Impacts of subsurface heat storage on aquifer hydrogeochemistry

The use of shallow aquifers for subsurface heat storage in terms of energy management and building climatisation can lead to a temperature rise in the aquifer to 70 °C and above. The influence of temperature changes on individual mineral and sorption equilibria, reaction kinetics and microbial activity is largely known. However, the impact of heating to temperatures as high as 70 °C on the aquifer overall system has not been quantified yet. Temperature-related changes in sediment ion exchange behaviour, dimension and rates of mineral dissolution and precipitation as well as microbially enhanced redox processes were studied in column experiments using aquifer sediment and tap water at 10, 25, 40, and 70 °C. At 70 °C, a change in sediment sorption behaviour for cations and organic acids was postulated based on temporal changes in pH, magnesium, and potassium concentration in the experimental solution. No clear changes of pH, TIC and major cations were found at 10–40 °C. Redox zoning shifted from oxic conditions towards nitrate and iron(III) reducing conditions at 25 and 40 °C and sulphate reducing conditions at 70 °C. This was attributed to (a) a temperature-related increase in microbial reduction activity, and (b) three times higher release of organic carbon from the sediment at 70 °C compared to the lower temperatures. The findings of this study predict that a temperature increase in the subsurface up to 25 °C and above can impair the usability of ground water as drinking and process water, by reducing metal oxides and thus possibly releasing heavy metals from the sediment. Generally, at 70 °C, where clear cation and organic carbon desorption processes were observed and sulphate reducing conditions could be achieved, a site-specific assessment of temperature effects is required, especially for long-term operations of subsurface heat storage facilities.     

Variation of wave velocity and thermal conductivity of concrete after high-temperature treatment

This paper studies the variation of mass, density, wave velocity and thermal conductivity of concrete after high-temperature heat treatment. The range of temperature to which the concrete specimens are exposed is 25–900 °C, in a heating furnace. The results are summarized as follows: three temperature ranges (20–300 °C, 300–600 °C and above 600 °C) corresponding to the moisture vaporization (i.e., adhered water, combined water or crystal water), decomposition of some minerals (i.e., Ca-hydroxide, Mg-hydroxide) and Ca-carbonate are obviously evident. The physical properties of concrete specimens change most significantly within the temperature range above 300 °C, which may be attributed to the transformation of concrete minerals. Moreover, within the temperature range of 300–900 °C, especially between 400 and 600 °C, the concrete structure has significant chemical changes basing on the variations of surface features, ultimately making the number and width of cracks and mass loss level increased, as well as the wave velocity and thermal conductivity changed.     

Thermally induced volume change and excess pore water pressure of soft Bangkok clay

This paper presents the results of an experimental investigation carried out to study thermally induced volume changes under drained heating condition and thermally induced excess pore water pressures under undrained heating condition of soft Bangkok clay. The clay was heated up from room temperature (25 °C) to 90 °C using a modified oedometer and a triaxial test apparatus, respectively. The clay was found to exhibit temperature induced volume changes and temperature induced excess pore water pressures, in the range of temperatures investigated, depending mainly on the stress history. A theoretical microstructure mechanism that can interpret the thermally induced volume change behaviour at different stress conditions is presented in this study. Furthermore, the thermally induced excess pore water pressure behaviour at different stress conditions is explained using the unloading–reloading hysteresis of the typical soil consolidation curve. The results of this study provide additional data that can enhance the understanding of the thermo-mechanical behaviour concepts of saturated clays.     

Thermal effect on fluorine emission in coal and clay minerals

Fluorine in coal is important because of not only its association with indicators of mineralization, but also environmental concerns relevant to the health of plants, animals and humans. In order to analyze the fluorine emission characteristics, this paper summarizes the laboratory data from a number of Chinese literatures of fluorine emission of coal and clay minerals for high-temperature heating. The range of temperature to which specimens have been exposed is room temperature to 1200 °C. The variation of fluorine emission has a significant increase with increasing temperature between the ranges 400–1200 °C, especially above 600 °C. Besides, environmental humidity has obvious influence on fluorine releasing.     

