Abstract: 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 swellâconsolidation 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.
Abstract: Boom clay formation, a deposit of slightly over-consolidated marine clay that belongs to the Oligocene series in the north east of Belgium, has been studied as a possible host material of nuclear waste disposal. In this context, the long-term deformation behavior of Boom clay is of crucial importance in the performance assessment of the whole storage system. In this study, low and high pressure oedometer tests are carried out; the e-log Ïâ²v (void ratioâlogarithm of vertical effective stress) and e-log t (void ratioâlogarithm of time) curves obtained are used to determine the compression index Cc*, swelling index Cs* and secondary deformation coefficient Cα during both loading and unloading. The relationship between Cα and the effective stress ratio (Ïâ²v/Ïâ²c, vertical effective stress to pre-consolidation stress) is analyzed, and it is observed that Cα increases linearly with log Ïâ²v/Ïâ²c. Examination of the ratio of Cα/Cc* for various soils shows that the secondary deformation behavior of Boom clay is similar to that of shale and mudstone. The relation between Cα and Cc* is linear; but the relation between Cα and Cs* is bi-linear. The bi-linearity observed is related to two different mechanisms: the mechanically dominated rebounding and the physico-chemically dominated swelling.
Abstract: The soil-atmosphere interaction was investigated by conducting physical model test in the laboratory and by performing numerical analysis using data from an experimental site.
For the physical model test, a large scale environmental chamber was developed. This chamber was instrumented by various sensors allowing the soil suction, volumetric water content, and temperature to be monitored at various depths. In the zone occupied by air, temperature, relative humidity and air rate (wind speed) were monitored. The soil surface temperature was also measured by an infrared thermometer. The soil sample was prepared by compaction and wetted from the soil surface before being dried under controlled conditions of relative humidity, temperature and flow rate of air. A camera fixed above the chamber allowed monitoring the development of cracks. The evaporation rate was calculated based on the temperature and relative humidity data at the inlet and the outlet of the chamber, allowing assessment of the performance of the chamber to simulate the evaporation phenomena. The results show that evaporation is a heat consuming process: in the test condition, both air and soil were cooled by the evaporation process. Cracks developed during soil drying. Analysis of the âcracking surface ratioâ and âweighted widthâ showed that the evolutions of these two parameters are similar: thus, in practice, only one is needed for evaporation analysis.
For the numerical analysis, a two-dimensional model of soil-atmosphere interaction was developed and implemented in a fully coupled thermo-hydro-mechanical (THM) code. The soil surface boundary conditions were determined from meteorological data using water balance and energy balance equations. The settlement due to soil-atmosphere interaction in an experimental site was simulated. Comparison between calculation and measurement showed that the THM codes can satisfactorily predict the soil settlement, provided that appropriate evapotranspiration models are implemented.
Abstract: The Callovo-Oxfordian (COx) argillite is a possible host rock for radioactive waste disposal in which the ANDRA underground laboratory of Bure (East of France) has been excavated. In this paper some aspects of the volume change behaviour of the COx argillite are investigated. To do so, high pressure oedometers with a maximum capacity of 113 MPa have been used. In a first stage, swelling tests were carried out on samples initially compressed at constant initial water content (unsaturated) that were afterwards soaked under vertical loads, respectively, smaller and higher than the in-situ vertical stress. All samples exhibited swelling, even at stress higher than the in-situ stress. In a second stage, standard step-loading compression tests were carried out on samples previously saturated under the in-situ vertical load, so as to investigate the volume change behaviour under load cycles. The strainâstress curves obtained appear to be different to what is currently observed in overconsolidated or cemented clays, with no clear appearance of yield and pre-yield reversible behaviour. The volumetric behaviour during both compaction and swelling is interpreted in terms of damage created by the collapse of pores within a fragile matrix. The amount of swelling observed is related to the extent of damage.
