Compacted Clay Research Articles

U(VI) retention in compact Callovo-Oxfordian clay stone at temperature (20–80 °C); What is the applicability of adsorption models?

In the context of the radioactive waste management in deep geological formations, U(VI) retention by intact Callovo-Oxfordian claystone (COx) was studied by percolation-type experiments at 20 and 80 °C. The experimental results were confronted with modelling prediction based on a published adsorption model developed from dispersed media in the 20–80 °C temperature range. For the experiments at 20 °C, the adsorption model allowed to explain the results for the intact system; the retention was weak (Rd ∼ 10 L•kg−1) and the analysis of the COx phases at the end of the experiment confirmed a retention of U by the clay fraction. The adsorption model in temperature also explained the observed trend of increasing retention with increasing temperature. However, it underestimated the temperature effect on the adsorption of U(VI) by the COx clay fraction, and other phases contributed to the retention. Solid-state analysis of the percolation-doped samples indicated a reactivity in the order pyrite>clay>calcite phases. The transposition of the knowledge at 20 °C from the dispersed system to the intact medium was therefore not possible at 80 °C for the studied U(VI)/COx system.

Experimental investigation on the effect of initial structure on the water retention behavior of Mile clay

Soil structure has strong effect on hydro-mechanical properties of soils, particularly in the case of expansive clays. Along the wetting–drying path, expansive clays undergo significant volume changes that can pose serious challenges for infrastructures built on them. Moreover, the hydro-mechanical behaviour of intact, compacted, and reconstituted expansive clay exhibits notable differences. In this study, the effect of natural soil structure of intact Mile expansive clay on water retention behaviour was studied by comparing two states of Mile clay, intact and compacted states. The corresponding water retention behavior along the main wetting and drying paths were analysed and compared. Utilizing scanning electron microscope and mercury intrusion porosimetry tests, the different water retention behaviours observed between intact and compacted Mile clay were further elucidated from a microstructural aspect. It shows that bonding between and within soil aggregates in intact Mile clay constrained volume changes in specimens along the drying and wetting paths. Along these paths, the volume of micropores in intact specimens was consistently larger than that of compacted specimens, resulting in a good water retention behavior, particularly along the main wetting path. Volume changes of intact and compacted specimens were primarily driven by changes in macropore volume. Only when specimens become unsaturated, the changes in micropore volume became significant. At very high suction, the effect of bonding between and within soil aggregates diminished, which was possibly due to the destruction of bonding, resulting in similar water retention properties for intact and compacted specimens.

A Review of Physicochemical Stabilization for Improved Engineering Properties of Clays

Severe climatic and environmental conditions warrant the use of stabilization agents in aid of compaction for sustainable improvement in engineering properties of clays. Physicochemical agents are a viable option because they are cost effective, environmentally friendly, and offer improved long-term performance of treated soils. This research developed a fundamental understanding of the clay–water–electrolyte admixtures relations. Based on a comprehensive literature review, the effect of nanomaterials, biopolymers, and geopolymers on the behavior of compacted clays was investigated. It was found that all of these admixtures facilitate the development of an aggregated soil microstructure through unique mechanisms. Biopolymers have the highest water adsorption capacity followed by geopolymers and then by nanomaterials. The effect of admixtures on optimum compaction properties follows a decreasing trend similar to untreated clays (S = 80% ± 20%). The variation of hydraulic conductivity, compression index, and compressive strength are largely within the family of curves identified by typical relationships for compacted clays. These preliminary findings indicate that not all engineering properties are improved to the same level by the different types of physicochemical admixtures. The specific nature of geotechnical engineering (soil type and site conditions) as well as the wide range of admixture types and potential biodegradation of some of the reagents are the major shortcoming of using this class of materials.