A nanoscopic hydro-thermo-mechanical model for nuclear waste shale/clay repositories

Shales or highly compacted engineered clay layers are being used as buffers in deep surface nuclear waste repositories. Due to the complex natural structure and fabric of clay and non-clay minerals associated with high in situ stresses, high temperatures, and the practical difficulties in the replication of the field stress and temperature conditions in the lab testing facilities, swell potential from the macro and micro investigations does not provide reliable and universally applicable results. In this study, a comprehensive molecular-level simulation-based volume change constitutive model has been developed for clay minerals incorporating the effects of cation exchange capacity, density, water content, in situ stress state, temperature, exchangeable-cations type and proportion, pore fluids, and the dissolved salts. The molecular simulations were performed using molecular mechanics, molecular dynamics, and Monte Carlo simulation techniques. Comparing the model predictions with the results of the lab tests, the model has been proven to be quite precise in the prediction of the swell potential of these strata under various overburden pressures and temperatures. There is several fold increase in swelling of clay samples at 80 °C as compared to the equivalent specimen tested at 25 °C. The effect of higher temperature is lesser at lower initial water content (higher density) while at higher water content (lower density) the structure has been found to be more vulnerable at higher temperatures. About 100 times higher confining pressure results in the same swell at 80 °C as in its counterpart specimen at 25 °C, the corresponding swell increase factor in case of 50 °C specimens is about 45. A sharp increase in swelling with a drop in in situ pressure emphasizes the probability of higher swell as a result of an accidental reduction in in situ pressure such as the higher concentration of nuclear reactions. In this study, the cohesive energy density (CED) was found to be highly sensitive to various volume change variables, such as water content, density, CEC, type, and percentage of exchangeable and non-exchangeable cations. Contrary to all the previous models, CED-based model developed in this study is universal in nature and can be adopted for any case with minimal basic material input parameters. The good agreement found between the predicted and real values for the swell potential of the undisturbed samples suggests that the model presented here can effectively be used for the assessment of the swelling potential of shale/clay deposits to be used as buffers to the nuclear waste storage.     

A comparative study of the thermal behaviour of length-fast chalcedony, length-slow chalcedony (quartzine) and moganite

The thermal behaviour of silica rocks upon heat treatment is dependent on the constituent minerals and petrographic texture types. These constituents can be shown to be mainly quartz in the form of two types of chalcedony (Length-fast (LF) chalcedony and Length-slow (LS) chalcedony, the latter also being termed quartzine) and moganite. Even though the thermal behaviour of LF-chalcedony is well understood, major uncertainties persist concerning the high-temperature behaviour of LS-chalcedony and moganite. We present here a comparative study of these three constituents of common silica rocks. Our results show that the chemical reaction is the same in all three, Si–OH + HO–Si → Si–O–Si + H₂O, but that the reaction kinetics and activation temperatures are very different. LS-chalcedony begins to react from 200 °C upwards, that is at temperatures 50 °C below the ones observed in LF-chalcedony, and shows the fastest reaction kinetics of this ‘water’ loss. Chemically bound water (SiOH) in moganite is more stable at high temperatures and no specific activation temperature is necessary for triggering the temperature-induced ‘water’ loss. Moganite is also found to act as a stabilizer in silica rocks preventing them from temperature-induced fracturing. These findings have implications for the study of potential heat treatment temperatures of silica rocks (in industry and heritage studies), but they also shed light on the different structures of SiO₂ minerals and the role of OH impurities therein.     

Low-temperature evolution of OH bands in synthetic forsterite, implication for the nature of H defects at high pressure

We performed in situ infrared spectroscopic measurements of OH bands in a forsterite single crystal between ?194 and 200 °C. The crystal was synthesized at 2 GPa from a cooling experiment performed between 1,400 and 1,275 °C at a rate of 1 °C per hour under high silica-activity conditions. Twenty-four individual bands were identified at low temperature. Three different groups can be distinguished: (1) Most of the OH bands between 3,300 and 3,650 cm^?1 display a small frequency lowering (<4 cm^?1) and a moderate broadening (<10 cm^?1) as temperature is increased from ?194 to 200 °C. The behaviour of these bands is compatible with weakly H-bonded OH groups associated with hydrogen substitution into silicon tetrahedra; (2) In the same frequency range, two bands at 3,617 and 3,566 cm^?1 display a significantly anharmonic behaviour with stronger frequency lowering (42 and 27 cm^?1 respectively) and broadening (~30 cm^?1) with increasing temperature. It is tentatively proposed that the defects responsible for these OH bands correspond to H atoms in interstitial position; (3) In the frequency region between 3,300 and 3,000 cm^?1, three broad bands are identified at 3,151, 3,178 and 3,217 cm^?1, at ?194 °C. They exhibit significant frequency increase (~20 cm^?1) and broadening (~70 cm^?1) with increasing temperature, indicating moderate H bonding. These bands are compatible with (2H)_Mg defects. A survey of published spectra of forsterite samples synthesized above 5 GPa shows that about 75 % of the incorporated hydrogen belongs to type (1) OH bands associated with Si substitution and 25 % to the broad band at 3,566 cm^?1 (type (2); 3,550 cm^?1 at room temperature). The contribution of OH bands of type (3), associated to (2H)_Mg defects, is negligible. Therefore, solubility of hydrogen in forsterite (and natural olivine compositions) cannot be described by a single solubility law, but by the combination of at least two laws, with different activation volumes and water fugacity exponents.     

