Cement-based solidification/stabilisation of soils is a common risk management technique for contaminated sites. The long-term performance of these monolithic systems and the role of mix design in the diffusion of contaminants from cement-stabilised soil is not well understood. In this study, a silty sand treated with 10% Portland cement and mixed at three different water-cement ratios (1.3, 1.5 and 2.5) was tested to examine how chloride diffused both into and out of the cured monolithic samples. When comparing specimens of the same mix proportions, the diffusive flux out of a soil-cement monolith pre-mixed with chloride was less than the diffusive flux into the uncontaminated monolith. Results showed that the ratio of the inward chloride diffusive flux to the outward chloride diffusive flux is 3.0, 2.5 and 1.4 for a water-cement ratio of 1.3, 1.5 and 2.5, respectively. This is attributed to more interconnected pores (less occluded pores) at higher water-cement ratios. From a design perspective, the findings indicate that using diffusive properties (ne and De) derived from conventional diffusion tests produces conservative estimates of long-term contaminant transport in cement-stabilised soils. However, this conservatism diminishes with increasing water-cement ratio, owing to a reduced fraction of occluded pores.
Biopolymers like xanthan gum (XG) are increasingly explored as an environmentally friendly soil binder; however, their combined use with conventional binders and response to moisture-induced effects have received limited attention. This study evaluates the mechanical properties of a silty sand treated with XG, cement, and their combinations under varying moisture states and durability conditions. Tests such as compaction, unconfined compressive strength, and wet–dry durability were conducted. Results revealed decreasing dry densities with increasing XG content, and XG-only treatments exhibited negligible strength with swelling on 24 h of water immersion. Conversely, cement–XG (3% cement + 0.5%–1.5% XG) treatments showed substantial wet strength and sustained six wet–dry durability cycles – exceeding XG-only (0.5%–1.5%) and cement treatments (3%). Fourier transform infrared spectroscopy indicated ionic-cross-linking interactions among biopolymer and Ca2+ ions, and X-ray diffraction analysis evidenced cementitious phase formations. Morphological insights into the strengthening mechanisms through field emission scanning electron spectroscopy exhibited distinct XG gel features due to cement incorporation. Overall, the findings demonstrate cement–XG combinations offering better moisture resistance compared to XG-only treatments, potentially presenting a viable soil treatment strategy for applications prioritising environmental sustainability.
Internal erosion induced by hydraulic loading poses a major threat to the long-term stability of subgrades constructed with clayey gravelly fills. Progressive migration and loss of fines continuously reconfigure the internal contact network and force transmission, making the associated strength degradation difficult to interpret solely from macroscopic responses. This study aims to clarify the coupled macro-micro evolution of clayey gravel during internal erosion under different confining pressures. Discrete element method simulations are conducted by progressively removing fine particles and quantifying coordination numbers, contact topology, force-chain characteristics, and the directional statistics of normal and tangential contact forces. The results indicate that higher confining pressure promotes compaction and interlocking and thereby forms denser, more continuous force-chain networks, whereas internal erosion shifts the load-bearing skeleton from fine-involved contacts to coarse-coarse contacts with reduced total contact forces but nearly unchanged mean per-contact forces and a more pronounced anisotropy in the coarse-coarse network, providing a micromechanical basis for subgrade stability assessment and erosion-resistant design.
Expanding the use of recycled construction sludge soils requires verification of workability, mechanical performance, and environmental compatibility, particularly the mitigation of high alkalinity caused by stabilisation. Carbon dioxide curing promotes neutralisation and carbon dioxide sequestration through carbonate formation; however, the influence of curing timing within the production process remains insufficiently understood. This study investigates how the timing of carbon dioxide curing affects the physicochemical and mechanical properties of recycled soils. Ao clay adjusted to 1.25 times its liquid limit was stabilised using either 10% blast furnace slag cement type B or 25% paper sludge ash-based stabiliser. Carbon dioxide curing was applied at 0, 7, or 13 days after mixing, or not applied, under controlled conditions with a total curing period of 14 days. The pH, carbonate content, particle crushing strength, cone index, and consolidated undrained triaxial behaviour were evaluated. Carbon dioxide curing effectively reduced pH and enhanced carbon dioxide sequestration in both soils, although partial pH rebound occurred during subsequent sealed curing, particularly when curing was introduced at an earlier stage. Mechanical responses varied depending on the stabiliser type and curing timing, with notable strength reductions observed primarily in the cement-stabilised soil. These findings highlight the importance of optimising carbon dioxide curing timing to balance environmental benefits and mechanical performance in recycled soil production.
