High Suction Range Research Articles

Comparative study of hydro-mechanical behaviors of compacted bentonite powder and granular bentonite

In the deep geological disposal repository of high-level radioactive waste, buffer/backfill materials typically consist of compacted bentonite block and granular bentonite. As these materials undergo a long-term hydration, it is anticipated that the two forms of bentonite materials (i.e. compacted bentonite powder (CBP) and granular bentonite (GB)) are expected to exhibit differing hydro-mechanical behaviors due to the differences in their structures. This work aims to investigate the differences in swelling pressure and compressibility through a series of swelling pressure tests, compression tests and mercury intrusion porosimetry (MIP) tests. The experimental results demonstrated that swelling pressure curves of the CBP specimens showed higher first peak values and more pronounced collapse than those of the GB specimens at a given dry density, regardless of vapor-water hydration or liquid-water hydration. The final swelling pressures of the two materials were similar at the same dry density, suggesting an independent correlation between swelling pressure and dry density. At the high suction range, the compression curves exhibited an obvious bi-linear pattern for the CBP specimens and a significant nonlinearity for the GB specimens. Meanwhile, the CBP specimens presented higher pre-consolidation pressures and larger compression indices than the GB specimens at a given suction. As suction decreased, the compression curves of the two materials gradually approached each other and their differences were reduced accordingly. After reaching saturation, a good consistency between them was observed whether for final swelling pressure or compressibility. Pore structure analysis revealed that the two materials both presented an initially double structure, and their differences were primarily manifested at the macrostructural level. Eventually, the differences in swelling pressure or compression curves of the two materials were well interpreted by combining microstructural evolutions.

Hydraulic conductivity over a wide suction range of loess with different dry densities

Experimental research into the hydraulic conductivity curve (HCC) of unsaturated soil is limited due to the inherent challenge associated with labor, cost, and time. Typically, the HCC is estimated using the soil water characteristic curve (SWCC) based models and saturated hydraulic conductivity (SHC). However, the efficiency of the SWCC-based model is rarely assessed, and the influence of soil density and pore structure on HCC remains incomplete due to limited experimental data. To address this gap, this study employs an innovative filter-paper-based column method, which can measure the HCC over a wide suction range (e.g. 0−105 kPa), to capture the HCCs of both intact and compacted specimens with varying dry densities. The efficiency of two typical SWCC-based models is assessed using the measured data. Meanwhile, the mercury intrusion porosity (MIP) technique is employed to obtain the pore characteristic (i.e. pore size distribution (PSD)) and a method of predicting the HCC using the PSD data is proposed, emphasizing the dominant role of the pore structure in shaping the HCC. The results reveal that the dry density's influence on the HCC is primarily observed within the low suction range, corresponding to variations in the dominant and large pores. In the high suction range, the HCCs align along a linear trajectory when plotted in a log-log format. A notable finding is the overestimation of the HCC obtained from the SWCC-based models using the measured SHC. When the SHC is regarded as a fitting parameter, good agreement is achieved. The adjusted SHC value is typically 0-1 order of magnitude lower than the measured value, and this discrepancy diminishes as dry density increases. On the other hand, the proposed PSD-based model performs well with the measured SHC data. Caution is exercised when using the SHC to estimate the HCC for modeling water movement in partially saturated soil.

Prediction of the hydraulic conductivity function for unsaturated soils over the entire suction range

The hydraulic conductivity function (HCF) refers to in this paper is the variation of hydraulic conductivity over the entire suction range from 0 to 106 kPa. Most of the available HCF models in the literature are derived based on capillary theory and have limitations in the explanation of HCF in the high suction range. Recently, several models have been proposed for accounting for the thin film flow dominated by the adsorbed water in the high suction range. However, these models require measured HCF data for describing the thin film flow, which is time-consuming. In this study, a mathematically continuous and simple model is proposed for predicting the HCF. The model has physically meaningful parameters for predicting the HCF. In this model, the residual suction estimated from the soil–water characteristic curve (SWCC) is used to distinguish the capillary and adsorption-dominated regions, and a correction operator that considers the thin film flow is introduced. The performance of the model was validated using the measured and published experimental data for various soils. In addition, the proposed model is successfully applied in a numerical investigation of the hydraulic behavior of a capillary barrier system, highlighting the role of the thin film flow.

