Final Dry Density Research Articles

Effects of synthetic site water on bentonite-concrete system for a potential nuclear waste repository

In high-level nuclear waste (HLW) repositories, concrete and compacted bentonite are designed to be employed as buffer materials, which may raise a problem of interactions between concrete and bentonite. These interactions would lead to mineralogy transformation and buffer performance decay of bentonite under the near field environment conditions in a repository. A small-scale experimental setup was established to simulate the concrete-bentonite-site water interaction system from a potential nuclear waste repository in China. Three types of mortars were prepared to correspond to the concrete at different degradation states. The results permit the determination of the following: (1) The macro-properties of Gaomiaozi (GMZ) bentonite (e.g. swelling pressure, permeability, the final dry density, and water content of reacted samples); (2) The composition evolution of fluids from the synthetic site water-concrete-bentonite interaction systems; (3) The sample characterization including Fourier transform infrared spectroscopy (FTIR) and X-ray powder diffraction (XRD). Under the infiltration of the synthesis Beishan site water (BSW), the swelling pressure of bentonite decreases slowly with time after reaching its second swelling peak. The flux decreases with time during the infiltrations, and it tends to be stable after more than 120 d. Due to the cation exchange reactions in the BSW-concrete-bentonite systems, the divalent cations (Ca and Mg) were consumed, and the monovalent cations (Na and K) were released. The dissolution of minerals in the bentonite such as albite causes Si increasing in the pore water. It was concluded that the hydro-mechanical property degradation of bentonite takes place when it comes into contact with concrete mortar, even under low-pH groundwater conditions. The soil dispersion, the uneven water content, and the uneven dry density in bentonite samples may partly contribute to the swelling decay of bentonite. Therefore, the direct contact with concrete has an obvious effect on the performance of bentonite.

Modelling the density homogenisation of a block and granular bentonite buffer upon non‐isothermal saturation

AbstractThis paper presents a numerical analysis of the mechanical performance of a bentonite clay buffer for the containment of nuclear waste in the context of deep geological disposal. The design of the buffer is based on the Swiss concept where the waste canisters are emplaced on pedestals of compacted bentonite blocks and the remaining space between the tunnel and the canister is backfilled with grains of highly compacted bentonite. A complete analysis of the long‐term performance of the repository requires a good understanding of the mechanical evolution of the bentonite upon heating from the radioactive waste and hydration from the host rock. Despite its importance, the implications of the initial heterogeneous bentonite layout, characterised by blocks and grains, on the final dry density at the repository scale in the steady state have not been previously studied. The present study aims to shed light into these processes by means of finite element modelling using an advanced constitutive model for the bentonite behaviour that considers several thermo‐hydro‐mechanical couplings. The constitutive model is shown to be able to reproduce several laboratory tests involving saturation of block and pellets at different dry densities. The model predictions, extended up to 100,000 years, indicate that the bentonite blocks and grains tend to homogenise in terms of dry density as the buffer reaches full saturation. Due to the different swelling potential of the block pedestal and the granular backfill, the canister is subjected to movements, although these remain relatively small. The impact of initial segregation of the granular bentonite is also studied and it is seen to not to affect substantially the mechanical evolution of the buffer, although it might reduce canister displacements.

On swelling behaviours of a bentonite under different water contents

This study starts from a question: how would initial water content (wi) affect equilibrium swelling pressure (peq) of compacted bentonites in swelling pressure tests (ps tests)? However, discussions based on experiments involving a bentonite, Kunigel V1 (K_V1), are extended to the following issues: (a) the effect of wi on peq; (b) pore water density (ρpw); (c) the co-existence of crystalline and osmotic swelling; and (d) relation between peq and compaction tests (pc tests). It is revealed that wi has an insignificant effect on peq in general, although a significant effect may appear for relatively high wi and dry density (ρd) conditions. Two swelling types, crystalline swelling with basal spacing of montmorillonite (d001 = 1·0–1·9 nm) and osmotic swelling (d001 ≥ ∼4·0 nm), co-exist in a w range from 36 to 65%. With the fact that ρpw of K_V1 may range from 1·1 to 1·2 Mg/m3, it is expected that crystalline swelling may mainly govern peq in ps tests if the final dry density of a specimen (ρdf) is larger than 1·46 Mg/m3, while osmotic swelling may mainly govern if ρdf < 1·05 Mg/m3. Together with the results of pc tests, the effects of wi on peq are explained by the proposed conceptual model.

