The traditional methods for determining the soil-water characteristic curve (SWCC) in the full suction range always takes several months. To better guide the engineering practice with the theory of unsaturated soil mechanics, the rapid determination of SWCC is particularly important. In this study, a rapid measurement method for SWCC in the wide (full) suction range was developed with the combination of pressure plate method, parallel filter paper method and dew point water potential meter method, and the proposed method was used to test two different soils. Results showed that: i) The experimental data of the parallel filter paper method and that of the serial filter paper method were in good agreement, so the parallel filter paper method can be used instead of the serial filter paper method to shorten the measurement time. ii) When the measured data meet the general shape of SWCC (or meet the requirements of engineering design), the number of tests can be reduced appropriately to improve the measurement efficiency. A complete SWCC can be determined by the proposed combined method with only 10 to 12 points. iii) The measurement method proposed in this study can shorten the measurement time of SWCC testing from several months to 7–10 days. The combined method proposed in this study achieved the rapid and accurate determination of SWCC in wide (full) suction ranges, which is expected to make the SWCC measurement become a routine geotechnical test.

In recent years, the remediation of heavy metal contaminated soil has become a hot topic, due to the rapid development of industry and technology. In this study, MICP technology combined with adsorption materials was used to repair the zinc-lead composite heavy metal contaminated soil. Through unconfined compressive strength test and toxicity leaching test, the solidification effect of contaminated soil and the stabilization effect of heavy metals before and after treatment were evaluated. Combined with scanning electron microscope (SEM) and X ray diffraction (XRD) detection methods, the repairing mechanism of zinc-lead heavy metal contaminated soil treated by MICP technology was revealed. The results showed that the solidification/stabilization of Zn-Pb heavy metal contaminated soil by MICP technology effectively reduced the leaching of harmful heavy metals from contaminated soil. When the mineralization time was 10 d, the unconfined compressive strength of the sample was 942.5 kPa. The leaching concentration of lead was 4.20 mg/L, which was 44.81% lower than that of the untreated sample. The leaching concentration of zinc was 4.31 mg/L, which was 46.19% lower than that without treatment. On this basis, by adding 10% porous silicon adsorption material, the unconfined compressive strength of the sample could reach 1 021 kPa, and the strength was increased by 8.3%. The leaching concentration of lead was 2.45 mg/L, which was 67.81% lower than that without treatment. Compared with the MICP method alone, the leaching concentration of lead was reduced by 41.67%. The leaching concentration of zinc was only 2.93 mg/L, which decreased by 63.4% compared with that without treatment. Compared with the MICP method alone, the leaching concentration of zinc was reduced by 31.9%. The addition of porous silicon adsorption material significantly improved the remediation effect of MICP technology on zinc-lead composite heavy metal contaminated soil. Due to the immobilization and adsorption of heavy metal ions by porous silicon adsorption material, the leaching concentration of heavy metals in contaminated soil decreased. At the same time, the adsorption material can also be used as the host site for calcium carbonate crystal deposition to accelerate the mineralization reaction. This study provides a new technology for the treatment of heavy metal contaminated soil and reveals its remediation mechanism, which provides a theoretical and experimental basis for the application of MICP technology combined with porous silicon adsorption materials to remediate Zn-Pb composite heavy metal contaminated soil.

Clayey rock is considered to be a reasonable geological barrier for high-level radioactive waste disposal. The research work is carried out from the aspects of both short- and long-term mechanical properties of clayey rock in the present work. Standard natural oedometer tests are carried out and basic physical and mechanical parameters such as pre-consolidation stress and compressibility coefficient are obtained. The hydro-mechanical coupling consolidation test shows that the saline has an important influence on the mechanical properties of clayey rock, and the swelling phenomenon in the hydro-mechanical coupling process is closely related to the content and type of clay minerals in clayey rock. Long-term hydro-mechanical coupled behavior of a clayey rock is studied by consolidation creep tests and the test results highlighted the creep potential of this clayey rock. Based on the test results, a creep constitutive model is established to describe the consolidated creep behavior of the clayey rock. The agreement of calculation results with the test data indicates that the model can well reflect one-demensional consolidation and reological properties of clayey rock. The research could provide significant guidance for the planning, design, site selection, and operation of the high-level radioactive waste repository built in clayey rock in China in the future.