Evaluation of sedimentary aquifer in tehsil Zehri area using VES technique, Balochistan Pakistan

A geophysical resistivity survey was conducted in Zehri tehsil in the district of Khuzdar for the metering of subsurface hydrogeological properties of aquifers. The Schlumberger electrode arrangement was employed with the vertical electrical sounding (VES) method for subsurface exploration. Two profile lines were designated for the survey. The distance between the two profile lines was about 1500 ​m and each profile contained four VES stations. The results of profile one show the aquifers were encountered in fractured limestone of the Jamburo Formation of Paleocene age at an average depth of 127 ​m, with an average resistivity of 39.7 ​Ωm, and the average aquifer thickness was 54.025 ​m. The result of profile two shows the aquifers were encountered only in VES four in the sandstone lithology of the Ziddi Formation with a resistivity of 79.9 ​Ωm, at a depth of 120 ​m with, and a thickness of 40 ​m. There was no aquifer encountered in the remaining all VES. The main lithology of these VES is thick impervious compacted limestone, shale, and clay of Jurassic age from the Ziddi Formation. The current tube wells were also rapidly depleting in the study area. The main factor behind the decline of aquifers in the study area is an uncontrolled installation of tube wells and continuous pumping of tube wells for agricultural purposes badly puts stress on aquifers and directly deteriorates the quality and quantity of groundwater.

A Theoretical and Experimental Investigation on the Fracture Mechanism of Center-Symmetric Closed Crack in Compacted Clay under Compression–Shear Loading

In this study, a modified maximum tangential stress criterion by considering T-stress and uniaxial compression tests have been utilized to theoretically and experimentally reveal the fracture initiation mechanism of a center-symmetric closed crack in compacted clay. The results show that wing cracks occur in the linear elastic phase of the stress-strain curve. In the plastic phase of the stress-strain curve, the wing cracks extend gradually and the shear cracks occur. The crack initiation stress and peak stress of compacted clay first decrease with the rise in pre-crack inclination angle (β = 0°–40°), and then increase with the rise in pre-crack inclination angle (β = 50°–90°). When the pre-crack inclination angle is relatively small or large (β ≤ 10° or β ≥ 70°), the crack type is mainly tension cracks. Secondary shear cracks occur when the pre-crack inclination angle is 10°–80°. When the dimensionless crack length is larger than 0.35, the crack types include wing-type tension cracks and secondary shear cracks. The experimental results were compared with the theoretical values. It was found that the critical size rc of compacted clay under compression-shear loading was 0.75 mm, smaller than the value calculated by the empirical formula (12 mm). The MTS criterion considering T-stress can be used to predict the compression-shear fracture behavior of compacted clay.

Influence of relative compaction and degree of saturation on the deformation characteristics of bentonite under freeze-thaw cycles

Bentonite, consisting of clay minerals of the montmorillonite group, has been widely used as an adsorbent and backfill material in nuclear waste disposal and groundwater remediation. It is challenging to use bentonite as a filling material in cold regions since bentonite is highly sensitive to thermal environmental changes, during which its bulk volume and microstructure change significantly. In this study, a series of one-dimensional and three-dimensional freeze-thaw tests were carried out within a closed system to investigate the influencing factors of the deformation of bentonite under freeze-thaw cycles. Results show that the initial soil water content greatly impacts bentonite's deformation during freeze-thaw cycles. For an initial higher degree of saturation (Sr), the expansion caused by the formation of ice lenses has a greater impact than the shrinkage induced by dehydration, ice-cementation, and so on. Conversely, bentonite tends to shrink at a lower degree of saturation during freezing. And the critical degree of saturation that determines bentonite's behavior of frost heave or frost shrinkage seems to be roughly 0.8. As the number of freeze-thaw cycles rises, initially uncompacted bentonite clay becomes more compacted, and initially compacted bentonite clay remains unchanged.