Sealing performance of compacted block joints backfilled with bentonite paste or a particle-powder mixture

Technology gaps are inevitable parts of the buffer barriers in the high-level radioactive waste (HLW) repositories. The mechanical properties, permeability and water-solute-radionuclide migration of buffer barrier are different when considering the gaps. The present study was designed to investigate the sealing effect of joint between the compacted blocks with two types of sealing materials, a bentonite paste (P) and a bentonite particles and powder mixture (P/P) both from the macro- and micro-aspects.A series of laboratory test was conducted to study the hydro-mechanical properties of bentonite blocks after backfilling the joint between blocks. Both the intake and outflow water volume, and the pressure on radial and axial direction were monitored during the hydration. Following the permeability test, the joint’s sealing was evaluated by a final dry density, water content distribution and microstructure evolution. The results showed that the axial and radial pressure of blocks backfilled with bentonite P and bentonite P/P mixture were higher than that of blocks with a blank joint. Pressure preferentially increased in the low-stress areas (the joint areas), contributing to sealing of the joint. The hydraulic conductivities for backfilled bentonite blocks were almost on the same order of magnitude as to intact bentonite blocks, revealing a clear sealed hydraulic behavior. The final dry density decreased in the block area and increased in the joint area after hydration, forming a transition zone with a certain width. A smaller difference was found between the block and joint area in the backfilled block. The mercury intrusion porosimetry (MIP) test and scanning electron microscope (SEM) test showed that backfill decreased the numbers of inter-aggregates pores ranged between 8 and 15 μm compared with that in the blank joint. The comprehensive data showed that a joint backfilled with a bentonite P/P mixture has a higher swelling capacity, lower permeability, and denser structure compared with joint backfilled with bentonite P. The results suggested that joint backfilling improved the integrity of the blocks and enhanced the swelling property and impermeability compared with that of the non-backfilled blocks. Both bentonite P and bentonite P/P has satisfactory sealing performance of compacted block joints, with better effect of the later.

Experimental Study on the Permeability of Compacted Loess

In the western regions of China, loess soils are widely used as fill materials in the construction of highways and building foundations. The hydraulic conductivity of this fill material is a significant parameter for initial engineering designs, but currently there are a very limited number of investigations into this aspect. In this study, a series of tests on compacted loess soils was conducted to determine how the dry density and water content affect loess permeability. The test results show that the hydraulic conductivity decreases gradually with the final water content or with increase in the final dry density. The phenomenon is induced by a decrease in cumulative volume or void ratio, which is verified by the results of mercury intrusion porosimetry and scanning electron microscopy images. A new mathematical model is proposed for predicting the value of k in compacted loess soils, expressed as a function of water content and void ratio.

Experimental study on the aeolotropic swelling behaviour of compacted bentonite/claystone mixture with axial/radial technological voids

Pre-compacted blocks of MX80 bentonite/Callovo-Oxfordian (COx) claystone mixture are chosen as sealing/backfilling materials in the French concept of deep geological disposal for radioactive waste. Creating technological voids is normally unavoidable during installation in the disposal galleries, in the form of axial voids between compacted blocks and radial voids between the compacted blocks and the host rock. In this work, the aeolotropic swelling behaviour of compacted mixture with axial/radial technological voids was experimentally investigated. Results showed that the aeolotropy coefficient (ratio of radial to axial pressures) of samples without voids increased from 0.46 to 0.88 with the increasing dry density from 1.6 to 2.0 Mg/m3 due to the relatively concentrated macro-pores which defined a relatively lower local dry density in the side part. For the samples with axial/radial voids, their final axial swelling pressures were in agreement with the unique correlation with final dry density, regardless of the soil density heterogeneity. Samples with axial voids had a larger aeolotropy coefficient than samples without voids, due to the reduction of the orientation of clay particles upon hydration, while samples with radial voids had a smaller aeolotropy coefficient, due to the relatively lower dry density near the cell wall.

