Radwaste must be isolated from the biosphere for a long periods. To isolate waste, engineered cla... more Radwaste must be isolated from the biosphere for a long periods. To isolate waste, engineered clay barriers can be constructed. The functional life of such engineered barriers must be much longer (factor 100–1000) than that of more common engineered structures. Can the responsible engineer guarantee a functional life of several thousand years? Probably not, therefore waste like radwaste is stored at great depths, in a combination of geological and engineered barriers. During the storage period the waste itself (radiation) or derived products (oxidation of containers) can interact with the engineered and geological clay barriers. In this contribution it is discussed how such interactions affect the hydro-mechanical properties of clay barriers. First some examples are given that show how properties of clays change at surface conditions if clays are exposed to typical waste products like different cations. Thereafter a method is proposed to classify the potential of hydro-mechanical modifications at three different scales: the particle level, the interlayer level and the tetrahedral–octahedral level. Next it is explained why the reactivity of clays at depth during long-term waste isolation is different (smaller) from the better known reactivity of clays at the surface. Instead of directing too much attention to such well known interactions, the focus for a safe deep disposal of radwaste should be directed to reactions that play a role at depth if, e.g. damage due to radiation occurs. Only if such reactions can be excluded, a safe storage of waste is possible.
Based on the concepts of the mechanics of unsaturated soils where capillary phenomena arise betwe... more Based on the concepts of the mechanics of unsaturated soils where capillary phenomena arise between the wetting fluid (water) and the non-wetting one (air), the subsidence of chalks containing oil (non-wetting fluid) during water injection (wetting fluid) is analysed. It is shown that the collapse phenomenon of unsaturated soils under wetting provides a physical explanation and a satisfactory prediction of the order of magnitude of the subsidence of the chalk. The use of a well established constitutive model for unsaturated soils allows a description of the hydro-mechanical history of the chalk, from its deposition to the oil exploitation.
Among some few others tests, the evaluation of the Atterberg limits is a very basic soil mechanic... more Among some few others tests, the evaluation of the Atterberg limits is a very basic soil mechanical test allowing a first insight into the chemical reactivity of clays. Basically, the liquid limit and the plasticity index are highly and mainly influenced by the ability of clay minerals to interact with liquids. In this contribution, a correlation between the Atterberg limits and clay mineralogy is proposed. This correlation increases the understanding between clay mineralogists and engineers in soil mechanics; additionally a wealth of information in clay mineralogy literature is now available to predict the mechanical behaviour of clays via index tests.
This paper presents an experimental study on the swelling pressure of heavily compacted crushed C... more This paper presents an experimental study on the swelling pressure of heavily compacted crushed Callovo-Oxfordian (COx) claystone at a dry unit mass ρd = 2.0 Mg/m3 using four different methods: constant-volume, swell-reload, zero-swell and adjusted constant-volume method. Results show that the swelling pressure varies in the range of 1–5 MPa and depends significantly on the test method. From the constant-volume tests, it is observed that the swelling behavior during wetting is a function of the suction and depends on both the hydration paths and wetting conditions (e.g. vapor-wetting or liquid-wetting). The swelling pressure decreases significantly with saturation time. To identify the microstructure changes of specimens before and after wetting, mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM) tests were performed. It is observed that, after wetting, the large inter-aggregate pores observed in the as-compacted specimen are no longer apparent; the whole pattern is characterized by a general swell of hydrated clay particles, rendering the soil more homogenous. Results from MIP indicated that wetting caused a significant reduction of the entrance diameter of the dominant inter-aggregate pores from 2.1 to 0.5 μm whereas intra-aggregate pores were not significantly influenced.► The swelling behavior was found to be hydration path and hydration process dependent. ► The new constant-volume cell gives good results of swelling pressure. ► Wetting under volume-constrained condition decreased the size of the dominant inter-aggregate pores.
The argillite extracted from Bure site (France) is proposed, after being crushed and compacted, a... more The argillite extracted from Bure site (France) is proposed, after being crushed and compacted, as a possible sealing and backfill material in the French geological high-level radioactive waste disposal. In this study, the effects of the grain size distribution and the microstructure on the hydro-mechanical behaviour of the compacted crushed argillite have been investigated. The volume change properties were investigated by running one-dimensional compression tests under constant water content (2.4–2.8%) with loading–unloading cycles. Under various vertical stresses, water flooding tests were carried out under constant–volume condition. Depending on the vertical stress level, either swelling or collapse behaviour was observed in the sense that vertical stress increased or decreased upon flooding respectively. A clear effect of grain size distribution has been also identified: finer samples exhibit stiffer compression behaviour and higher swelling potential. To provide a microstructure insight into the macroscopic behaviour feature observed, both mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM) observations were performed, evidencing that: (i) at the same dry density, the size of inter-aggregate pores is larger for the coarser crushed material; (ii) mechanical compression only reduces the inter-aggregate porosity in the stress range considered; (iii) the micro-mechanisms governing the flooding under constant–volume condition include the swelling of the clay particles, the increase of the intra-aggregate pores and the collapse of the inter-aggregate pores. The results show a strong effect of the grain size distribution on the hydro-mechanical behaviour and thus the close link between the microstructure and the hydro-mechanical behaviour.► The compression curve is strongly dependent on the grain size distribution. ► The finer soil exhibited higher swelling potential when water flooded. ► The soil volume changes during compaction was mainly due to the decrease of the sizes of inter-aggregate pores. ► Water flooding under constant–volume conditions also decreased the inter-aggregate pores. ► Water flooding separated the clay particles from the aggregates.