Abstract: A new hollow cylinder triaxial cell (60 mm internal diameter and 100 mm external diameter) has been designed to study the behavior of low permeability saturated geo-materials such as stiff clays or argillites under controlled temperature and pore pressure conditions. The main advantage of this device is the short drainage path allowed by the hollow cylinder geometry that is reduced to half the thickness of the sample (10 mm)âfour times less than that of standard full cylinder samples of 78 mm height. The reduced drainage path allows a significantly faster resaturation procedure of initially unsaturated samples compared to conventional full cylindrical samples. It also permits the achievement of drained conditions (i.e. negligible excess pore pressure during testing) with a higher loading rate. A numerical simulation of the saturation process demonstrates that the resaturation of the hollow cylinder clay sample can be achieved almost 30 times faster than in standard full samples that are drained at one end, and about seven times faster than in samples drained at both ends. Appropriate loading rates to be used in drained tests on stiff clays and argillites are also discussed based on a numerical simulation of the isotropic compression test. The effect of the deformability of the drainage system on the measured parameters during undrained tests is also analyzed. A correction method is proposed, based on the work of Bishop, by considering the deformability of the porous elements and of the drainage system.
Abstract: Soil air permeability is one of the most important parameters which govern the aeration in agricultural soils and thus has a significant effect on the plant growth and crop production. Therefore, it appears important, when analysing the effect of soil compaction due to agricultural machinery, to correlate air permeability with soil capacity parameters such as air-filled porosity, degree of saturation, water content, etc. In the present work, the relationship between air permeability, soil capacity parameters and vertical stress was analysed by performing confined uniaxial compression tests accompanied by air permeability measurements. Three French soils having different textures were studied. Tests were performed on remoulded and undisturbed soils, at various initial dry bulk densities and water contents. For the remoulded soils, the air permeability has been found strongly correlated with the applied vertical stress for sandy loam; by contrast, no obvious correlation could be established for clay. As far as the undisturbed soils are concerned, the air permeability could be correlated with the air-filled porosity for sandy loam and silty-clayey loam but also no evident correlation could be established for clay. Examination of an existing model predicting the air permeability from the air-filled porosity using one tortuosity/connectivity parameter showed that this parameter varies in a small range for sandy soils and in a larger range for clayey soils.
Abstract: The argillite extracted from Bure site (France) is proposed, after being crushed and com-pacted, as a possible sealing and backfill material in the French geological high-level ra-dioactive waste disposal. In this study, the effects of the grain size distribution and the mi-crostructure 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 behavior was observed in the sense that vertical stress increased or decreased upon flooding respectively. A clear effect of grain size distribution has been also identi-fied: 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) observa-tions were performed, evidencing that: (i) at the same dry density, the size of inter-aggregates 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-aggregates pores. The results show a strong effect of the grain size distribution on the hy-dro-mechanical behaviour and thus the close link between the microstructure and the hy-dro-mechanical behaviour.
Abstract: Rainfall is the most frequent triggering factor for landslides and the development of early warning systems has to take account of this. It is suggested that direct measurement of pore pressure gives the most reliable prediction of failure of a slope. The amount of rainfall can be only a crude indicator of failure as the processes that occur between rain falling on a slope and the resulting pore water pressure change are complex, highly non-linear and hysteretic. The paper describes high-capacity tensiometers developed within the EU-funded MUSE Research Training Network that have been used for measuring suctions in slopes. High-capacity tensiometers are capable of direct measurement of pore water pressure down to â2 MPa and are also able to record positive pore water pressures. Two methods of field installation are discussed; one developed by ENPC in France uses a single tensiometer per hole, and the second technique, developed by Durham University in the UK, allows multiple tensiometers to be used at different depths within a single borehole. Continuous monitoring of pore water pressure has been carried out over several months and shows the responses to climatic events.
Abstract: In the context of research into deep nuclear waste disposal, various works have concerned the hydromechanical behavior of Boom clay, a stiff plastic clay extracted in the SCK-CEN Underground Research Laboratory near the Mol City (Belgium), at a depth of 223 m. Due to some amount of smectite minerals in the clay fraction, Boom clay exhibits swelling properties when hydrated under low stresses. To investigate some aspects of the hydromechanical behavior of Boom clay, oedometer compression tests were carried out on samples of Boom clay close to saturation and submitted to an initial suction. During oedometer compression, the changes in suction with increased vertical stress are monitored by means of a high capacity tensiometer installed at the bottom of the sample. Some aspects related to hydromechanical couplings are examined through the investigation of the changes in suction during oedometer compression, a somewhat delicate and poorly documented experimental approach. A comparison is also made with a completely different soil sample under suction, i.e. a statically compacted unsaturated low plasticity silt. Some technical difficulties typical of this new experimental approach are first described in detail so as to optimize the interpretation of the data obtained. The experiment allows the determination of the point at which suction is changed to positive pressure during compression. Below this point, the ratio between the vertical stress and the change in suction are determined. Above this point, the data show that positive pore pressures are dissipated in a common way. The suction/stress behavior during unloading is also described and discussed. Finally, an interpretation in terms of microstructure effects is provided for both samples. The experimental approach initiated here seems to provide interesting further application to better understand hydromechanical couplings in natural soils in relation with suction increase during stress release.