Temperature and Damage Impact on the Permeability of Opalinus Clay

The effects of temperature on the water transport properties of intact and damaged Opalinus (OPA) clay are investigated by using a recently developed hollow cylinder triaxial cell [Monfared (Int J Rock Mech Min Sci 48:637–649, 2011b)] that allows full saturation and drainage conditions in low-permeability clays and shales. The volumetric response of saturated OPA clay sample during a drained heating test shows an irreversible contraction after a temperature threshold. The permeability tests which are performed before and after the heating test show that the induced irreversible sample contraction by thermal loading reduces the permeability of OPA clay sample. In order to study the effect of temperature on the permeability of a damaged sample of OPA clay, the permeability tests are performed on a saturated sample previously sheared by a standard drained triaxial loading. The test results show no significant effect of shear-type damage on the permeability of the sample at 25 °C and 80 °C. The experimental results presented in this paper show the crucial role of the thermally induced strains on sample permeability. Thermo-elastic dilation leads to a slight increase of the permeability, whereas thermoplastic contraction leads to a reduction.     

Aquifer heat storage: sulphate reduction with acetate at increased temperatures

Aquifer thermal energy storage in urban and industrial areas can lead to an increase in subsurface temperature to 70 °C and more. Besides its impacts on mineral and sorption equilibria and chemical reaction kinetics in an aquifer, temperature sensitively influences microbial activity and thus redox processes, such as sulphate reduction. Microorganism species can only operate within limited temperature ranges and their adaptability to temperature is a crucial point for the assessment of the environmental consequences of subsurface heat storage. Column experiments with aquifer sediment and tap water at 10, 25, 40, and 70 °C showed that under the constant addition of acetate sulphate reduction could be initiated after 26–63 pore volumes exchanged at all temperatures. Fastest initiation of sulphate reduction with the highest reduction rates was found at 40 °C. Maximum rate constants during experimental run-time were 0.56 h^?1 at 40 °C and 0.33, 0.36, and 0.25 h^?1 at 10 and, 25, and 70 °C, respectively. Hence, microbial activity was enhanced by a temperature increase to 40 °C but was significantly lowered at 70 °C. At 25 °C methane was found in solution, indicating the presence of fermenting organisms; at 10, 40, and 70 °C no methane production was observed. It could be shown that redox processes in an aquifer generally can adapt to temperatures significantly higher than in situ temperature and that the efficiency of the reduction process can be enhanced by temperature increase to a certain limit. Enhancement of sulphate reduction in an aquifer due to temperature increase could also allow enhanced degradation of organic ground water contaminants such as BTEX, where sulphate is an important electron acceptor.     

Influence of Pore Fluid Chemistry on the Mechanical Properties of Clay-Based Materials

The producers of nuclear waste, within all countries exploring options, including Canada, have determined the long-term solution to be a deep geological repository. In the Canadian concept, within the deep geologic repository a number of clay-based barriers will separate the containers from the surrounding geosphere. Following placement the surrounding groundwater will infiltrate into the repository. In order to analyze the performance of the repository under very complex conditions, accurate material properties are required. The chemistry of the host rock is an important aspect as the behaviour of clay-based barrier materials could be affected by the saturating saline groundwater. This paper investigates the saturated mechanical behaviour of light backfill (composed of 50 % silica sand and 50 % Na-bentonite clay) and dense backfill (composed of 70 % crushed granite, 25 % glacial lake clay and 5 % Na-bentonite clay) and the quantifying the effect of pore fluid chemistry on the strength and compressibility behaviour of the materials. The results indicate that light backfill behaviour is strongly influenced by its pore fluid chemistry while dense backfill shows limited effects. The material parameters of light backfill and dense backfill are interpreted for input into numerical simulations. These results and interpretation enrich the understanding of the mechanical response of light and dense backfill, two components of the sealing system of the Canadian deep geologic repository.     