Effect of Initial Conditions on the Pore Structure and Bimodal Soil–Water Characteristic Curve of Compacted Granite Residual Soil

Granite residual soil typically forms complex pore structures and exhibits high water sensitivity due to physical and chemical weathering processes. Changes in initial compaction conditions significantly affect the mechanical and hydraulic properties of in situ granite residual soil subgrades, with these variations fundamentally related to changes in pore structure and soil–water characteristics. This study investigates the pore structure and bimodal soil–water characteristic curve (SWCC) of a compacted granite residual soil through laboratory tests and mercury intrusion porosimetry tests. Nine initial conditions were selected based on potential in situ compaction conditions of subgrades, and their effects on the pore size distribution (PSD) and SWCC were thoroughly analyzed. The results show strong correlations between bimodal pore structure and SWCC. The size and volume of inter-aggregate pores exhibit noticeable changes with initial conditions, affecting SWCC within the low and middle suction range. Conversely, the intra-aggregate pores, which constitute over 60% of the pore structures, remain nearly intact across different initial conditions, resulting in similar SWCCs within the high suction range. As the compaction energy increases, the inter-aggregate pores are compressed and lead to a higher water retention capacity. In addition, as the compaction water content increases, the SWCC becomes less sensitive to compaction energy after the aggregates in the pore structure are fully saturated. Additionally, a three-dimensional bimodal SWCC equation is proposed and validated using test data with an R2 value above 0.98. These findings offer valuable insights for the design and quality control of granite residual soil subgrades.

Effect of granular structure and initial suction on shear strength of GMZ bentonite for deep geological disposal

Granular bentonites have recently attracted increasing attention to serve as buffering/backfilling material alternatives in deep geological repository for disposing high-level radioactive waste (HLRW). In this paper, GMZ bentonite was prepared in the forms of compacted powder (CP), powder/pellet mixture (PPM) and pellet mixture (PM), and a series of direct shear tests, compression tests, soil-water retention tests and microstructural observation were conducted. Results show that the water retention capacity is independent of granular structure and dry density at suction s > 10 MPa. Below this suction, variation in water content should be related to the difference in microstructure. As s decreased, the specimen became more compressible and its shear resistance decreased while showing unimportant changes at s > 100 MPa. Besides, differences in compression and shear behavior between three types of specimens became gradually less obvious with decreasing suction. This indicates that wetting-induced comparable fabric could gradually cause similar compression and shearing response. Both high-density pellets and gradation are considered to affect the internal friction mode and mechanical interlocking phenomenon, so that the peak shear strength and relevant parameters varied. The unified effective stress equation considering both capillary and adsorptive mechanisms was used to predict the peak shear strength, and the predicted values agreed well with test data mostly for the high suction range. The results and ideas obtained in this study are conceived to provide helpful reference for material selection and assessment of bentonite barriers for deep geological disposal.

Determining the Bimodal Soil–Water Characteristic Curve of Fine-Grained Subgrade Soil Derived from the Compaction Condition by Incorporating Pore Size Distribution

The soil–water characteristic curve (SWCC) is a key constitutive relationship for unsaturated soil which can be unimodal or bimodal. For the fine-grained compacted subgrade soil with a bimodal pattern, the determination of SWCC is complicated and needs a wide-range suction measurement. In this paper, the bimodal SWCC of a subgrade soil derived from the compaction condition was measured and determined by incorporating pore size distribution. For this purpose, a series of laboratory tests were conducted, including the pressure plate method, filter paper method, and vapor equilibrium method, which were used to measure SWCC at the low, medium, and high suction range, respectively. The pore size distribution (PSD) data were obtained by mercury intrusion porosimetry (MIP) tests and used to predict SWCC. Based on the analysis of hydraulic paths and SWCC-PSD correlations, the SWCC of the subgrade soil should be determined to follow the actual hydraulic path. SWCC within a low suction range can be filled by PSD-based data to improve the fitting accuracy. Then, a graphical method is applied to predict the bimodal SWCC by combining the filter paper method, vapor equilibrium method, and PSD-based data. The prediction curves fit well with the test data for all selected compaction conditions. Furthermore, the prediction method can still provide good prediction performance in the absence of high suction section data, which is beneficial for the application of bimodal SWCC.