Impact of Fe(II) on 99Tc diffusion behavior in illite

A comprehensive understanding of the geochemical behavior of 99Tc is of great importance for safe disposal of radioactive waste and remediation of contaminated environmental sites. Illite is one of the most common constituents of clay rocks, and thus used in this work as a model system for studying the retention and transport of 99Tc in clay-rich systems. In this study, a through-diffusion technique was applied to investigate the diffusion behavior of Tc in compacted illite clay under oxic and anoxic conditions. Particular focus of this investigation was on the role of Fe(II) on the redox state and mobility of Tc in clay. As the diffusion cells contained stainless steel filters for confining the clay plug, 99Tc-diffusion in the filters was assessed first, followed by 99Tc diffusion in the illite column with or without Fe-loading. Two types of Fe(II) loadings in illite were considered, surface complexed Fe(II) on illite edge sites and Fe(II) added as pyrite grains. COMSOL Multiphysics was used to analyze the diffusion data. The measured filter porosity was about 0.2 and the effective diffusion coefficient, De, for Tc (as TcO4−) in the filter was 0.59 × 10−10 m2/s. Tc diffusion in illite under ambient conditions showed a typical anion diffusion behavior with De in the range 0.38-0.44 × 10−10 m2/s and the anion accessible porosity ε of approximately 0.2. In the presence of Fe(II), De for Tc migration in illite was one order of magnitude lower, showing that Fe(II) has a significant effect on the migration of 99Tc. Analysis of the Tc distribution in the system suggests that most Tc was retarded at the filters, especially the ones connected to the high concentration reservoir. The remaining Tc quantities were immobilized at sample boundaries next to the filters with higher concentrations observed in the domains close to the reservoirs. Almost no Tc was immobilized in the middle part of the sample, where the Fe(II) was preloaded. This observation contradicts the anticipation, because Tc was expected to be immobilized in the middle of the sample preloaded with Fe(II). A possible explanation is that a delocalized redox reaction may occur, which means that electrons from the oxidation of the preloaded Fe(II) in the central part of the cell are transferred to the filter via the walls of the steel diffusion cell. Tc reacts with the electrons in the filter at a relative slow rate, which results in most of the Tc retarded in the filter, while some small amounts of it may diffuse through the clay.

Study on Osmotic Consolidation and Hydraulic Conductivity Behavior of an Expansive Soil Inundated with Sodium Chloride Solution

Canals are a very imperative source of irrigation for the agricultural sector in India. Seepage causes major water loss in canals, and hence, the installation of liners becomes necessary. Compacted clay soils are commonly used as liners in the canals. This structure will most probably be subjected to salinization and desalinization cycles throughout its life. Because of the interaction between the pore liquid and clay particles, physico-chemical influences considerably impact the behavior of clay barriers. In this paper, the effect of interacting fluid on volume change, consolidation parameters, and hydraulic conductivity of compacted clay soil is investigated with the help of a one-dimensional consolidation test. The compacted clay specimens were immersed alternatively with distilled water (DW) and sodium chloride (NaCl) solutions (SW) at constant loading of 10 kPa, which replicates the load conditions in the field canal due to 1 m head of water and incremental loading as per IS 2720 part 15 standards. The experimental results proved that there is a percentage volume change increase of about two times for each stage inundated with 4M NaCl solution than its preceding stages inundated with distilled water at constant loading of 10 kPa. The consolidation rate was accelerated with 4M NaCl solution than the normal consolidation at incremental loading. The permeability coefficient in the salt water-induced sample increased by 217% more than the distilled water-induced sample at incremental loading. Therefore, the soil specimen subjected to alternate salinization and desalinization cycles significantly affects the volumetric and consolidation behavior, leading to decreased life of clay barrier structures.

Impact of drying-wetting cycles on the small strain behaviour of compacted clay

Small strain shear modulus (Gmax) is an important parameter for assessing the performance of compacted soils that underlie typical transport infrastructure assets such as railway tracks. This is particularly important when considering changes in climate patterns, which are expected to yield larger seasonal soil-atmosphere moisture fluctuations. This in turn results in the progressive variation of the small strain properties of compacted soils during their service life (i.e. drying and wetting). In this study, the small strain shear behaviour was evaluated for an intermediate plasticity clay (i.e. kaolin) in a series of drying and wetting cycles. Four different dying and wetting boundaries were considered to explore a wide range of moisture amplitudes during 10 drying-wetting cycles. Drying and wetting was controlled using gravimetric water content in order to mimic realistic field conditions typically observed at substructure level. An ultrasonic pulse transmission method was used to capture the change in small strain stiffness and volume at discrete points during the drying-wetting cycles. The results reveal clear distinctions in behaviour for all four boundaries considered, with wetting boundaries having the greatest influence on behaviour. In this instance, specimens brought to full saturation during wetting exhibited an increase in the small strain shear modulus during progressive drying-wetting cycles. However, a reduction was observed when the wetting boundary was restricted to the compacted state. Measured volume changes were also in agreement with these findings, however there was some evidence of volume increase when drying to residual conditions. The results suggest that this is associated with the formation of the partial pendular state where a loss of capillary contacts between particles occurs. Furthermore, when all data for 10 drying-wetting cycles is plotted in the e-Gmax space, a linear relationship is observed for different constant water content levels. Remarkably, this trend is shown to be independent of the boundary conditions considered in this study or number of drying-wetting cycles.