Abstract: Lime treatment is a well-known technique to improve the mechanical response of clayey subgrades of road pavements or clayey soils used for embankment. Several studies show that lime treatment significantly modifies the physical and hydro-mechanical properties of compacted soils. Nevertheless, studies on the scale effect under climatic changes are scarce. Actually, wetting-drying cycles might significantly modify the microstructure of treated soils, giving rise to changes in hydro-mechanical properties. This modification could be dependent on the size of soil aggregates before lime treatment. In the present work, this scale effect was studied by investigating the stiffness of a compacted lime-treated clayey soil using bender elements. The studied soil was first air-dried and ground into a target maximum soil aggregates size (Dmax). For each aggregates size, the soil was humidified to reach the target water contents wi, then mixed with 3% of lime powder (mass of lime divided by mass of dried soil) prior to the static compaction at a dry density of 1.60 Mg/m3. Two initial water contents (wi = 14 and 18%) and four maximum soil aggregates sizes (Dmax = 0.4, 1.0, 2.0 and 5.0 mm) were considered. After the compaction, the soil specimen (50 mm in diameter and 50 mm in height) was covered by plastic film in order to prevent soil moisture changes. The soil stiffness was then monitored at variable time intervals until reaching stabilisation. Afterwards, the soil specimen was subjected to full saturation followed by air-drying to come back to its initial water content. The results show that: i) the soil stiffness after lime-treatment is significantly dependent on the aggregates size: the finer the aggregates the higher the soil stiffness; ii) the effect of initial water content on the stiffness is negligible and iii) the wetting-drying cycles seem to slightly increase the soil stiffness in the case of lime-treated specimens and decrease the soil stiffness in the case of untreated specimens. Furthermore, when an intensive drying was applied reducing the soil water content lower than the initial one, the soil stiffness decreased drastically after the subsequent wetting.
Abstract: The hydraulic conductivity is a key parameter in geotechnical engineering practice for the seepage and consolidation analysis. Experimental results show that the hydraulic conductivity is mainly governed by the soil porosity, and the correlations with void ratio are usually proposed. The validity of these correlations has been verified for soft clays and sands. However, few studies were involved in stiff clays. In this work, the hydraulic conductivity of Boom clay, a stiff clay taken from the Essen site in Belgium, was determined from both consolidation and constant-head percolation tests. The data obtained was then analyzed to evaluate the existing correlations to predict the hydraulic conductivity. In addition, as these correlations usually require a referred hydraulic conductivity at a known void ratio, it is often difficult to be used in practice. Thus, a new method was developed allowing the prediction of hydraulic conductivity without the referred value, which was based on two existing correlations and involved the void ratio and the liquid limit. The proposed correlation was verified using the experimental results obtained from Boom clay samples which were collected from various locations in Belgium.
Abstract: The THM behavior of compacted GMZ bentonite has been investigated using a suction-temperature controlled isotropic cell. The results obtained were compared with the existing results on other reference bentonites (MX80, FEBEX, FoCa, and Kunigel-V1). It has been observed that the coefficient of thermal expansion of the compacted GMZ bentonite is 2 x 10-4°C-1, similar to the values of compacted MX80 and FEBEX bentonites. The heating tests of the GMZ bentonite also show that the suction is an important parameter that governs the thermal volumetric behavior of unsaturated soils. Unlike temperature, suction has a significant effect on the compressibility parameters. Examination of the mineralogy of various bentonites showed that a good correlation can be generally established between the montmorillonite content and the cations exchange capacity (CEC) or the specific surface area (S). Nevertheless, both the basic geotechnical properties and the swelling potential seem to depend not only on the montmorillonite content but also on other factors such as the nature of base exchangeable cations. The quartz content of the GMZ bentonite is relatively high (11.7%). This could explain its relatively large values of thermal conductivity.