Radwaste must be isolated from the biosphere for a long periods. To isolate waste, engineered cla... more Radwaste must be isolated from the biosphere for a long periods. To isolate waste, engineered clay barriers can be constructed. The functional life of such engineered barriers must be much longer (factor 100–1000) than that of more common engineered structures. Can the responsible engineer guarantee a functional life of several thousand years? Probably not, therefore waste like radwaste is stored at great depths, in a combination of geological and engineered barriers. During the storage period the waste itself (radiation) or derived products (oxidation of containers) can interact with the engineered and geological clay barriers. In this contribution it is discussed how such interactions affect the hydro-mechanical properties of clay barriers. First some examples are given that show how properties of clays change at surface conditions if clays are exposed to typical waste products like different cations. Thereafter a method is proposed to classify the potential of hydro-mechanical modifications at three different scales: the particle level, the interlayer level and the tetrahedral–octahedral level. Next it is explained why the reactivity of clays at depth during long-term waste isolation is different (smaller) from the better known reactivity of clays at the surface. Instead of directing too much attention to such well known interactions, the focus for a safe deep disposal of radwaste should be directed to reactions that play a role at depth if, e.g. damage due to radiation occurs. Only if such reactions can be excluded, a safe storage of waste is possible.
Based on the concepts of the mechanics of unsaturated soils where capillary phenomena arise betwe... more Based on the concepts of the mechanics of unsaturated soils where capillary phenomena arise between the wetting fluid (water) and the non-wetting one (air), the subsidence of chalks containing oil (non-wetting fluid) during water injection (wetting fluid) is analysed. It is shown that the collapse phenomenon of unsaturated soils under wetting provides a physical explanation and a satisfactory prediction of the order of magnitude of the subsidence of the chalk. The use of a well established constitutive model for unsaturated soils allows a description of the hydro-mechanical history of the chalk, from its deposition to the oil exploitation.
Among some few others tests, the evaluation of the Atterberg limits is a very basic soil mechanic... more Among some few others tests, the evaluation of the Atterberg limits is a very basic soil mechanical test allowing a first insight into the chemical reactivity of clays. Basically, the liquid limit and the plasticity index are highly and mainly influenced by the ability of clay minerals to interact with liquids. In this contribution, a correlation between the Atterberg limits and clay mineralogy is proposed. This correlation increases the understanding between clay mineralogists and engineers in soil mechanics; additionally a wealth of information in clay mineralogy literature is now available to predict the mechanical behaviour of clays via index tests.
This paper presents an experimental study on the swelling pressure of heavily compacted crushed C... more This paper presents an experimental study on the swelling pressure of heavily compacted crushed Callovo-Oxfordian (COx) claystone at a dry unit mass ρd = 2.0 Mg/m3 using four different methods: constant-volume, swell-reload, zero-swell and adjusted constant-volume method. Results show that the swelling pressure varies in the range of 1–5 MPa and depends significantly on the test method. From the constant-volume tests, it is observed that the swelling behavior during wetting is a function of the suction and depends on both the hydration paths and wetting conditions (e.g. vapor-wetting or liquid-wetting). The swelling pressure decreases significantly with saturation time. To identify the microstructure changes of specimens before and after wetting, mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM) tests were performed. It is observed that, after wetting, the large inter-aggregate pores observed in the as-compacted specimen are no longer apparent; the whole pattern is characterized by a general swell of hydrated clay particles, rendering the soil more homogenous. Results from MIP indicated that wetting caused a significant reduction of the entrance diameter of the dominant inter-aggregate pores from 2.1 to 0.5 μm whereas intra-aggregate pores were not significantly influenced.► The swelling behavior was found to be hydration path and hydration process dependent. ► The new constant-volume cell gives good results of swelling pressure. ► Wetting under volume-constrained condition decreased the size of the dominant inter-aggregate pores.
The argillite extracted from Bure site (France) is proposed, after being crushed and compacted, a... more The argillite extracted from Bure site (France) is proposed, after being crushed and compacted, as a possible sealing and backfill material in the French geological high-level radioactive waste disposal. In this study, the effects of the grain size distribution and the microstructure on the hydro-mechanical behaviour of the compacted crushed argillite have been investigated. The volume change properties were investigated by running one-dimensional compression tests under constant water content (2.4–2.8%) with loading–unloading cycles. Under various vertical stresses, water flooding tests were carried out under constant–volume condition. Depending on the vertical stress level, either swelling or collapse behaviour was observed in the sense that vertical stress increased or decreased upon flooding respectively. A clear effect of grain size distribution has been also identified: finer samples exhibit stiffer compression behaviour and higher swelling potential. To provide a microstructure insight into the macroscopic behaviour feature observed, both mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM) observations were performed, evidencing that: (i) at the same dry density, the size of inter-aggregate pores is larger for the coarser crushed material; (ii) mechanical compression only reduces the inter-aggregate porosity in the stress range considered; (iii) the micro-mechanisms governing the flooding under constant–volume condition include the swelling of the clay particles, the increase of the intra-aggregate pores and the collapse of the inter-aggregate pores. The results show a strong effect of the grain size distribution on the hydro-mechanical behaviour and thus the close link between the microstructure and the hydro-mechanical behaviour.► The compression curve is strongly dependent on the grain size distribution. ► The finer soil exhibited higher swelling potential when water flooded. ► The soil volume changes during compaction was mainly due to the decrease of the sizes of inter-aggregate pores. ► Water flooding under constant–volume conditions also decreased the inter-aggregate pores. ► Water flooding separated the clay particles from the aggregates.
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Papers by Christian Schroeder