Cracks evolution and micro mechanism of compacted clay under wet-dry cycles and wet-dry-freeze-thaw cycles

Desiccation crack is a significant factor contributing to numerous engineering and environmental disasters. Under complex climate conditions, soils undergo not only seasonal wet-dry (WD) cycle, but also freeze-thaw (FT) cycle, which can alter the structural characteristics of soil, and influence the formation and development of cracks. In this study, the laboratory WD cycle and wet-dry-freeze-thaw (WDFT) cycle tests were carried out on the compacted lean clay samples which were dried to different dryness degrees to analyze the formation and development of cracks under different conditions and to discuss the mechanisms involved from a microstructural perspective. A high-resolution camera and scanning electron microscope were utilized to capture pictures of the macroscopic cracks on the sample surface and microscopic pores within the samples at each stage, respectively. The quantitative analyses were then conducted on the macroscopic cracks and microscopic pores. The results indicate that, with higher dryness degree, the crack ratio and crack width of the sample decrease with consecutive WD cycles. The samples subjected to WDFT cycles exhibit a higher crack ratio, wider cracks, and a greater proportion of primary cracks compared to the samples subjected solely to WD cycles. With lower dryness degree, the crack ratio and crack width remain relatively unchanged after each WD cycle, mainly consisting of small, shallow, and densely distributed sub-cracks. The overall crack ratio of samples subjected to WDFT cycles is lower than that of samples subjected solely to WD cycles. This is attributed to the aggregation of aggregate particles during the drying process and the dispersion promoted by the wetting process. Additionally, under high drying degrees, the FT process stabilizes the aggregate structure, while under low drying degrees, the FT process promotes the dispersion, rearrangement, and even fragmentation of aggregates, resulting in differences in crack development between WD and WDFT at different dryness degrees. These research findings provide valuable insights into the study of soil crack evolution and its mechanisms under complex environmental conditions.

Numerical simulation of the anti-freezing performance of a phase change clay core-wall with loose covering

Clay core-wall of rockfill dam may endure short-term freezing or freeze–thaw cycles owing to negative temperature conditions during winter construction in cold regions, resulting in the degradation of the clay compaction, frost swelling, and cracking performance. The most commonly used insulation cover measures interfere with the continuous construction of clay core-walls, which may shorten the effective time available for construction and delay the dam construction progress. To address this problem, we proposed using the upper layer of the loose-covering clay and the latent heat of phase change materials (PCMs) to control the core-wall temperature. We implemented a numerical model for simulating the anti-freezing performance of the PCM mixed clay (PCM-clay) with loose covering. First, a heat transfer model was established using the finite element software COMSOL, and the effectiveness of the model was verified by comparing it with indoor temperature control test results. Subsequently, numerical models including different construction conditions were established, and the temperature control effect was analyzed under different loose-covering thicknesses, initial soil temperatures, and PCM contents. The results showed that at the design ambient temperature and a 2% PCM content, an effective construction time availability of two consecutive days was maintained under a 20-cm-thick loose covering. In contrast, under the same temperature conditions, but for a 4% PCM content, the effective construction time available under a 5-cm-thick loose covering was 43.1 h. Thus, the use of PCM-clay combined with the upper layer loose covering for temperature control proved feasible and effective. Additionally, by increasing the thickness of the loose covering and the initial temperature of the PCM-clay, the PCM content can be reduced. This provides a theoretical basis and a potential technical solution to maximize the time available for construction and accelerate the construction progress of core-wall rockfill dams.