Abstract: Physical degradation of agricultural and forest soils due to compaction is an important concern for the actors in agriculture, forest
and environment. Many questions are still pending on the importance of this degradation, the evaluation of its impact on environment,
the effect of changes of climate and practices, its natural remediation. The DST (Agricultural and Forest Soil Degradation induced by
Compaction ?) project has proposed a research action on soil compaction in metropolitan France in order to : quantify the impacts of
soil compaction on important soil environmental functions ; identify the conditions leading to compaction ; prevent the occurrence of
these conditions ; monitor the evolution of soil structure after compaction. The project was structured in five working packages : impact,
prediction, prevention, follow-up and cartography. It was based on modelling (hydrology, soil deformation, soil-crop interactions models)
to study the occurrence of compaction and its impacts on crop growth and the environment. It also aimed at developing non-destruc-
tive methods to monitor soil structure deformation. The project associated research laboratories working in agriculture, geophysics or
civil engineering and extension services members and was also based on field and laboratory experimentations, performed in various
conditions (field crops, vineyards, forests). Mains results concerned : (i) impacts of compaction, as we demonstrated the existence of
a threshold value for the structural void ratio (0,15 m3m3 in loamy soils) beyond which crop performances and hydrological properties
are systematically affected ; (ii) modelling, with a 2-D model of soil deformation ; (iii) compaction assessment, based on non-destructive
geophysical methods ; (iv) prevention of compaction, with a predictive model of soil water content and (v) mapping of compaction risks
in France, based on the knowledge acquired during the project.
Abstract: Boom clay has been selected as a potential host rock formation for the geological disposal of radioactive waste in Belgium. In the present work, the hydro-mechanical behaviour of Boom clay samples from the borehole Essen-1 at a depth of 220 - 260 m and from HADES that is the underground rock laboratory at Mol in Belgium, at 223-m depth was investigated in the laboratory by performing low pressure oedometer tests (vertical effective stress ranging from 0.05 to 3.2 MPa), high pressure oedometer tests (vertical effective stress ranging from 0.125 to 32 MPa), isotropic consolidation tests (confining effective stress ranging from the in-situ stress to 20 MPa) and triaxial shear tests. It has been observed that the mineralogy, geotechnical properties and hydro-mechanical behaviour of Boom clay from Essen at 227-m, 240-m and 248-m depths are similar to that of Boom clay from Mol. As in the case of Boom clay at Mol, the failure envelope of Boom clay at Essen in the pâ-q plane is not linear. The slope of the portion beyond the pre-consolidation stress of Boom clay from Essen is almost the same as that from Mol, suggesting a similar internal friction angle of about 13°. The compression curves (void index Iv versus logarithm of vertical stress) beyond the pre-consolidation stress are the same for both samples from Mol and Essen, and situated between the intrinsic compression line (ICL) and the sedimentation compression line (SCL). The yield stress determined from oedometer tests seems to be stress-path dependent and lower than the pre-consolidation stress. Thus determining the over-consolidation ratio (OCR) using the yield stress value would lead to an incorrect estimate. From a practical point view, the laboratory test results from Essen and their comparison with those from Mol provide important information regarding the transferability of knowledge on Boom clay at different sites, taking into account the fact that most investigations have been carried out on Boom clay at Mol.
Abstract: This paper presents an experimental study on the swelling pressure of heavily compacted crushed Callovo-Oxfordian (Cox) claystone at a dry unit mass Ïd = 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 behaviour during wetting is a function of the suction and depends on both the hydration paths and wetting conditions (e.g. vapour-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 homogeneous. 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.
Abstract: Laboratory tests were conducted to investigate the effect of wettingâdrying (WâD) cycles on the initiation and evolution of cracks in clay layer. Four identical slurry specimens were prepared and subjected to five subsequent WâD cycles. The water evaporation, surface cracks evolution and structure evolution during the WâD cycles were monitored. The effect of WâD cycles on the geometric characteristics of crack patterns was analyzed by image processing. The results show that the desiccation and cracking behaviour was significantly affected by the applied WâD cycles: the measured cracking water content θc, surface crack ratio Rsc and final thickness hf of the specimen increased significantly in the first three WâD cycles and then tended to reach equilibrium; the formed crack patterns after the second WâD cycle were more irregular than that after the first WâD cycle; the increase of surface cracks was accompanied by the decrease of pore volume shrinkage during drying. In addition, it was found that the applied WâD cycles resulted in significant rearrangement of specimen structure: the initially homogeneous and non-aggregated structure was converted to a clear aggregated-structure with obvious inter-aggregate pores after the second WâD cycle; the specimen volume generally increased with increasing cycles due to the aggregation and increased porosity. The image analysis results show that the geometric characteristics of crack pattern were significantly influenced by the WâD cycles, but this influence was reduced after the third cycle. This is consistent with the observations over the experiment, and indicates that the image processing can be used for quantitatively analyzing the WâD cycle dependence of clay desiccation cracking behaviour.