The frequent occurrence of dry spells, heatwaves, and water scarcity takes a toll on green infrastructure, particularly in arid and semi-arid regions. Water-absorbing polymers (WAPs) have the potential to minimise the effects of water stress on plant growth. It can absorb and store large amounts of water and release it when water is unavailable in the soil matrix. This study conducted experimental and numerical research on a novel water-absorbing polymer derived from waste-generated fly ash, aiming to improve the drought resistance of vegetated soils, with a focus on root water uptake () dynamics. The fundamental goal of this research is (1) to establish the efficacy of WAP-amended soil on grass species under water stress conditions and (2) to estimate the from soil under the effect of . The results demonstrate that the improved water retention behaviour by altering water storage in the soil matrix. The water retention characteristics, such as plant-available water content and wilting time, were improved by more than 2 times and 1.2 times, respectively, compared to the control soil. Furthermore, it is observed that WAP-amended soil has higher from the soil matrix. This study suggests that can be utilised for a variety of applications, including green infrastructure and agricultural practices.
To optimise the utilisation of cementation solution and reinforcement effects in clay bio-bricks, this study compared four biological treatment methods: surface alternate infiltration bacterial suspension, surface first infiltration bacterial suspension, surface infiltration combined mixing, and pre-mixing. The performance of these methods was evaluated based on three key metrics: compressive strength, calcium carbonate content, and cementation solution utilisation efficiency. The pre-mixing method exhibited the highest measured compressive strength under the tested conditions. In addition, pre-mixing achieved the highest calcium carbonate content, reached 9.91%. It indicates that pre-mixing promotes a more uniform bacterial distribution within the bio-bricks and maximises utilisation of the cementation solution, reaching 69.6%. In contrast, the surface alternate infiltration method demonstrated the weakest performance, producing only of the calcium carbonate content at 6.93%, with minimal reinforcement and the lowest utilisation rate of only 47.9%. This deficiency was attributed to rapid surface crystal formation, which blocked solution penetration into the interior. Furthermore, compressive strength was greatest when the cementation solution concentration was 0.5 mol/L. Both macroscopic and microscopic analyses indicate that the mechanical performance of clay bio-bricks is closely linked to calcium carbonate content, underscoring the importance of optimising bacterial distribution and solution efficiency.
This study investigates the effects of waste hemp stalks on expansive clay soils. Hemp, a biodegradable and sustainable material, was used at 5%, 7.5%, and 10% by dry weight to stabilise a clay mixture (70% kaolin, 30% bentonite). Laboratory tests, including Atterberg limits, compaction, and consolidation, were conducted in accordance with standards. Results show that hemp additives significantly reduced liquid limit, plasticity index, swelling percentage, and swelling pressure. The optimal performance was achieved with 10% T1 hemp, reducing swelling from 25% to 0.10% and swelling pressure from 40.64 kPa to 5.08 kPa. These findings demonstrate the potential of waste hemp as an alternative bio-based stabilisation material to traditional stabilisers. A comparative analysis with cement- and lime-based stabilisers highlights hemp's advantages in reducing costs and mitigating carbon footprints.
The final experiment (full canister test, FCT) of the large-scale gas injection test (LASGIT) aimed to explore the impact of gas volume on gas transport behaviour. Unlike previous tests, the FCT involved pressurising a full-scale KBS-3 canister up to ∼7100 kPa when pressure was held, allowing excess water to drain and to establish gas entry. Once drained, pressure gradually decreased by 220 kPa, indicating gas moving into the fully saturated clay buffer. Observations showed different pore pressure and total stress behaviour compared with earlier tests, but no major changes were seen in the buffer’s response, suggesting gas migration was unaffected by gas volume. The slow pressure decay shows gas travelled through a limited number of narrow pathways, which were not formed through tensile fracturing. A gas leak early in the FCT led to depressurisation and later pressurisation of the canister, causing a 50 μm expansion in its radius. This mechanical loading on the buffer, greater than the bentonite’s drainage capacity, caused pore pressure and radial stress changes. The expansion of the canister by pressurisation or thermal effects should therefore be considered in performance assessments.
Gas generation is an inevitable consequence of radioactive waste storage and disposal. The rate at which gas diffuses through host rocks and overlying strata is an important consideration in safety assessment. To examine the impact of material variability, British Geological Survey developed a methodology to manufacture 'synthetic rock' samples from mixtures of clay, sand, and silt. Diffusion experiments were conducted on these, and natural samples of Boom Clay and Eigenbilzen Sands. Samples were tested under an isotropic stress equivalent to 400 m burial, assessing anisotropy by measuring intrinsic permeability and diffusion normal and perpendicular to bedding. Boom Clay exhibited permeability and gas diffusion anisotropy ratios of 4.5 and 1.5, respectively. A semi-log relationship between permeability and diffusivity was observed. No significant correlation to mineralogy changes was found, indicating fabric and pore morphology are more important. This was supported by a correlation between porosity and diffusivity. X-ray computed tomography analysis suggested diffusivity was inversely proportional to bioturbation features and that stress promoted micro-crack closure which may also help to explain previous data dispersion. The study confirmed the usefulness of synthetic samples in exploring complex processes, generated data relevant to natural clay systems and showed diffusivity was not highly sensitive to large changes in permeability.