Effects of Support Friction on Mixed-Mode I/II Fracture Behavior of Compacted Clay Using Notched Deep Beam Specimens under Symmetric Fixed Support

This paper investigates the effects of support friction on mixed-mode I/II fracture behavior of compacted clay using notched deep beam (NDB) specimens under symmetric fixed support. Numerical models of 330 NDB specimens were established considering the crack inclination angle, crack length, support span, and support friction coefficient, and the normalized fracture parameters (YI, YII, and T*) of NDB specimens were calibrated. The numerical results showed that the values of YI, YII, and T* decreased at different degrees after considering the support friction. Notably, the support friction coefficient could significantly change the loading pattern at the crack tip. To verify this phenomenon, 12 compacted clay NDB specimens were prepared, and a mixed-mode I/II fracture test was performed under fixed support conditions; the phenomenon of asymmetric crack propagation was studied. The test data were processed using the numerical calibration results of YI, YII, and T* with and without consideration of friction. Afterward, the test data were compared and analyzed by combining the generalized maximum tangential stress (GMTS) and the maximum tangential stress (MTS) criteria. The analysis indicated that the real fracture characteristics of compacted clay NDB specimens could not be reflected when conducting mixed-mode I/II fracture tests under symmetric fixed support conditions if the test results were analyzed by YI, YII, and T* without considering support friction, as in previous studies.

Swelling of compacted bentonite in organic solvents: Correlation of rate and extent of swelling with solvent properties

The swelling of clay minerals within shale formations during oil and gas exploration, and within compacted bentonite barriers for radioactive waste containment, presents a number of challenges to operators. Whilst much work has been devoted to understanding the interlayer swelling properties of clay mineral crystals, significantly less has been devoted to understanding coupled pore and interlayer swelling in reactive shale/compacted clay minerals. Here we study the swelling of compacted clay mineral tablets on exposure to a range of organic solvents, selected so that the effect of key solvent properties such as dielectric constant, density, octanol-water partition coefficient, viscosity and surface tension can be correlated with the swelling observed. We use a novel non-contact swelling meter to carry out the swelling tests, allowing us to access information on rate of swelling. Short-term swelling rate showed the strongest correlation to the solvent octanol-water partition coefficient. In long term swelling, good correlation was found between total linear swelling and viscosity, and the octanol-water partition coefficient.

Fate and Transport of Coronavirus Surrogate through Compacted Clays for Pathogenic Waste Disposal.

An increased pathogenic waste post-COVID-19 pandemic forced policymakers to treat biomedical waste (BMW) similar to municipal solid waste (MSW) to dispose into dumpsites and MSW landfills across the globe. The granular bentonite of geosynthetic clay liners (GCLs) does not completely seal the macro-voids upon saturation due to the loss of osmotic potential in the salt environment from the leachate. Such behavior of GCLs can lead to advection-dominant virus migration through the liner system. A knowledge of the fate and transport of coronavirus and other viral pathogens in compacted clays is essential for safe disposal of the viral pathogens in MSW landfills. Although the attenuation and transport parameters for coronavirus have been recently evaluated theoretically, experimental backup is currently lacking. The present work uses Newcastle disease virus (NDV) as a surrogate to coronavirus due to structural similarities for studying the fate and transport in the compacted natural clays. This study also implicitly addresses the waste management facilities for waste generated from NDV outbreaks through poultry litter and carcasses. The interaction of bentonite and kaolin clays with the NDV was studied by varying the virus concentration, interaction time, and clay dose using batch sorption tests. The studied clays showed excellent attenuation efficiency for the NDV. Design parameters, viz., the diffusion coefficient and retardation factor, were evaluated, affirming the suitability of these clays for exclusive pathogenic waste disposal protocols that are discussed in this article.

Experimental investigation on water/gas resistance and strength properties of clinoptilolite/Na-PAA amended compacted clay cover

Compacted clay is an effective and economical engineering barrier for seepage prevention and gas blocking in landfills (including tailings pond, landfill, etc.). In addition to the lack of durability such as desiccation cracks of compacted clay modified by clinoptilolite and sodium polyacrylate, it is also necessary to consider the changes of engineering properties such as water holding capacity, water and gas resistance, and strength of compacted clay. Therefore, through oven evaporation test, flexible wall permeability test and unconfined compressive strength test, this paper studies the water retention, permeability and strength characteristics of compacted clay with different amounts of clinoptilolite and sodium polyacrylate. Orthogonal experiment L16(45) was used to optimize the amount of clinoptilolite and sodium polyacrylate and the compaction conditions, and further combined with the analytic hierarchy process (AHP), the factors affecting the water retention, permeability and strength of the compacted clay before and after the modification of clinoptilolite and sodium polyacrylate were discussed simply and quickly, and the applicability of the orthogonal experiment traditional analysis method and the analytic hierarchy process in the modified compacted clay of clinoptilolite and sodium polyacrylate was studied.

