arXiv subjects

Yu-Jun Cui

Publications and source records attributed to Yu-Jun Cui.

18 recordsLinked to original sources

arXiv2013

On-sample water content measurement for a complete local monitoring in triaxial testing of unsaturated soils

To provide a complete local monitoring of the state of an unsaturated soil sample during triaxial testing, a local water content measurement device was adapted to a triaxial device comprising the measurement of local displacements (Hall effect transducers) and suction (High capacity transducer). Water content was locally monitored by means of a resistivity probe. The water content/resistivity calibration curves of an intact natural unsaturated loess from Northern France extracted by block sampling at two depths (1 and 3.3 m) were carefully determined, showing good accuracy and repeatability. The validity of two models giving the resistivity of unsaturated soils with respect to their water content was examined.

physics.geo-ph↗

arXiv2012

On The Thermal Consolidation Of Boom Clay

When a mass of saturated clay is heated, as in the case of host soils surrounding nuclear waste disposals at great depth, the thermal expansion of the constituents generates excess pore pressures. The mass of clay is submitted to gradients of pore pressure and temperature, to hydraulic and thermal flows, and to changes in its mechanical properties. In this work, some of these aspects were experimentally studied in the case of Boom clay, so as to help predicting the response of the soil, in relation with investigations made in the Belgian underground laboratory at Mol. Results of slow heating tests with careful volume change measurements showed that a reasonable prediction of the thermal expansion of the clay-water system was obtained by using the thermal properties of free water. In spite of the density of Boom clay, no significant effect of water adsorption was observed. The thermal consolidation of Boom clay was studied through fast heating tests. A simple analysis shows that the hydraulic and thermal transfers are uncoupled. Experimental results from fast heating tests showed that the consolidation coefficient does not change significantly with increased temperature, due to the opposite effect of increasing permeability and decreasing porosity. The changes of permeability with temperature were investigated by running constant head measurements at various temperatures. An indirect analysis, based on the estimation of the mv consolidation parameter, showed that the indirect method of estimating the permeability from consolidation tests should be considered carefully. Intrinsic permeability values were derived by considering the change of the viscosity of free water with temperature. A unique relationship between the intrinsic permeability and the porosity was observed, with no dependence on temperature, confirming that the flow involved in the permeability test only concerns free water.

physics.geo-ph↗

arXiv2011

Permeability changes in Boom clay with temperature

In the framework of research into radioactive waste disposal, this paper presents some tests carried out to investigate the effects of temperature on the changes in permeability of Boom between 20 and 90 deg C. Constant head permeability tests were carried out in a high pressure isotropic compression tests at various temperature and isotropic stresses. The results show that the changes in permeability of Boom clay with temperature are only due to the changes in viscosity of free water with temperaure. This demonstrates on the one hand that the water involved in transfer at various temperatures is free water and on the other hand that there is apparently only few changes in the status of adsorbed water with respect to temperature between 20 and 90 deg C.

cond-mat.other↗

arXiv2009

Experimental study on the cyclic resistance of a natural loess from Northern France

In order to analyze the instability phenomenon observed along the Northern High Speed Line of Réseau Ferré de France (RFF), soil blocks were taken at a site near the railway, at four different depths (1.2, 2.2, 3.5 and 4.9 m). Cyclic triaxial tests were carried out on saturated and unsaturated soil specimens. The results from tests on initially saturated specimens showed that the soil taken at 2.2 m depth has the lowest resistance to cyclic loading, in relation to its highest porosity and lowest clay fraction. This soil was then studied at unsaturated state with various initial water contents. Unsaturated soil specimens were first subjected to cyclic loadings to decrease their volume. These cyclic loadings was stopped when the volume decrease was approximately equal to the initial pore air volume, or when the pores filled by air were eliminated and the soil was considered to become saturated. Afterwards, the back-pressure tubing was saturated with de-aired water and cycles were applied under undrained condition. Significant effect of initial water content was evidenced: the lower the initial water content, the higher the cyclic resistance. This can be explained by the densification of the soil during the initial cyclic loadings.

physics.class-ph↗

arXiv2009

Analysis of the railway heave induced by soil swelling at a site in southern France

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.

physics.class-ph↗

arXiv2009

Modelling the thermo-mechanical volume change behaviour of compacted expansive clays

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.

physics.class-ph↗

arXiv2009

Investigating the time-dependent behaviour of Boom clay under thermo-mechanical loading

Among the various laboratory studies to investigate the Thermo-Hydro-Mechanical (THM) 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. Significant effects of temperature as well as of compression and heating rates were observed on the volume change behaviour. After being loaded to a stress lower than the pre-consolidation pressure (5 MPa) at a low temperature of 25\degree 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 pre-consolidation pressure. Due to 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.