Analysis of boundary friction effect on the homogenization process of compacted bentonite/claystone mixture with technological voids upon hydration

Pre-compacted MX80 bentonite/Callovo-Oxfordian (COx) claystone mixture has been proposed to backfill and seal the underground galleries for radioactive waste disposal in France. While emplacing these pre-compacted blocks, technological voids are created between the blocks and the host rock and among the blocks themselves. It is expected that homogenization process will take place over time for the structure constructed with pre-compacted blocks upon hydration. This study investigated the boundary friction effect on such a process. Results showed that after the filling of technological voids, the soil far from the technological voids would swell further, while those near the voids would be compressed under the welling pressure generated by the soil behind, resulting in an increase in homogeneity in terms of dry density distribution. However, this homogenization process would stop after a certain time. Further examination showed that the homogenization process ended when the maximum boundary friction force became equal to or higher than the vector sum of swelling forces in the radial direction. Based on the force equilibrium and the mass conservation, the final dry density distribution was estimated. Comparison between the estimation and the measurement showed a good agreement, indicating the relevance of the identified mechanism related to boundary friction.

Effect of technological voids on swelling behaviour of compacted bentonite–claystone mixture

The effect of technological voids on the swelling pressure of compacted MX80 bentonite–Callovo-Oxfordian (COx) claystone mixture was investigated by simulating the technological voids with a pre-defined space between the compacted disks of the mixture. Both axial and radial swelling pressures were monitored. After the tests, the microstructure of samples at different positions was investigated using mercury intrusion porosimetry (MIP), together with the determination of dry density and water content. Results showed that two main processes, filling and homogenization, occurred during soil hydration. In the filling process, the initial technological voids were gradually filled and the axial swelling pressure tended to increase. In the homogenization process, the samples had a sealing zone and a swelling zone. The sealing zone was characterized by a lower dry density than the expected final dry density while the swelling zone was characterized by a larger one. From the MIP results, the sealing zone showed larger inaccessible-pore, medium-pore, and large-pore void ratios and a lower small-pore void ratio than the swelling zone. Over time, the medium and large pores in the sealing zone were compressed, while the small pores in the swelling zone decreased. The stabilized axial swelling pressure followed a unique relationship with the expected final dry density. Moreover, the swelling pressure anisotropy was found to decrease as the technological voids increased.

Effects of a simulated gap on anisotropic swelling pressure of compacted GMZ bentonite

Gaomiaozi (GMZ) bentonite is a promising buffer material for Chinese high-level radioactive waste (HLW) disposal, and will be either compacted in situ or used as dense prefabricated material when constructing engineered barriers. Uniaxial compaction can induce anisotropy of the bentonite structure, resulting in anisotropic swelling. In this work, a series of swelling pressure tests were performed on compacted GMZ bentonite to investigate the anisotropy of its property and highlight the dry density effect. Two kinds of technological gaps were simulated to analyze the change in anisotropy of properties with limited swelling deformation. The relationship between swelling pressure and dry density was found to be logarithmic, and axial pressure increased more rapidly than radial pressure with dry density. The ratio of radial to axial pressure decreased significantly in the dry density range 1.395 Mg/m3–1.706 Mg/m3; beyond this, it remained at about 0.76. For the swelling volume ratio of less than 16.7%, the stable swelling pressure was controlled by the final dry density. For a larger swelling volume ratio, the axial gap induced a smaller swelling pressure compared with the case of no gap at the same final dry density.

Time- and density-dependent microstructure features of compacted bentonite

Pre-compacted bentonite bricks are often considered as sealing/backfill elements in deep geological repositories for high level radioactive waste. A good understanding of their microstructure changes upon hydration is essential as the microstructure changes are directly related to the macroscopic hydro-mechanical behaviour. In this study, the microstructure features of the compacted MX80 bentonite used as a sealing material in a field experiment were characterized by means of both mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM). Emphasis was put on the effects of final dry density (density after swelling) and hydration time. The results obtained show that the changes in soil porosity upon swelling are mainly due to the increase in large-pores of about 50µm diameter and medium-pores of 1µm diameter. In addition, the microstructure changed over time due to the water re-distribution that occurred among each level of pores: the volume of both the large-pores and small-pores decreased along as the volume of the medium-pores increased. A uniform microstructure can be then expected in the long term. Furthermore, it was observed that the higher the final dry density, the slower the microstructure changes.