This study investigates the adsorption performance and characterisation of mesoporous silica-modified Bangkok clay (BKC) as a geosynthetic clay liner (GCL) for removal of heavy metal in aqueous solution. BKC was modified with mesoporous SBA-15 to create a mesoporous silica-coated clay (5SBS), enhancing its surface area, porosity, and adsorption efficiency. The materials were characterized using Fourier-transform infrared spectroscopy, scanning electron microscopy-energy-dispersive X-ray spectroscopy, X-ray diffraction, Brunauer-Emmett-Teller (BET), and X-ray photoelectron spectroscopy techniques while adsorption experiments of Cu(II), Zn(II), and Cd(II) ions under controlled conditions in ternary systems. The 5SBS composite exhibited superior physicochemical characteristics, including a BET surface area of 67.45 m(2)/g and well-distributed mesopores. Adsorption kinetics followed a pseudo-second-order model, indicating chemisorption as the dominant mechanism. Equilibrium isotherm data fit best with the Langmuir and Sips models, suggesting monolayer adsorption on homogenous surfaces. The maximum uptake capacities for 5SBS were 31.74, 17.96, and 14.26 mg/g for Cu(II), Zn(II), and Cd(II), respectively, outperforming unmodified BKC and closely matching bentonite. Enhanced thermal stability and minimal pore structure degradation post-adsorption confirmed its suitability for harsh environmental conditions. Metal adsorption has mainly occurred at the surface of the mesoporous silica-modified clay by bonding with surface functional groups. Hydraulic conductivity results further indicate that SBA-15 modification effectively reduces permeability and chemical sensitivity of BKC, maintaining performance comparable to bentonite through stable pore-blocking mechanisms. These findings highlight 5SBS as sustainable alternative to bentonite in GCL, with potential implications for contaminant protection.
Biogeotechnics provides a sustainable engineering alternative for soil reinforcement, yet the long-term durability of microbially induced calcium carbonate precipitation () in granite residual soil remains insufficiently understood. This study investigates the influence of wetting-drying (W-D) cycles and curing times on the mechanical behaviour and microstructural evolution of MICP-treated granite residual soil through integrated mechanical, mineralogical, and pore-scale analyses. The results show that W-D cycles progressively reduce strength, with unconfined compressive strength () and peak deviatoric stress decreasing by 10.3% and 12.9% every 4 cycles, respectively, while the secant and shear moduli undergo the most severe deterioration. Nevertheless, after 20 cycles, the treated specimens still exhibited significantly higher and peak deviatoric stress than the untreated soil. Longer curing substantially improves durability, with strength tending to stabilise after 9-12 days due to sustained calcite precipitation, which strengthens interparticle bonding and improves resistance to W-D-induced degradation. Microstructural characterisation suggests that pore enlargement, calcite dissolution, and feldspar disintegration are major contributors to cementation loss and strength decline. These findings help clarify the degradation mechanisms of biocemented granite residual soil and provide a useful reference for the design and long-term performance assessment of MICP-based soil improvement strategies from a sustainable engineering perspective.
The Gaoligongshan Tunnel in Yunnan Province, China, serves as a representative example of a high ground-temperature and high-altitude tunnel. The tunnel's construction through four heat-water conduction zones presented significant safety risks, which formed the primary motivation for this research. Correlation and regression analyses between on-site measured wet-bulb globe temperature (WBGT) values and environmental parameters, including air temperature, atmospheric pressure, wind speed, and relative humidity, reveal that air temperature, atmospheric pressure, and relative humidity exhibit significant correlations with . Based on Pearson correlation coefficients, the degree of correlation ranks in descending order as follows: air temperature > relative humidity > atmospheric pressure. A method utilising a Sparrow search algorithm (SSA)-BP neural network is proposed for the rapid and accurate prediction of values from conventional tunnel environmental parameters. This approach facilitates thermal environment assessment based on WBGT, supporting the development of a three-dimensional '3-D' tunnel cooling system. Compared with predictions from a standardised regression equation, the SSA-BP neural network improves the R-2 value between predicted and measured by 7.7%. Technical measures such as ventilation, cryocoolers, and ice cooling are demonstrated to reduce to approximate to 26 degrees C, thereby effectively improving the tunnel thermal environment.