Abstract: The hydro-mechanical behaviour of compacted expansive Romainville clay was investigated. The soil was air-dried, crushed, and passed through a 2 mm sieve before being statically compacted to a dry density of 1.35 Mg/m3. The mechanical behaviour was investigated by tests in oedometer with controlled suction using the vapor equilibrium technique (suction s = 0, 9, 39, and 110 MPa). The vertical stress was applied in the range of 0â800 kPa. The experimental results are shown as follows: 1) wetting-induced swelling was higher at lower vertical stresses; 2) the vertical stress under which no swelling occurred during water flooding was estimated at 60 kPa, which can be considered as the swelling pressure of the soil tested; 3) the soil compressibility (changes of volume upon stress increases) was strongly influenced by the soil suction: the lower the suction, the higher the compressibility. The hydraulic behaviour was investigated using a large-scale inltration chamber (800 mm à 1000 mm in section and 1000 mm high). The large size of the soil column allowed burying the volumetric water content sensors (ThetaProbe) without signicantly affecting the water transfer and the soil swelling during inltration. The soil suction was monitored along the soil height (every 100 mm) using various relative humidity sensors and psychrometers. In the inltration test, water was kept on the soil surface and changes in suction and volumetric water content were monitored for 338 d. The wetting front has reached the bottom of the soil column at the end of the test. The data from the simultaneous monitoring of suction and water content were used to determine the water retention curve and the unsaturated hydraulic conductivity using the instantaneous prole method. It has been observed that the soil water retention curve depends on the soil depth; that is to be related to the soil depth-dependent swelling. The unsaturated hydraulic conductivity was found to be quite low, comprised between 3 à 10â11 m/s (at saturated state) and 10â14 m/s (at about 100 MPa suction).
Abstract: 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.
Abstract: 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/m3
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 â 18 MPa (at an initial suction of 110 MPa)
to 0.2 MPa at saturated state.
Abstract: The osmotic technique was calibrated at various temperatures (20 â 40°C) using a high-capacity tensiometer. The effect of temperature on the calibration curve of the high-capacity tensiometer in the positive range has been found to be insignificant, about 0.03%/°C. The measurement at ambient temperature shows that the suction value is not significantly dependent on the molar mass of poly-ethylene glycol (PEG) nor on the molecular weight cut-off (MWCO) of the semi-permeable cellulose membrane. On the other hand, the matric suction measured in the present work by tensiometer was lower than the total suction found in the literature measured by psychrometer. This shows that the so-called membrane effects must be characterised by not only the crossing of PEG molecules but also other complex phenomena. The calibration at controlled temperatures showed a slight suction decrease by 1%/°C. One possible explanation for this decrease is that increasing temperature decreases the viscosity of PEG solutions, possibly accelerating the PEG molecules crossing. It is also possible that increasing temperature would change the physico-chemical properties of the PEG solutions, resulting in a suction decrease.
Abstract: When drying a clayey soil, shrinkage and then cracking on soil surface occur due to water loss by evaporation, this phenomenon seems to be temperature-dependent. In the present work, experimental tests were conducted on saturated slurry to investigate the desiccation cracking behavior at three temperatures (22, 60 and 105 °C). The initiation and propagation of desiccation cracks during drying was monitored using a digital camera. By applying computer image processing technique, the surface crack ratio (RSC) which is the ratio of the surface area of cracks to the total surface area of specimen, was defined to quantify crack networks at different water contents. The experimental results show that the initial critical water content (wIC), which corresponds to the initiation of desiccation crack, increases with temperature rise. After the initiation of a crack, the ratio RSC increases with decreasing water content and then keeps almost constant when the water content becomes lower than the critical water content (wFC). By comparing the cracking curve with shrinkage curve, it has been found that the cracking curve, to some extent, reflects the shrinkage properties of soil since the wFC is related to the shrinkage limit and slightly influenced by temperature.