Exploring soil water retention hysteresis in the entire suction range and microstructure evolution of loess: The influence of sediment depths

Sediment depths can influence both soil structure and hydraulic processes, resulting in diverse soil-water retention properties. This study aimed to investigate the impact of sediment depths on soil-water retention hysteresis across the entire suction range, as well as the microstructure evolution of loess during the drying-wetting cycle. Laboratory tests were conducted to achieve this objective. The test results demonstrated that the water retention properties of the natural loess were significantly affected by sediment depths over the entire suction range. And the tested samples shrank considerably due to drying, while the swelling curves were almost linear induced by the wetting. Besides, the commonly used four water retention models were adopted to fit the test data, and Zhou's model showed the best-fitting performance. Furthermore, the local degree of hysteresis in the low suction range was generally larger than that in the high suction range for all samples, and the minimum local degree of hysteresis corresponded to the in-situ suction of tested samples. The average degree of hysteresis of JY-5 m, JY-15 m, and JY-30 m samples were 0.12, 0.14, and 0.13 respectively. Regarding microstructure change, the void ratios (which were smaller than the actual void ratios of soil samples) measured by the mercury intrusion porosimetry (MIP) tests became larger after undergoing the drying-wetting cycle, and most of the increased pores were primarily attributed to the formation of small mesopores in the vicinity of the main capillary pores. The results of this study contribute valuable insights into the dynamics of soil-water retention hysteresis and the associated microstructure evolution of loess.

Investigation on strength and deformation properties of lateritic clay

Lateritic clay is often used as a construction material for roads in tropical and subtropical areas. However, these materials exhibit high compressibility, high rate of creep, and susceptibility to severe cracking due to swelling and shrinkage behavior. These traits are closely linked to its hydro-mechanical and deformation properties. In this study, firstly, a boundary line between the swelling and compression deformation zone was determined based on the results of wetting tests. This boundary line is crucial for identifying the specific deformation mechanisms observed in unsaturated lateritic clays under varying water conditions. Secondly, an analysis of the relationship between pore size distribution and the soil water retention curve (SWRC) were conducted. A simple bimodal SWRC model, using the normal distribution function, was proposed. Additionally, the strength characteristics of lateritic clay were investigated over a wide suction range. It was observed that the existing strength model significantly underestimated the tested values in the medium to high suction range. To address this, a segmented strength equation was introduced based on unsaturated effective stress analysis. This approach allows enables enhanced predictions of the strength properties of lateritic clay. Altogether, these findings have greatly contributed to a better understanding of the engineering properties of lateritic clay.

Correction Functions for Soil-Water Characteristics Curves Extending the Principles of Thermodynamics

The soil-water characteristics curve (SWCC) that is typically measured from laboratory tests is fit using mathematical models that are based on capillary law and used along with the saturated soil properties for predicting the hydro-mechanical behavior of unsaturated soils. Such SWCC models are valid for coarse-grained soils. However, the same models are also extended for modeling the SWCC of fine-grained soils over suction range from 0 to 106 kPa with a correction function. Due to this reason, SWCC models with correction functions have limitations in rigorous analyses of complex thermo-hydro-mechanical-chemo (THMC) behaviors of unsaturated soils, especially in the high suction range. In the present study, correction function is proposed for modeling the SWCC behavior using two widely models based on a theoretical framework extending the principles of thermodynamics. The relationships between the traditional and the proposed correction functions are discussed. Finally, comparisons are provided between the proposed and the traditional correction functions on the SWCC behavior to highlight their differences. In addition, the effects of temperature and salinity on the SWCC with correction functions are also summarized. The proposed correction function is a valuable tool for rigorous analyses and reliable prediction of the complex THMC behaviors of unsaturated soils.

Prediction of the Soil–Water Retention Curve of Loess Using the Pore Data from the Mercury Intrusion Technique

The soil–water characteristic curve (SWCC) is a crucial input parameter for describing the distribution of soil moisture and water movement in various environmental and geotechnical challenges. It is widely recognized that the SWCC is controlled by the soil’s pore structure. Attempts to estimate the SWCC using pore data obtained through the mercury intrusion porosity (MIP) technique have been conducted. However, the performance of MIP estimation remains uncertain and requires further validation with experimental data. In this study, the accuracy of MIP estimation is validated using intact and compacted loess samples prepared at different water contents, specifically dry of optimum (8%), optimum (17%), and wet of optimum (19%). The results reveal that intact and dry of optimum specimens exhibit relatively good pore connectivity, with more point-to-point contacts between particles. Conversely, specimens compacted under optimum and wet of optimum conditions exhibit poor pore connectivity, with more isolated pores, particularly in the wet-of-optimum specimen. The SWCC predictions based on MIP data are accurate for intact and dry-of-optimum compacted specimens, but significant errors occur for the optimum and wet-of-optimum specimens. Prediction accuracy using MIP data is closely linked to the soil’s pore connectivity. Despite a tenuous theoretical basis in the high suction range where adsorption forces dominate, a strong consistency between predictions and measured data across a wide suction range (e.g., 10–104 kPa) instills a high level of confidence in using MIP data to predict the wetting SWCC. The contact angle required for the prediction is suggested as a fitting parameter.