The treatment of marine sediment has been a global-scale challenge. Portland cement (PC) is widely-used binder in the conventional stabilization/solidification method. Use of PC can cause serious environmental pollution. In this context, the environment-friendly binder (blend of quicklime and ground granulated blast-furnace slag (GGBS)) has been adopted to replace PC in the soil remediation field. This study investigated the quicklime-activated GGBS for the stabilization of marine sediment at high water content. The physicochemical and unconfined compression measurements were performed to analyze the physical, chemical, and strength characteristics of the quicklime-GGBS stabilized sediments. The results were compared with that of PC-stabilized sediment. As compared to the PC-stabilized sediment, the quicklime-GGBS stabilized sediment would generate larger volume shrinkage, lower water content, and slightly higher density. With reducing quicklime proportion and continuing curing time, the pH of the quicklime-GGBS stabilized sediment gradually decreases. The unconfined compressive strength of the lime-GGBS stabilized sediment shows a trend of first increasing (quicklime-binder ratio of 0.05–0.15) and then decreasing (0.15–0.3) and finally increasing again (0.3–0.4). The maximum strengths appear at the quicklime-binder ratios of 0.15 and 0.4. The maximum strength at the quicklime-binder ratio of 0.15 is 1.4 times than the corresponding PC-stabilized sediment under the same condition. The findings indicate that the combination of GGBS with little quicklime has the potential to replace PC for stabilizing natural sediment at high water content.

The mobility of nano zero-valent iron (nZVI) will greatly affect its potential application as a remediation material for polluted groundwater. One-dimensional (1D) column tests are commonly used in previous works to study the transport behavior of nZVI. However, the reports about the two-dimensional (2D) transport behavior of nZVI are still very limited. In this study, model test systems were developed to simulate the transport and retention of nanoparticles in porous media. Glass beads of different sizes (fine, medium and coarse) were used to simulate the porous media. The motion behavior of phosphate-sorbed nZVI (PS-nZVI) in porous media was obtained by sampling and image analysis. Results of 1D column tests show that PS-nZVI has certain mobility in medium and coarse glass beads with liquid-phase recoveries equal to 50.3% and 41.0%, but has poor mobility in fine glass beads. Results of 2D model tests show that the retention of PS-nZVI in medium glass beads decreases with the distance, while the retention in coarse glass beads increases first and then decreases with the distance. Results show that the transport and retention process of PS-nZVI is affected by both the particle size of porous media and the flow velocity. The combined influence of clogging and surface deposition leads to the different transport and retention behaviors of PS-nZVI in glass beads of different sizes. The results of this study can be used to evaluate the mobility of PS-nZVI and analyze its retention mechanism in porous media, and provide a reference for engineering application and environmental risk assessment of nZVI technology.

Soil cementation technology based on microbially or enzyme induced calcium carbonate precipitation (MICP/EICP) is one of the hot topics in the field of geotechnical and geological engineering in recent years. In this study, a systematic review was performed on this technology, focusing on advances in the cementation mechanisms of MICP/EICP and the influence of soil pore structures, properties of bacterium, urease and cementation solution, and cementation methods on characteristics of calcium carbonate (CaCO3). The results indicate that the smaller the soil pores are, the more difficult the infiltration of microorganisms or urease is and the worse the cementation uniformity is. More contact points among soil particles will produce more deposition points for CaCO3, resulting in stronger bonding and bridging effects and better cementation effects. The generation rate and total amount of CaCO3 increase as the concentration of bacterium or urease and the activity of urease increase in a certain range. However, too high concentration or activity will induce a too high generation rate of CaCO3, resulting in clogging near the injection end. The calcium carbonate crystals obtained from low concentration cementation solution are relatively small and evenly distributed in the soil. The use of appropriate grouting saturation can increase the proportion of CaCO3 with bonding effect. Multilayer alternating injection or one-phase low pH injection can improve the distribution uniformity of CaCO3 in the sample. Based on influencing factors of CaCO3 precipitation characteristics, the improvement of cemented soil uniformity, durability verification, and the adaptability and improvement scheme of applying the laboratory test results to the field scale should be the focus points of the future research.