Heating–freezing effects on the pore size distribution of a kaolinite clay

This study aims to quantify the effect of heating and freezing temperatures on the pore size distribution of saturated clays. Three kaolinite clay specimens were subjected to different temperatures: 20, 70, and ? 10 °C. Upon achieving the desired temperature for each specimen, the specimens were flash frozen in liquid nitrogen to preserve their microstructure. Each specimen was, then, freeze-dried for 24 h after which consecutive two-dimensional (2-D) SEM images were taken using a dual focused ion beam/scanning electron microscope. The produced 2-D images of each specimen were used to reconstruct three-dimensional tomographies of the specimens, which were analyzed to determine the pore size distribution at each temperature. Compared to the specimen at room temperature, the pores in the specimen subjected to ? 10 °C were larger; this is believed to be due to the formation of ice lenses inside the pores upon freezing and potential merging between initial pores to form larger pores. On the other hand, the heated specimen showed an increase in the volume of the smaller pores and a decrease in the volume of the larger pores compared to the specimen at room temperature. This opposite behavior between the small and large pores in the heated specimen is justified considering (1) the easier flow of water out of the larger pores compared to that in the smaller pores and (2) the anisotropic nature of the thermal expansion of the clay particles.     

Landscape features and potential heat hazard threat: a spatial–temporal analysis of two urban universities

Urban universities are a microcosm of urban built-up areas, such as cities, but with a much smaller scale of spatial resolution. Within universities, there are many types of landscape features exhibiting different heat absorption and transmission capacities. These landscape features generate spatial–temporal heat signatures, and the knowledge about landscape features and urban heat hazard on university campuses is limited. The objective of this research is an assessment of landscape features and the potential heat hazard threats of two urban universities in ASEAN, located in the centre of the equatorial region. The focus of this research is on urban heat hazards in two urban universities in ASEAN, the University of Malaysia in Kuala Lumpur and the University of Indonesia in Jakarta, within the context of the spatial–temporal behaviour of urban heat and the urban heat effects on the environment and human well-being on campuses. The spatial and temporal analysis used to answer the objective of this research via data-gathering methods from image satellite, ground trough, and human perception study. The UM campus and UI campus, both urban campuses, had similar landscape features but had different total percentage areas of these features. The UM campus was 59.1% covered by the densely vegetated surface landscape feature, a percentage lower than that of the UI campus, which was 65.3% vegetation covered. The temporal results for the UHS of the UM campus in 2013–2016 show a maximum temperature of 39 °C. Therefore, the UHS of the UI campus demonstrated temporal behaviour in 2013–2016, with a maximum temperature of 38 °C. The UHS behaviour of the UM campus and UI campus had an air surface temperature with a maximum average temperature of 33 °C. The air surface temperatures exceeding 32 °C at the UM campus (12 pm until 6 pm?=?5 h) lasted for a longer time than those at the UI campus (12 pm until 3 pm?=?3 h). This study showed that, based on the perceptions on both campuses, if temperatures exceeded 30 °C, respondents were very hot and very uncomfortable, which will impact health and decrease work or academic achievements, as perceptions of heat intensity impact human well-being. Students perceived that heat intensity impacted their health and they reported becoming tired and lethargic under maximum temperatures and were very hot and very uncomfortable, and this condition impacted their work activity. These results indicated that, at both the UM and UI campuses, heat intensity impacts human well-being, with risks associated with hot temperatures. These two urban campuses are significant for ASEAN university awareness of the urban heat hazard of the equatorial area.     

Freezing of water confined in porous materials: role of adsorption and unfreezable threshold

It has been widely accepted in scientific communities that water confined in porous materials gradually freezes from large pores to small pores at subfreezing temperatures (< 0 °C), though we still describe a soil as frozen or unfrozen in engineering practice and daily life. Therefore, it is more accurate to say “how frozen” instead of “whether frozen.” This gradual freezing process is temperature-dependent because water in pores of different sizes has different energy levels, which requires different temperatures for its phase transition, leading to a relationship between unfrozen water content and temperature in soils. However, the understanding of this relationship, i.e., the Phase Composition Curves (PCC), is still incomplete, especially in the low-temperature range. We still lack answers to even the most fundamental questions for frozen soils and their PCCs: (1) How much pore water could be frozen? (2) How do capillarity and adsorption control the freezing of pore water? This study investigates two basic physical mechanisms, i.e., unfreezable threshold and adsorption, for their dominant roles in the low-temperature range of the PCC. To quantify the effects of the unfreezable threshold, molecular dynamics simulation was employed to identify the unfreezable threshold of cylindrical pores. The simulation results, for the first time, revealed that the unfreezable threshold corresponds to a pore diameter of 2.3 ± 0.1 nm and is independent of the wettability of the solid substrates. Combining this unfreezable threshold with a modified Gibbs–Thomson equation, a mathematical model was proposed to predict the melting temperature in pores of different sizes, which considers both unfreezable threshold and adsorption. Comparisons of the results calculated with the new model and other two conventional equations against experimental results indicated that the model can improve conventional equations which have been used for centuries by including the two mechanisms, which significantly improved our understanding of frozen soils.     