Water-retention behaviour and microscopic analysis of two biopolymer-improved sandy soils

Soil erosion in the Yellow River basin in China has seriously affected the stability of the foundations of civil engineering structures. Traditional treated materials used for preventing the soil erosion are often harmful to the environment. Biopolymers have been widely used for soil improvement because of their low cost, environmental friendliness, and feasible application. However, most studies on the water-retention behaviour of treated soils conducted to date have focused on the low or high suction range, while few have examined the behaviour over a wide suction range. Converting the saturated state of treated soils into unsaturated state until it is completely dry will lead to changes in the water retention behaviour of biopolymer-treated soils. Therefore, this study investigated variations in the water-retention behaviour and micromechanism of biopolymer-treated sandy soils over a wide range of suction. Specifically, the water-retention behaviour of treated soil was tested using a pressure plate apparatus in the low suction range and a WP4C apparatus in the high suction range. Scanning electron microscopy was also used to evaluate water-retention mechanism of soils treated with different ratios of biopolymer. Biopolymers were found to absorb water to form hydrogels and biofilms, which reduced the pore spaces between soil particles and improved the water storage ability of sandy soils. The water-retention capacity of both Xanthan gum and Gellan gum-treated sandy soils was higher than that of untreated sandy soils, and increased as the biopolymer ratio increased. The water-retention behaviour of the Gellan gum-treated sandy soil were better than that of the Xanthan gum-treated sandy soil. Finally, a prediction model of the soil–water retention curve that considered the biopolymer ratio was proposed based on the Fredlund-Xing model. This study provides a scientific basis for the application of biopolymers to control soil erosion in the Yellow River basin in China.

Investigating the Impacts of Biochar Amendment and Soil Compaction on Unsaturated Hydraulic Properties of Silty Sand

The application of biochar as an environmentally friendly additive for agricultural soils has recently gained significant attention. However, the influence of biochar addition on unsaturated hydraulic behavior at high suction ranges (i.e., exceeding 100 kPa) remains largely understudied. This study investigates the impact of biochar addition on the unsaturated hydraulic properties of biochar amended soil (BAS). The effects of biochar content, particle size, and soil compaction on the unsaturated hydraulic properties of BAS were also considered. Peanut shell biochar was utilized in this investigation and was amended into a compacted silty sand with distinct particle size groups. Soil water retention curves and unsaturated permeability were measured through a series of evaporation tests. Results demonstrate that the impact of soil compaction on the unsaturated hydraulic properties of BAS diminishes at high suction range, regardless of biochar particle size and content. A high degree of compaction reduces the saturated permeability of BAS by minimising soil macropores. On the other hand, incorporating high biochar contents with fine particles into the soil enhances the reduction of unsaturated permeability and the improvement of water holding capacity, thereby making biochar an effective application in soil for sustainability of the agroecological environment.

An extended model based on the general Van Genuchten model for the “wetting” process of GMZ Na-bentonite subjected to alkali-heat treatment

During the long-term operation of a nuclear waste repository, the buffer/backfill material in the shield of the repository can eventually lose the ability to retain water owing to factors such as groundwater chemistry and heat released from nuclides. In this paper, a test platform was constructed to investigate the “wetting” behavior of GMZ Na-bentonite (a local bentonite from Gaomiaozi county, China) after being subjected to different alkali-heat treatments (AHTs). The experimental results show that (i) montmorillonite primarily decomposes and dissolves under the alkali-heat conditions, (ii) the mass moisture content and equivalent crystalline interlayer (ECI) space of montmorillonite are negatively correlated with the duration and temperature of AHT and (iii) independent of AHT, the evolutionary relationship between the mass moisture content and suction of montmorillonite exhibits a rapid decrease in the low-suction range and a gradual stabilization in the high-suction range. Significantly, an extended VG (EVG) model was developed to quantitatively characterize impact of temperature and alkaline solution on weakening the ability of bentonite to retain water. The developed model considers the contribution of the spatial perturbation factor to the saturation, so as to clearly characterize the three “wetting” states of bentonite in ascending order of the matric potential: water-linked & air-closed, water–air fully open, and water-closed & air-linked.