Measured swelling, hydraulic and thermal properties of MX-80 bentonite: Distinguishing between material variability and measurement limitations

The Canadian Nuclear Waste Management Organization (NWMO) is investigating Sedimentary and Crystalline media as potential hosts for a Deep Geological Repository (DGR) for permanent isolation of used nuclear fuel and fuel wastes. In NWMO's concept, densely compacted bentonite clay is used to surround the used fuel containers and to backfill the placement rooms.Development of sealing concepts and performance prediction models requires many materials properties inputs. These inputs are important to the evaluation of hydraulic conductivity (k), swelling pressure (Ps) and heat transfer. Evaluation of these and other properties is complicated by factors such as unidentified variability in material composition (mineralogy) and differences in test methodologies. In order to eliminate the effects of technique, data obtained using similar methods are used in this study. This should result in data scatter being more related to materials variability and limitations than measurement method.In an effort to assess the variability in the reported Ps, k and thermal properties and determine representative values or relationships for input into performance models, sufficiently large databases are needed to allow for statistical evaluation for a full range of potential repository conditions. Unfortunately, in many cases these databases are not extensive, particularly for materials in a highly saline environment.This paper presents statistical evaluations of the relationships between density, k, Ps and thermal properties using a database generated from literature sources for a well-known bentonite (MX-80) under freshwater and saline porefluid conditions. Also included is previously unpublished work by NWMO where repository-relevant groundwaters were used in testing. To limit uncertainty, the data used were obtained from sources reporting use of MX-80 bentonite and providing at least basic mineralogical information. From these data, best-fit, confidence and prediction bands and intervals were generated.

Sustainable utilization of chemically depolymerized polyethylene terephthalate (PET) waste to enhance sand-bentonite clay liners

Polyethene terephthalate (PET) waste poses major environmental harm which can be minimized by reusing it in clay soil stabilization. In general, various polymers are known to reduce hydraulic conductivity and increase the shear strength of clays. However, the application of the effect of a chemically depolymerized form of PET, i.e., Bis (2-Hydroxyethyl) terephthalate (BHET) has not been performed as an additive in Compacted Clay Liners (CCLs) for landfills. This research focuses on the effect of the air curing period (1 and 28 days) on the hydromechanical behavior of BHET-treated SBM (0, 1, 2, 3, and 4 % by dry weight). Results from One Dimensional Consolidation tests showed that an increase in BHET content reduced both compressibility and hydraulic conductivity of SBM due to pore clogging mechanism of swollen BHET hydrogel, however, hydraulic conductivity reduced over 28 days of curing due to loss in re-swelling availability of the hydrogel, thereby allowing less tortuous paths to flow. Results from Consolidated-Drained Direct Shear tests showed that for 1 and 28-days curing, BHET treatment to SBM increased the cohesion (c’) due to strong polymer interparticle bridging, however, polymer coating over the sand grains causes a reduction in its surface roughness to decrease the frictional angle (ϕ'). SEM (Scanning Electron Microscopy) and EDX (Energy-dispersive X-ray spectroscopy) analysis on BHET-treated specimens support the flocculation of bentonite, polymer bridging of sand and clay-sand polymer links. A significant Pb2+ removal capacity was also observed with BHET-treated SBM from the batch tests. FTIR (Fourier Transform Infrared Spectroscopy) analysis on batch sorption specimens confirms the role of the carbonyl groups (C = O) and hydroxyl groups (OH) present in the BHET structure indicating the possibility to adsorb Pb2+. The findings of the study suggested that a mechanism of interaction exists between sand-bentonite and BHET polymer and it can be adopted in CCLs design.