Abstract: Abstract: 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 oedometer 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/m3
), 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 1ï´10ï10
m/s.
Abstract: 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.
Abstract: Boom clay, a stiff clay, has been selected as a potential host formation for the geological disposal of radioactive waste in Belgium. The underground research facility HADES has been constructed to enable various in situ experiments to be performed on Boom clay so as to study the feasibility of high-level radioactive waste disposal, and to provide reliable data on the performance of Boom clay as a host formation. Among the various laboratory studies performed on samples extracted from the HADES facility to investigate the thermo-hydro-mechanical behaviour of Boom clay, relatively few were devoted to the time-dependent behaviour, limiting any relevant analysis of the long-term behaviour of the disposal facility. The present work aims at investigating the time-dependent behaviour of Boom clay under both thermal and mechanical loading. High-pressure triaxial tests at controlled temperatures were carried out for this purpose. The tests started with constant-rate thermal and/or mechanical consolidation and ended with isobar heating and/or isothermal compression at a constant stress rate or by step loading. The results obtained confirmed the effect of the overconsolidation ratio (OCR) on the thermal volume changes (i.e. thermal dilation under high OCRs and thermal contraction with OCR close to unity). Significant effects of temperature as well as of compression and heating rates were also observed on the volume change behaviour. After being loaded to a stress lower than the preconsolidation pressure (5 MPa) at a low temperature of 25°C and at a rate lower than 0·2 kPa/min, the sample volume changes seemed to be quite small, suggesting a full dissipation of pore water pressure. By contrast, after being subjected to high loading and heating rates (including step loading or step heating), the volume changes appeared to be significant, particularly in the case of stresses much higher than the preconsolidation pressure. Because of its low permeability, full consolidation of Boom clay required a long period of time, and it was difficult to distinguish consolidation and creep from the total volume change with time.
Abstract: The soil compaction by vehicles is a major factor responsible for physical degradation of
cultivated soils. Uniaxial confined compression tests are usually performed to characterise
the compaction properties of soil. Two main forms of compression curve have been
observed: (i) the bi-linear curve having an elastic rebound curve at low stresses and a linear
virgin compression curve at higher stresses; (ii) the S-shaped curve having deviation of the
virgin compression curve at high stresses. In the present work, uniaxial confined
compression tests were performed on four soils having various textures and different
plasticity. Tests were performed on undisturbed and remould samples, at various initial dry
bulk densities and water contents. The S-shaped compression curves were observed more
frequently when the clay content and/or the initial water content were high. In addition, the
S-shaped curves were observed more frequently on remould soils than on undisturbed soils.
The difference between the compression of air-filled pores and that of meso-pores storing
water subjected to high capillary forces could explain the observed S-shaped curves.
Abstract: In order to better understand the heave observed on the railway roadbed of the French high-speed train (TGV) at Chabrillan in southern France, the swelling behaviour of the involved expansive clayey marl taken from the site by coring was investigated. The aim the study is to analyse the part of heave induced by the soil swelling. First, the swell potential was determined by flooding the soil specimen in an oedometer under its in-situ overburden stress. On the other hand, in order to assess the swell induced by the excavation undertaken during the construction of the railway, a second method was applied. The soil was first loaded to its in situ overburden stress existing before the excavation. It was then flooded and unloaded to its current overburden stress (after the excavation). The swell induced by this unloading was considered. Finally, the experimental results obtained were analyzed, together with the results from other laboratory tests performed previously and the data collected from the field monitoring. This study allowed estimating the heave induced by soil swelling. Subsequently, the part of heave due to landslide could be estimated which corresponds to the difference between the monitored heave and the swelling heave.
Abstract: Compacted expansive clays are often considered as a possible buffer material in high-level deep radioactive waste disposals. After the installation of waste canisters, the engineered clay barriers are subjected to thermo-hydro-mechanical actions in the form of water infiltration from the geological barrier, heat dissipation from the radioactive waste canisters, and stresses generated by clay swelling under almost confined conditions. The aim of the present work is to develop a constitutive model that is able to describe the behaviour of compacted expansive clays under these coupled thermo-hydro-mechanical actions. The proposed model is based on two existing models: one for the hydro-mechanical behaviour of compacted expansive clays and another for the thermo-mechanical behaviour of saturated clays. The elaborated model has been validated using the thermo-hydro-mechanical test results on the compacted MX80 bentonite. Comparison between the model prediction and the experimental data show that this model is able to reproduce the main features of volume changes: heating at constant suction and pressure induces either expansion or contraction; the mean yield stress changes with variations of suction or temperature.