physics.class-ph↗

arXiv2008

Analysing the form of the confined uniaxial compression curve of various soils

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.

physics.geo-ph↗

arXiv2008

Determining the unsaturated hydraulic conductivity of a compacted sand-bentonite mixture under constant volume and free-swell conditions

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.

physics.geo-ph↗

arXiv2008

A study on the thermal conductivity of compacted bentonites

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 others existing methods using experimental data on several compacted bentonites.

physics.geo-ph↗

arXiv2008

A constitutive model for unsaturated cemented soils under cyclic loading

On the basis of plastic bounding surface model, the damage theory for structured soils and unsaturated soil mechanics, an elastoplastic model for unsaturated loessic soils under cyclic loading has been elaborated. Firstly, the description of bond degradation in a damage framework is given, linking the damage of soil's structure to the accumulated strain. The Barcelona Basic Model (BBM) was considered for the suction effects. The elastoplastic model is then integrated into a bounding surface plasticity framework in order to model strain accumulation along cyclic loading, even under small stress levels. The validation of the proposed model is conducted by comparing its predictions with the experimental results from multi-level cyclic triaxial tests performed on a natural loess sampled beside the Northern French railway for high speed train and about 140 km far from Paris. The comparisons show the capabilities of the model to describe the behaviour of unsaturated cemented soils under cyclic loading.

physics.class-ph↗

arXiv2008

Geotechnical problems related with loess deposits in Northern France

Special problems were encountered in some areas in Northern France where the high speed railways (TGV Nord) crossed some loess deposits that appeared to be specially sensitive to change in water content and susceptible to collapse. Numerous sinkholes appeared along some sections of the line following wet climatic periods. After a general geological and geotechnical presentation of loess deposits and collapse susceptibility, in which some tools of the mechanics of unsaturated soils are reconsidered with special application to loess collapsibility, this paper describes the results of a geotechnical study carried out on block samples of intact loess. Collapse susceptibility is examined in the light of microstructure observation. The dependence of collapse to water content changes is examined and the validity of various existing collapse criteria is investigated.

physics.geo-ph↗

arXiv2008

Thermo-mechanical behaviour of a compacted swelling clay

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 degrees 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.

physics.class-ph↗

arXiv2008

Monitoring field soil suction using a miniature tensiometer

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.

physics.class-ph↗

arXiv2008

Suction effects in deep Boom clay block samples

Extensive investigations have been and are being carried out on a stiff clay from an underground research laboratory located at Mol (Belgium) called Boom clay, in the context of research into deep nuclear waste disposal. Suction effects in deep Boom clay block samples were investigated through the characterisation of the water retention and of the swelling properties of the clay. The data obtained allowed an estimation of the sample initial suction that was reasonably compatible with the in-situ state of stress at a depth of 223 m. The relationship between suction and stress changes during loading and unloading sequences were also examined by running oedometer tests with suction measurements. A rather wide range of the ratio s/sigma 'v (being s the suction and sigma 'v the effective vertical stress) was obtained (0.61 - 1), different from that proposed by Bishop et al; (1974). Finally, the effect of suction release under an isotropic stress close to the estimated sample suction was investigated. A slight swelling (1.7%) was observed and further compression provided a satisfactory value of the overconsolidation ratio confirming the suggestions of taking some precautions before putting a swelling sample in contact with water as suggested by Graham et al. (1987). The various experimental data gathered in this study finally evidenced a relatively good state of conservation of the block sample used.

physics.geo-ph↗

arXiv2007

Controlling suction by vapour equilibrium technique at different temperatures, application to the determination of the water retention properties of MX80 clay

Problems related to unsaturated soils are frequently encountered in geotechnical or environmental engineering works. In most cases, for the purpose of simplicity, the problems are studied by considering the suction effects on volume change or shear strength under isothermal conditions. Under isothermal condition, very often, a temperature independent water retention curve is considered in the analysis, which is obviously a simplification. When the temperature changes are too significant to be neglected, it is necessary to account for the thermal effects. In this paper, a method for controlling suction using the vapour equilibrium technique at different temperatures is presented. First, calibration of various saturated saline solutions was carried out from temperature of 20 degrees C to 60 degrees C. A mirror psychrometer was used for the measurement of relative humidity generated by saturated saline solutions at different temperatures. The results obtained are 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 is shifting down with increasing of temperature.

physics.class-ph↗

arXiv2007

A new isotropic cell for studying the thermo-mechanical behavior of unsaturated expansive clays

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.

physics.class-ph↗

arXiv2007

Use of a differential pressure transducer for the monitoring of soil volume change in cyclic triaxial test on unsaturated soils

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.

physics.class-ph↗