stress state and the bonding play significant role in their volume change behaviour. In nonexpansive fine grained unsaturated undisturbed soils, the shearing resistance at particle level (including the matrix suction and bonding) and fabric play a significant role in influencing the volume change behaviour. While both the mechanism co-exist, one of them play a dominant role depending upon the primary clay mineral is swelling or non swelling.

The main goal of this paper is to present the first results of a study performed to provide insights into the relationship between the hydromechanical stress path experienced by a compacted soil and the modification of its pore space geometry. A new oedometer employing the axis translation technique was used to characterize the hydromechanical behaviour of the tested material. The fabric of the tested samples was determined using the mercury intrusion porosimetry technique under 4 stress levels and two different suctions. From this information, the macro and micropore volume variations were then determined. The results showed that mechanical loading produced a progressive reduction of the macropore volume and a significant increase of the micropore volume beyond a stress of 250 kPa. The obtained results tend to demonstrate that suction strengthens soil fabric, as the initial “double structure” of the tested material was not destroyed in the case of the unsaturated sample by the loading up to 1 000 kPa, whereas, the saturated sample exhibited a more homogenous fabric.

This paper presents preliminary results of research on hydro-mechanical characterisation of a highly compacted bentonite-sand mixture for its potential use as buffer material in nuclear waste disposal system. Development of swelling pressure with change in suction is presented. Wetting curve of the material was derived from the test as suction was reduced from its initial value using vapour equilibrium technique as well as via axis-translation technique. For comparison, three-dimensional free swell tests were also performed by inducing different suction paths into several specimens such that wetting and drying curves could be obtained. No significant swelling pressure development was observed as the specimens were wetted from as-prepared and oven-dried states to about 10000 kPa suction. Comparison with the three-dimensional free swell test results leads to a conclusion that from dry state to about 10000 kPa suction, the maximum swelling pressure upon saturation is suction independent. It is also shown that swelling pressure curve, wetting-drying curves and void ratio-suction relationship generally exhibit hysteresis. Moreover, the results indicate that even at high suction, void ratio change plays an important role in determining the shape of the wetting curves.

Most countries favour compacted air-dry bentonite for engineered barriers in final repositories for nuclear waste. While the hydrophilic properties of bentonite appear to be ideally suited for minimising any contact of water with the waste the process of resaturation is not fully understood yet. In order to investigate the dynamics of water uptake several resaturation experiments with liquid water as well as with water vapour have been performed. A series of tests concerning the uptake of liquid water in compacted MX-80 bentonite samples has been completed recently. The experiments provide uptake rates and moisture distributions as a function of time. Analogous experiments with water vapour are presently running. Some first results are presented. The data gained is used to check new conceptual models which explain resaturation by flow of liquid water and by diffusion of water vapour, respectively. In the new models the effects of hydration on the local pore water content and the change of porosity corresponding to the amount of hydrated water are considered. The results strongly suggest that vapour diffusion plays a significant - if not dominant - role in the resaturation process of bentonite.