Abstract: Abstractï¼In high-level radioactive waste(HLW) repositoryï¼backfilling material should be pre-compacted. The
compression characteristics are very important for the design and security of HLW repository. The Callovo-
Oxfodian(COx) argillite(France) is now proposedï¼after being crushed and compactedï¼as an alternative backfilling
material in the geological HLW repository. In this investigationï¼the compression behavior of COx argillite with
two different grain size distributions(GSDs)ï¼which were obtained by two different crush proceduresï¼was studied
by running 1D compression tests with several loading-unloading cycles. It is found that the compression curves
are significantly influenced by the GSD of the soils. To obtain the same degree of compactionï¼the axial stress
applied to finer soil is much higher than that of coarser soil. In additionï¼the compression index at initial water
content of the finer soil is bigger than that of coarser soil. During the compressionï¼both the compression curves of
the finer and coarser soils tend to be close to each other with increasing dry density. It is shown that the effect of
GSD on the compression behavior is weakened after the soil is highly compacted. The swelling index at initial
water content increases with the dry density and seems to be independent of the GSD. During saturationï¼ the highly
compacted coarser soil sample(ï²d
= 2.0 g/cm3
) shows evident collapse behavior under 7 MPa axial stress. The
compression index decreases and the swelling index increases after the saturation.
Abstract: A large-scale infiltration tank was developed to study the water transfer in compacted expansive clay. Volumetric water content sensors were buried in a soil column for water content monitoring during infiltration. In addition to water content, soil suction and temperature at various locations and the heave at the soil surface were also monitored. Emphasis was put in minimizing the effect of sensors installation on water transfer and soil deformation. The results obtained for 338 days of infiltration were presented in terms of changes in suction, volumetric water content, temperature, and the soil heave. Based on the recorded data, the performance and limitation of different suction and volumetric water content sensors and the adopted test procedure were analyzed. The recorded data on soil suction and volumetric water content were finally analyzed for determining the unsaturated hydraulic properties of soil, such as the water retention curve and the unsaturated hydraulic conductivity. Note also that the results constitute useful data for further physical analysis or numerical modelsâ calibration.
Abstract: Identifying the vulnerability of soils to compaction damage is becoming an increasingly important issue when planning and performing farming operations. Soil compaction models are efficient tools for predicting soil compaction due to agricultural field traffic. Most of these models require knowledge of the stress/strain relationship and of mechanical parameters and their variations as a function of different physical properties. Since soil compaction depends on the soil's water content, bulk density and texture, good understanding of the relations between them is essential to define suitable farming strategies according to climatic changes. In this work we propose a new pedotransfer function for 10 representative French soils collected from cultivated fields, a vineyard and forests. We investigate the relationship between soil mechanical properties, easily measurable soil properties, water content and bulk density. Confined compression tests were performed on remoulded soils of a large range of textures at different initial bulk densities and water contents. The use of remolded samples allowed us to examine a wide range of initial conditions with low measurement variability. Good linear regression was obtained between soil precompression stress, the compression index, initial water content, initial bulk density and soil texture. The higher the clay content, the higher the soil's capacity to bear greater stresses at higher initial water contents without severe compaction. Initial water content plays an important role in clayey and loamy soils. In contrast, for sandy soils, mechanical parameters were less dependent on initial water content but more related to initial bulk density. These pedotransfer functions are expected to hold for the soils of tilled surface layers, but further measurements on intact samples are needed to test their validity.
Abstract: Compacted unsaturated swelling clay is often considered as a possible buffer material for deep nuclear waste disposal. An isotropic cell permitting simultaneous control of suction, temperature and pressure was used to study the thermo-mechanical behaviour of this clay. Tests were performed at total suctions ranging from 9 to 110 MPa, temperature from 25 to 80 °C, isotropic pressure from 0.1 to 60 MPa. It was observed that heating at constant suction and pressure induces either swelling or contraction. The results from compression tests at constant suction and temperature evidenced that at lower suction, the yield pressure was lower, the elastic compressibility parameter and the plastic compressibility parameter were higher. On the other hand, at a similar suction, the yield pressure was slightly influenced by the temperature; and the compressibility parameters were insensitive to temperature changes. The thermal hardening phenomenon was equally evidenced by following a thermo-mechanical path of loading-heating-cooling-reloading.