Using soil survey data for series-level environmental phosphorus risk assessment

Choosing soil series scale for assessing phosphorus (P) retention and release characteristics may help relate routinely collected series-specific soil survey data with P retention and aid in designing series-specific P management strategies. Phosphorus retention and release characteristics of pedons collected from two benchmark upland soil series (Berks and Monongahela) and two floodplain (Huntington and Lindside) soil series of West Virginia (USA) were assessed by evaluating P sorption capacity (PSC, Langmuir method) and its major determinants, and effect of different levels of degree of P saturation (DPS) and soil test P (STP, Mehlich-1 P) on the desorbable P (0.01 M CaCl₂-extractable) concentrations. The PSC of the two floodplain soils, Huntington and Lindside, was similar but lower than PSC of upland Berks and Monongahela soils. However, thicker A horizons of Huntington and Lindside soils may compensate for their lower PSC. The B horizons exhibited higher PSC than A horizons. However, slow permeability and thinness of such horizons may discount the higher PSC effect. Relationship of PSC with ammonium oxalate extractable Al (AOX-Al) and Fe (AOX-Fe), dithionite–citrate–bicarbonate extractable Al (DCB-Al) and Fe (DCB-Fe), total C, clay content, and pH [soil:water ratio 1:1 (pH-water) and soil:0.01 M CaCl₂ solution ratio 1:2 (pH-CaCl₂)] showed that in general all except Fe and total C influenced PSC significantly. Aluminum associated with crystalline clay minerals particularly affected PSC, especially of upland soils. Most of the soils did not release considerable P even beyond the conventional critical limit of 25 % DPS for well-drained soils. DPS-desorbable P relationships, though, reflected poor reliability of DPS as an environmental index. At a given DPS and STP, surface horizons released more P than their subsurface counterparts and thus reflected the net sink character of subsurface horizons. Most of the soils did not show considerable release of P even beyond agronomically high STP levels (>23 mg kg^?1). The study provides an economical alternative to time and money-intensive lysimetric studies for assessing subsurface P loss. It reveals the workability of integrating environmental P studies with soil survey data and superiority of integrated assessment of environmental indices of P over the use of any single index.     

Past methane release events and environmental conditions at the upper continental slope of the South China Sea: constraints by seep carbonates

Authigenic carbonates and seep biota are archives of seepage history and record paleo-environmental conditions at seep sites. We obtained the timing of past methane release events at the northeastern slope of the South China Sea based on U/Th dating of seep carbonates and seep bivalve fragments from three sites located at 22°02′–22°09′N, 118°43′–118°52′E (water depths from 473 to 785 m). Also, we were able to reconstruct the paleo-bottom water temperatures by calculating the equilibrium temperature using the ages, the corresponding past δ¹⁸O of seawater (δ¹⁸O_sw) and the δ¹⁸O of the selected samples formed in contact with bottom seawater with negligible deep fluid influence. A criterion consists of mineralogy, redox-sensitive trace elements and U/Th-isotope systematics is proposed to identify whether the samples were formed from pore water or have been influenced by deep fluid. Our results show that all methane release events occurred between 11.5 ± 0.2 and 144.5 ± 12.7 ka, when sea level was about 62–104 m lower than today. Enhanced methane release during low sea-level stands seems to be modulated by reduced hydrostatic pressure, increased incision of canyons and increased sediment loads. The calculated past bottom water temperature at one site (Site 3; water depth: 767–771 m) during low sea-level stands 11.5 and 65 ka ago ranges from 3.3 to 4.0 °C, i.e., 1.3 to 2.2 °C colder than at present. The reliability of δ¹⁸O of seep carbonates and bivalve shells as a proxy for bottom water temperatures is critically assessed in light of ¹⁸O-enriched fluids that might be emitted from gas hydrate and/or clay dehydration. Our approach provides for the first time an independent estimate of past bottom water temperatures of the upper continental slope of the South China Sea.     