Effects of MWCNTs content and dry density on hydro-structural characteristics of bentonite during drying and wetting

Bentonite has been an essential component of geosynthetic clay liners due to its low hydraulic conductivity, which has been used as a barrier against hazardous pollutants in landfills. During the drying and wetting processes caused by high temperatures and leachate in landfills, the hydro-structural characteristics of bentonite vary as the moisture content change. This article investigates the effect of dry density and multi-walled carbon nanotubes (MWCNTs) content on the soil shrinkage characteristic curve (SSCC), the soil free-swelling characteristic curve (SFSCC), the soil water characteristic curve (SWCC), and microstructure of bentonite during the drying and wetting processes. Experimental results indicate that the addition of MWCNTs can slow down the volume variation of bentonite at the normal shrinkage stage and at the initial swelling stage and improve the water retention capacity in the low suction range. Meanwhile, reducing bentonite’s dry density can increase the void ratio during drying, decrease the void ratio during wetting, and improve the water retention capacity in the low suction range. Dry density and MWCNTs content both have no influence on the SSCC at the residual shrinkage stage and the zero shrinkage stage, the SFSCC at the residual swelling stage, and the SWCC at the high suction range.

Quantifying Coupling Effects Between Soil Matric Potential and Osmotic Potential

AbstractSoil matric potential and osmotic potential are widely accepted as two independent components of total soil water potential. However, laboratory observations repeatedly demonstrated that matric potential can vary with salt concentration, implying a potential coupling between matric potential and osmotic potential. To date, it remains elusive whether matric potential and osmotic potential are independent or not and why so, and a theoretical theory for quantifying the coupling between them is still missing. Herein, a theoretical model is developed to quantitatively explain this problem via a lens provided by a recent concept of soil sorptive potential (SSP). The proposed model substantiates that matric potential and osmotic potential are not independent. The increasing salt concentration can notably depress two variables underpinning SSP, namely relative permittivity and electrical double layer thickness, leading to non‐negligible decreasing (more negative) of matric potential in the high suction range, and increasing (less negative) of it in the low suction range. In turn, the soil‐water interactions redistribute ions in soil water, raising osmotic potential especially for clay with high cation exchange capacity. The proposed model shows excellent performance in capturing experimental data, validating its accuracy. A parametric study implies that the neglection of coupling effects can lead to a significant underestimation of soil hydraulic conductivity in the film flow regime.

Microstructure and hydro-mechanical behaviour of compacted granular bentonite

In recent years, granular bentonite GB has become a reference material since it facilitates the backfilling operation due to high-density compacted granules with extended grain size distribution(maximum sizes around 10 mm). This investigation explores the initial microstructure of MX80-type GB and the hydro-mechanical HM response observed at the phenomenological scale. Samples were statically compacted at a fixed dry density (1.55 Mg/m3 ), encompassing dry and wet sides of the optimum. Themicrostructural study was performed using a combination of X-ray micro-computed tomography and mercury intrusion porosimetry. The results indicate that the pore size density function of compacted GB can be simplified as a double-porosity network with micropores (intra-granular/aggregate pores) and macropores(inter-granular/aggregate pores). Compacting at the dry side increases macropores, making the sample more compressible on loading under as-compacted states despite the higher matric suction. The time evolution of the swelling pressure displays a double-peak pattern attributed to the particular microstructure of the samples. Moreover, the initial microstructure does not significantly influence the water retention properties without a marked hysteresis in the high-suction range under confined wetting/drying cycles. This work’s outcomes highlight the microstructure’s significance in the HM behaviour of compacted GB and provide further insight into the geotechnical properties of this material.

Stable expression and control parameters in the numerical simulation of unsaturated flow

The Richards’ equation describes the flow phenomenon in unsaturated porous media and is essential to hydrology and environmental science. This study evaluated the numerical stability of two different forms of the Richards’ equation. Sensitivity analyses were performed to investigate the control parameters of the equation. The results show that the h-form Richards’ equation has better applicability for calculating variable saturation flows than the θ-form Richards’ equation. For the h-form Richards’ equation, the hydraulic conductivity of the soil in the low-suction range and the specific moisture capacity in the high-suction range primarily influenced the solution. In addition, sensitivity analyses indicated that the saturated hydraulic conductivity, initial condition, and air-entry pressure have a higher sensitivity to the simulation results than the saturated water content, rainfall intensity, and decline rate of hydraulic conductivity. Moreover, their correctness needs to be guaranteed first in numerical simulations. The research findings can provide a helpful reference for improving the reliability of numerical simulations of unsaturated flows.