Experimental study on the swelling behaviour of bentonite/claystone mixture

A mixture of the MX80 bentonite and the Callovo-Oxfordian (COx) claystone were investigated by carrying out a series of experiments including determination of the swelling pressure of compacted samples by constant-volume method, pre-swell method, zero-swell method and swell–consolidation method. Distilled water, synthetic water and humidity controlled vapour were employed for hydration. Results show that upon wetting the swelling pressure increases with decreasing suction; however, there are no obvious effects of synthetic water chemistry and hydration procedure on the swelling behaviour in both short and long terms. For the same initial dry density, the swelling pressure decreases with increasing pre-swell strain; whereas there is a well defined logarithmic relation between the swelling pressure and final dry density of the sample regardless of the initial dry densities and the experimental methods. It was also found that swelling pressure depends on the loading-wetting conditions as a consequence of the different microstructure changes occurred in different conditions. Furthermore, it was attempted to elaborate a general relationship between the swelling pressure and the final dry density for various reference bentonites.

Thermo-hydro-mechanical model of the Canister Retrieval Test

The paper presents a 2-D axisymmetric numerical simulation of the Canister Retrieval Test (CRT). The main objectives of this analysis are the study of the canister–buffer and buffer–host rock interfaces; and the evaluation of the mathematical models used to predict the THM behaviour of the buffer and interface. The THM simulation was performed using the finite element program Code-Bright. The constitutive law adopted to represent the mechanical behaviour of bentonite and pellets is the Barcelona Basic Model (BBM) with a non-linear elastic component accounting for swelling. A joint element with a non-linear elastic mechanical law was implemented in the code for the simulation of the opening–closure of the interface canister–buffer. The formulation of this element also incorporates appropriate thermal and hydraulic laws. The evolution of temperatures, relative humidities and stresses recorded during the test were compared with the simulation results. Comparing the results measured in situ with the results of numerical analysis shows that the formulation used is able to reproduce satisfactorily the phenomena involved in the test as well as their interactions. In addition, the final dry density and degree of saturation measured in samples of bentonite and pellets extracted during the test dismantling were also in agreement with the calculated values.

Influence of exchangeable cations on hydraulic conductivity of compacted bentonite

Compaction and hydraulic conductivity tests were conducted using deionized (DI) water to determine how exchangeable cations affect the hydraulic conductivity of compacted bentonite. Regardless of the compaction pressure, the maximum dry density decreased and the optimum water content increased with increasing equivalent fraction of Na + ions ( X Na). The hydraulic conductivity decreased with increasing X Na. The initial dry density of bentonite generally had no effect on the hydraulic conductivity, regardless of the type of exchangeable cations. The hydraulic conductivity of bentonite decreased with increasing a final dry density. The decrease in hydraulic conductivity was more significant at higher X Na. The difference in hydraulic conductivity of the specimens at a given final dry density decreased with increasing a final dry density. When permeated with DI water, compacted Ca-bentonite with a very high dry density might have a very low hydraulic conductivity (< 10 − 9 cm/s) similar to compacted Na-bentonite, if no volume change is allowed (i.e. very high effective stress).

Frost heave and dry density changes during cyclic freeze‐thaw of a silty clay

AbstractOpen‐system frost heave tests were performed on a silty clay using a lower boundary condition of +2°C and unidirectional freezing at −10°C from the surface, or freezing and thawing with an upper boundary temperature that cycled from −10°C to +10°C or from −10°C to 0°C. The amount of frost heave, the distribution of moisture at the end of the experiment and the final dry density profiles all differed for the three cases. The experiments demonstrate that freeze‐thaw history is important in assessing or predicting the frost heave of silty clays. Copyright © 2009 John Wiley &amp; Sons, Ltd.