Cross-anisotropic stiffness characteristics of a compacted lateritic clay from very small to large strains

Lateritic clay is widely distributed in tropical and subtropic regions, with distinct engineering properties from other clays. Its stiffness properties, which are often anisotropic and crucial for analysing the serviceability limit state of earthen structures, have not been well understood. This study investigated the cross-anisotropic stiffness of compacted lateritic clay through comprehensive isotropic compression and consolidated drained and undrained shear tests. Both vertically and horizontally cut specimens were tested using triaxial apparatus equipped with bender elements and local strain measurements. The results show that compared with other clays, the lateritic clay shows a weaker dependency of shear modulus to confining pressure, a higher shear modulus at the same confining pressure, and a higher degradation rate of stiffness with strain. The lateritic clay behaves like granular material due to its large-size aggregated microstructure and 42% sand content. The widely used correlations between stiffness parameters and plasticity index seem unsuitable for it. Furthermore, the elastic shear modulus in the vertical direction is slightly higher than that in the horizontal direction, and the anisotropy evolves during shearing. A complete set of cross-anisotropic stiffness parameters for both effective and total stress analysis were determined. They are useful for developing constitutive models and analysing earth structures' serviceability involving lateritic soils.

Experimental study on mode I fracture characteristics of compacted bentonite clay

To explore the pure mode I fracture characteristics of compacted Gaomiaozi bentonite clay, notched semi-circular bend specimens of compacted Gaomiaozi bentonite with different dry densities and moisture contents were prepared by using the self-designed compaction and slitting molds, and then three-point bending tests were carried out, real-time monitoring was also conducted with the help of digital image correlation method. Based on the test results, the effects of dry density (1.5 g·cm−3, 1.6 g·cm−3, 1.7 g·cm−3, 1.8 g·cm−3) and moisture content (6.20%, 9.48%, 13.65%, 16.47%) on the mode I fracture toughness of compacted Gaomiaozi bentonite were analyzed, and the development of fracture process zone (FPZ) was also revealed. The results show that the pure mode I fracture toughness of compacted Gaomiaozi bentonite is in the magnitudes of 30 to 180 kPa·m1/2 within the testing range of dry density and moisture content. The mode I fracture toughness increases with increasing the dry density. At a moisture content of 9.48%, the mode I fracture toughness obtains a peak value, and with the decrease or increase of moisture content, the mode I fracture toughness presents a general decreasing trend. The length of the fully developed fracture process zone and the critical crack tip opening displacement decrease first and then increase with the increase in moisture content, while there is no obvious regularity in the variations of fully developed FPZ length and critical crack tip opening displacement with respect to dry density.

Land Subsidence in Tianjin, China: Before and after the South-to-North Water Diversion

The South-to-North Water Diversion (SNWD) is a multi-decadal infrastructure project in China aimed at alleviating severe water shortages in north China. It has imposed broad social, economic, environmental, and ecological impacts since 2015, particularly in the Beijing-Tianjin metropolitan area. Sentinel-1A/B Interferometric Synthetic Aperture Radar (InSAR) (2014–2021), Global Positioning System (GPS) (2010–2021), and hydraulic-head data are used to assess the impacts on ongoing land subsidence in Tianjin in this study. Additionally, the Principal Component Analysis (PCA) is employed to highlight primary factors controlling the recent land subsidence. Our results show that the reduced groundwater pumping has slowed down the overall subsidence since 2019 due to SNWD. As of 2021, the subsiding area (>5 mm/year) has reduced to about 5400 km2, approximately 85% of the subsiding area before SNWD; the areas of rapid subsidence (>30 mm/year) and extremely rapid subsidence (>50 mm/year) have reduced to 1300 km2 and 280 km2, respectively, approximately 70% and 60% of the areas before SNWD. Recent subsidence (2016–2021) was primarily contributed by the inelastic compaction of clays in deep aquifers of Aquifers III and IV ranging from approximately 200 to 450 m below the land surface. The ongoing rapid subsidence (>30 mm/year) in Tianjin is limited to border areas adjacent to large industrial cities (e.g., Langfang, Tanshan, Cangzhou) in Hebei Province. Ongoing subsidence will cease when hydraulic heads in the deep Aquifers (IV and V) recover to the new pre-consolidation head, approximately 45 m below the land surface, and subsidence will not be reinitiated as long as the hydraulic heads remain above the new pre-consolidation head. This study reveals the importance of coordinating groundwater and surface water uses at local, regional, and national scales for land subsidence mitigation.