Investigation into the hydro-mechanical behaviour and microstructural evolution of MX80 bentonite pellet upon wetting/drying

Bentonite pellet/powder mixtures have been proposed as the candidate sealing plugs of boreholes or galleries in deep geological disposal of high-level radioactive waste. During the long-term operation of repository, the filled bentonite pellets will be subjected to a coupled wetting/drying process, and their hydro-mechanical behaviors are accordingly expected to undergo complex changes. To clarify this point, the water retention and volume change behaviors of bentonite pellets were investigated by conducting hydration/dehydration tests under free-swell condition. Afterwards, the microstructure of bentonite pellets was studied by mercury intrusion porosimetry (MIP) tests. Results revealed that the water retention curves of pellets exhibited an obvious hysteresis in the high-suction range. The volume change of pellet was attributed to the combined actions from the expansion of micro-pores and the propagation of cracks. Meanwhile, the swelling of bentonite pellets exhibited a significant anisotropy during hydration, with a greater swelling strain in the axial direction. But this phenomenon became insignificant during the subsequent drying/rewetting processes. After a wetting/drying cycle, the bentonite pellets yielded an obvious plastic deformation, which was primarily related to the irrecoverable wetting-induced cracks.

Assessing temperature dependence of soil water adsorption strength by molecular simulation

Soil water adsorption strength represents the changing rate of adsorptive water content with regard to soil suction or water potential changes, dictating the magnitude of the soil water-retention curve at the dry end. Non-isothermal conditions are frequently encountered in energy and environmental geotechnics, posing the requirement to assess temperature effects on soil water adsorption strength. Yet, it remains challenging in assessing the temperature effects at the high suction range due to limitations in available experimental techniques. Here, the grand canonical Monte Carlo (GCMC) simulation was explored as a method to address this challenge. A series of GCMC simulations has been performed to assess the temperature effect on the external surface adsorption of three representative soil minerals – that is, K-muscovite, Na-montmorillonite and α-quartz. Molecular simulation results preliminarily reveal that the water adsorption strength of the external surface of these soil minerals only demonstrates marginal dependence on temperature. This observation is consistent with experimental results, substantiating the feasibility of the proposed method.

Temperature effects on water retention behaviour and structural evolution of GMZ bentonite pellet mixtures

Mixtures of high-density, multi-sized bentonite pellets are considered as a promising buffer/backfill material for deep geological disposal of high-level radioactive waste (HLW). During the operation of a HLW repository, the pellet mixtures will be subjected to ground water infiltrated from surrounding rock and elevated temperatures induced by nuclide decay heat, leading to a complex hydro-mechanical behaviour. In this paper, water retention tests with suction controlled by vapour equilibrium and osmotic techniques, as well as structural observations by mercury intrusion porosimetry (MIP) were performed on three kinds of specimens of Gaomiaozi (GMZ) bentonite at different temperatures (20, 40, 60 and 80 °C), including constant-volume pellet mixtures (PM) and compacted blocks (CB) with an identical dry density (1.45 Mg/m3) and free-swelling single pellets with the same initial dry density (1.95 Mg/m3) as those involved in the PM specimen. For the three kinds of specimens, the water retention capacity decreased with increasing temperature. From a perspective of thermal dynamics theory, it is concluded that in high suction range (> 10 MPa), increasing temperature inhibited the water adsorption and promoted the water desorption process consequently leading to a decreasing water retention capacity. According to the capillary law (Young – Laplace equation), in the low suction range (< 10 MPa), increasing temperature reduces the capillary water content in terms of decreasing the surface tension, contact angle and density of the capillary water, as well as expanding the entrapped pore air. Meanwhile, the MIP test results illustrate that during hydration, most of the large pores (2000–360,000 nm) were gradually converted into medium pores (100–2000 nm) while the change of small pores (6.4–100 nm) and inaccessible pores (< 6.4 nm and > 360,000 nm) were limited. Compared to the effect of suction, the temperature influence on this structural evolution process was relatively slight. Therefore, the temperature-dependent water retention behaviour was mainly caused by temperature-induced changes of water-clay interactions as well as physical properties of adsorbed and capillary water during hydration, rather than the limited changes of pore structure of the specimen.