Novel sensible thermal storage material from natural minerals

Novel sensible thermal storage materials (TSM) were first synthesized via thermally treating the green compact obtained using clay, kaolin tailings, and hematite as major raw materials. The samples were characterized using differential scanning calorimetry and thermogravimetric, X-ray diffraction, thermal conductivities, petrography analysis, Fourier transformation infrared spectroscopy, and scanning electron microscopy. The thermal conductivity of the green compact reached 1.11–1.64 W m^?1 K^?1 after thermally treated at 200–1,000 °C. The clay component was proven to have a predominant effect on the thermal conductivity of the green compact. Kaolin tailings could act as a “modulator” for adjusting the thermal conductivity from 1.42 to 1.92 W m^?1 K^?1. Affecting mechanism of microstructural change of main components during sintering on thermal conductivity of TSM was prominently investigated. TSM could provide a potential candidate for thermal energy storage systems of concentrated solar power.     

Hydrogeological and mixing process of waters in deep aquifers in arid regions: south east Tunisia

This study addresses the hydrogeochemistry of thermal and cold waters from south east Tunisia. Temperature intervals are 38.5–68 °C and 22–27.8 °C for thermal water and cold water, respectively. Three distinct hydrogeological systems supply water either for irrigation or for drinking; they are: (1) the Continental Intercalaire geothermal aquifer (CI), (2) the Turonian aquifer and (3) the Senonian aquifer. A synthetic study including hydrochemical, hydrogeological and geothermal approaches have been applied in order to evaluate the inter-aquifers water transfer in south east of Tunisia. By using silica geothermometers and saturation indices for different solid phases, estimated thermal reservoir temperature varies between 52 and 87 °C and between 75 and 110 °C, respectively. Based on chemical and thermal data, mixing, which occurs between the ascending deep geothermal water and shallow cold water, is about 57 % cold water.     

The seabed characteristics of the bottom floor of Damietta Branch,Egypt, using acoustic signals

The study area lies between 31.32°N, 31.70°E and 31.41°N, 31.78°E along Damietta branch, Nile River. It is about 24-km long. Acoustic classification (Quester Tangent Corporation—QTC) is used as a powerful tool to study seabed characteristics which is confirmed by the sediment analyses. Sediment characteristics of the study are presented by three acoustic classes: sand, mud and organic matter intercalated by clay. The depth varies from about ?12 m at Faraskour Bridge (southern part of the study area) to about ?2.5 m at Faraskour Dam (northern part of the study area). The average current velocity is detected as 4 cm/s and it has a very low effect on the transport of both sediment and waste debris. The sediment in the northern part characterized by organic matter reaches about 70 cm thickness under and around fish cages. This huge amount of organic matter deposit leads to the reduction of the dissolved oxygen and increase pH values. This study shows that the water quality in the northern part of the study area is at risk (drinking water for Damietta city) due to the presence of the huge amount of waste debris intercalated by organic matter. The rises of temperature in summer enhance oxygen consumption and the decomposition of the organic matter. This is rapidly increasing the growth of bacteria and phytoplankton, causing turbidity and algal blooms. Those affect the water quality and raise the water toxicity (drinking water).     

Experimental study of thermal effects on the mechanical behaviour of a clay

The paper presents the results of an experimental study of thermal effects on the mechanical behaviour of a saturated clay. The study was performed on CM clay (Kaolin) using a temperature-controlled triaxial apparatus. Applied temperatures were between 22 and 90°C. A comprehensive experimental program was carried out, including: (i) triaxial shear tests at ambient and high temperatures for different initial overconsolidation ratios; (ii) consolidation tests at ambient and high temperatures; and (iii) drained thermal heating for different initial overconsolidation ratios. The obtained results provide observations concerning a wide scope of the thermo-mechanical behaviour of clays. Test results obtained at 90°C were compared with tests performed at ambient temperature. Based on these comparisons, thermal effects on a variety of features of behaviour are presented and discussed. Focus is made on: (i) induced thermal volume change during drained heating; (ii) experimental evidence of temperature influence on preconsolidation pressure and on compressibility index; (iii) thermal effects on shear strength and critical state; and (iv) thermal effects on elastic modulus. Thermal yielding is discussed and yield limit evolution with temperature is presented. The directions of the induced plastic strains are also discussed. Several remarks on the difference in the mechanical behaviour at ambient and high temperatures conclude the paper. Copyright © 2004 John Wiley & Sons, Ltd.