Abstract: An experimental device was developed to monitor the field soil suction using miniature tensiometer. This device consists of a double tube system that ensures a good contact between the tensiometer and the soil surface at the bottom of the testing borehole. This system also ensures the tensiometer periodical retrieving without disturbing the surrounding soil. This device was used to monitor the soil suction at the site of Boissy-le-Châtel, France. The measurement was performed at two depths (25 and 45 cm) during two months (May and June 2004). The recorded suction data are analyzed by comparing with the volumetric water content data recorded using TDR (Time Domain Reflectometer) probes as well as the meteorological data. A good agreement between these results was observed, showing a satisfactory performance of the developed device.
Abstract: Thermal conductivity of compacted bentonite is one of the most important properties in the design of high-level radioactive waste repositories where this material is proposed for use as a buffer. In the work described here, a thermal probe based on the hot wire method was used to measure the thermal conductivity of compacted bentonite specimens. The experimental results were analyzed to observe the effects of various factors (i.e. dry density, water content, hysteresis, degree of saturation and volumetric fraction of soil constituents) on the thermal conductivity. A linear correlation was proposed to predict the thermal conductivity of compacted bentonite based on experimentally observed relationship between the volumetric fraction of air and the thermal conductivity. The relevance of this correlation was finally analyzed together with other existing methods using experimental data on several compacted bentonites.
Abstract: Highly compacted sand-bentonite mixtures are often considered as possible engineered barriers in deep high-level radioactive waste disposals. In-situ, the saturation of these barriers from their initially unsaturated state is a complex hydro-mechanical coupled process in which temperature effects also play a role. The key parameter of this process is the unsaturated hydraulic conductivity of the barrier. In this paper, isothermal infiltration experiments were conducted to determine the unsaturated hydraulic conductivity according to the instantaneous profile method. To do so, total suction changes were monitored at different locations along the soil specimen by using resistivity relative humidity probes. Three constant volume infiltration tests were conducted showing, unexpectedly, a decrease of the hydraulic conductivity during infiltration. One test performed under free-swell conditions showed the opposite and standard trend. These observations were interpreted in terms of microstructure changes during wetting, both under constant volume and free swell conditions.
Abstract: This paper presents a new suction-temperature controlled isotropic cell that can be used to study the thermo-mechanical behavior of unsaturated expansive clays. The vapor equilibrium technique is used to control the soil suction; the temperature of the cell is controlled using a thermostat bath. The isotropic pressure is applied using a volume/pressure controller that is also used to monitor the volume change of soil specimen. Preliminary experimental results showed good performance of the cell.
Abstract: A new experimental set-up using a differential pressure transducer was developed, that enables the monitoring of volume changes in cyclic triaxial tests on unsaturated soils. Calibration tests were performed in order to analyze the performance of the set-up, especially in terms of loading frequencies. Based on calibration results, a low frequency of 0.05 Hz was adopted for the tests carried out on the unsaturated loess from northern France. Five water contents were considered in the tests. The obtained results have confirmed the efficiency of the new system for volume change monitoring under cyclic loading. The effect of water content on the cyclic behavior of loess was clearly evidenced. Finally, some suggestions were made to improve the accuracy of the system.
Abstract: Problems related to unsaturated soils are frequently encountered in geotechnical or environmental engineering works. In most cases, for simplicity, one can study the problems by considering the suction effects on volume change or shear strength under isothermal conditions. Often, under these conditions, a temperature-independent water retention curve is considered in the analysis â obviously a simplification. When the temperature changes are too significant to be ignored, it is necessary to account for the thermal effects. This note presents a method for controlling suction with the vapour equilibrium technique at different temperatures. First, calibration of various saturated saline solutions was carried out at temperatures ranging from 20 °C to 60 °C. A mirror psychrometer was used to measure relative humidity generated by saturated saline solutions at different temperatures. The results obtained were in good agreement with the data from the literature. This information was then used to determine the water retention properties of MX80 clay, which showed that the retention curve shifts downward with increasing temperature.