To examine the hydraulic effect of vegetation, soil column experiments were used to investigate how alfalfa (Medicago sativa L.), the dominant grass species on the Chinese Loess Plateau, affects water content and suction dynamics of loess under rainfall and drought conditions, with emphasis on planting density and soil compaction. The results suggested that under rainfall conditions, the effect of planting density on water content and suction dynamics varied depending on soil compaction. During drought, water content in vegetated soils generally decreased with planting density, due to the increased water uptake by roots under high planting densities; as plant growth progressed, the differences caused by varying planting density diminished. Soil compaction generally positively correlated with water content, owing to the coupled effects of plant roots and soil compaction. In addition, numerical models were developed using Hydrus-2D to simulate water migration in soil columns and validated with experimental data. The close agreement between predicted and measured water content values demonstrated the feasibility of using Hydrus-2D to derive soil hydraulic properties through inverse modelling and to simulate water dynamics in vegetated soils.
Conventional stabilisation with lime or cement enhances soil performance but entails significant environmental costs. This study explored three widely available waste materials, steel slag (SS), construction and demolition waste (CDW) and eggshells (ES) as non-conventional binders. These materials were selected for their comparable chemical or physical properties to those of conventional binders and for their abundance, low cost and their capacity to reduce the environmental impact of landfilling through effective reuse. Experimental results demonstrate that a 3% addition of ES increased unconfined compressive strength (UCS) to values exceeding 4000 kPa after 56 days of curing, outperforming the reference lime-stabilised sample (2000 kPa). Likewise, the corresponding secant modulus (Es) reached 254.9 MPa with ES, compared with 187.7 MPa for lime. For granular additions, 30% SS yielded UCS values above 1500 kPa, while 24% CDW achieved over 500 kPa. Microstructural analyses confirmed that hydration and pozzolanic reactions are responsible for the improvement. Overall, the stabilised samples exhibited a mechanical performance comparable to that of conventional binders, which would promote the reuse of substantial waste streams and contribute to reductions in greenhouse gas emissions. These findings indicate a viable pathway towards more sustainable ground improvement practices within the framework of a circular economy.
The escalating accumulation of industrial solid wastes and carbon-intensive cement production has intensified climate change and extreme weather events, driving the demand for sustainable construction materials that synergise waste utilisation with carbon sequestration. This study systematically investigates the effects of magnesium oxide (MgO) content, durian shell biochar (DSB) dosage, and its modification on the compressive strength, carbon dioxide (CO2) capture and mineralisation performance, phase evolution, and microstructure of copper slag-granulated ground blast furnace slag (CS-GGBS) based geopolymer composites. The results reveal that 5% magnesium oxide incorporation in CS-GGBS geopolymers facilitates magnesium silicate formation, while optimal biochar addition enhances both geopolymerisation and carbon dioxide capture. Among them, the alkaline-modified biochar with larger pores (45.9678 nm) facilitates polymer gel encapsulation of unreacted particles, improving structural integrity. Co-grinding magnesium oxide with biochar enhances early strength and preparation-stage carbonation. Furthermore, carbon dioxide mineralisation predominantly occurs within the matrix during preparation and curing, while surface deposition dominates during carbon dioxide exposure, filling microstructural defects that become interwoven with the geopolymer network. The CS-GGBS composite with 5% magnesium oxide and 1.5% alkaline-modified biochar (24-h treatment) determined as optimal mixture, exhibiting 40.2 MPa compressive strength, 0.88981 mg/g carbon dioxide capture, and 4.25 Wt.% of mineralisation potential at least.
Rainfall-induced slope failures pose a growing threat to infrastructure, especially in clay-rich embankments. Previous studies have advanced understanding through field monitoring, numerical modelling, and physical testing; however, many approaches lack generalisability and often fail to capture the coupled hydraulic-mechanical interactions in unsaturated soils under varying rainfall conditions. Predictive frameworks also tend to rely on large or site-specific datasets, limiting their practical use for rapid assessment. This study presents a proof-of-concept experimental-predictive framework to evaluate the hydromechanical response of compacted unsaturated slopes under varying rainfall intensities. Using small-scale flume tests on sand-clay mixtures, rainfall intensities ranging from 20 to 120 mm/h were applied over a 120-min period to investigate time-dependent changes in volumetric water content and crest settlement. Results revealed critical thresholds between 85 and 90 mm/h for the test material, beyond which infiltration accelerated and deformation intensified. Seven regression-based machine learning models, namely, support vector regression, K-nearest neighbours, random forest, gradient boosting, decision tree, and polynomial regression (degrees two and three), were employed to forecast crest settlement based on rainfall intensity and soil water content. These models demonstrated strong predictive capabilities (R2 up to 0.987), effectively capturing the coupled hydraulic-mechanical behaviour of unsaturated slopes. The study contributes to ongoing efforts by quantifying rainfall thresholds and demonstrating the potential of interpretable, data-driven models for practical applications.