The laterite may form cracks due to shrinkage for dehydration. These cracks not only reduce the overall strength, but also provide infiltration channel for rainwater, which intensifies the weakening of its bearing capacity. Therefore, how to inhibit laterite shrinkage is a key problem for engineering applications. The nano-silica particles are extremely fine in size and belong to the nano category. It is proposed to fully utilize the size advantage of nano-silica, so that nano-silica particles can enter the laterite aggregates and resist the shrinkage behavior of laterite caused by dehydration. Therefore, different dry mixing ratios (i.e. nano-silica: laterite = 0: 100, 2: 100, 3.5: 100, 5: 100, and 6.5: 100) were programmed, and nano-silica was mixed with laterite for compaction (dry density is 1.44 g/cm3 and 1.46 g/cm3 respectively). Shrinkage characteristics and pore distribution of compacted laterite-nano-silica mixture were compared. It was found that nano-silica could inhibit the shrinkage of laterite, and the shrinkage limit was also increased by the mixing ratio rising. Besides, plenty of nano-silica particles were found in the pores while the mixing ratio was greater than 5% by means of apparent morphology images. Meanwhile, the pore distribution curve also showed that the pores with diameter at 0.03 ?m reduced significantly, which indicated that nano-silica mainly filled the pores with diameter greater than 0.03 ?m. Adding nano-silica into laterite is a physical method to improve the shrinkage properties, which is different from the chemical methods such as lime treatment and has potential advantages to environmental protection.

Textured geomembrane (GMX) and non-woven geotextile (GT) are important components of the liner system in MSW landfills. GMX-GT interface characteristics are sensitive to the stability of landfills. However, the dynamic behavior of the GMX-GT interface is not well understood. A series of cyclic direct shear tests was conducted on the GMX-GT interface under dry and fully saturated conditions. The effects of vertical stress, displacement amplitude, and cycle times on the dynamic behavior of the GMX-GT interface were studied. The dynamic behaviors of the GMX-GT interface under dry and fully saturated conditions were compared. The results demonstrated that the GMX-GT interface changed from shear hardening to shear softening with the increase of displacement amplitude. Meanwhile, the internal friction angle of the GMX-GT interface increased with the increase of displacement amplitude due to the cyclic shear. The GMX-GT interface was mainly characterized by shear shrinkage. The total shear shrinkage increased with the increase of vertical stress, displacement amplitude and cycle times. The shear stiffness increased with the increase of vertical stress and cycle times, while decreased with the increase of displacement amplitude. The damping ratio increased with the increase of displacement amplitude and decreased with the increase of cycle times, indicating that the displacement amplitude increased the energy dissipation of GMX-GT interface. The failure pattern was obvious different under the two conditions. The internal failure of GT was more significant under dry conditions, and the surface failure of GMX was more obvious under fully saturated conditions.

In order to study the effect of supercritical carbon dioxide (ScCO2) on the mechanical properties of granite located in the CO2 based enhanced geothermal system (EGS) region, fluid-rock interaction experiments were conducted at 210 ℃, 240 ℃ and 270 ℃. Three experimental conditions were used: i) ScCO2 and dry granite; ii) ScCO2, water vapor and dry granite; iii) ScCO2 and granite that is soaked in water for 24 hours. The P-wave velocities, uniaxial comprehensive strength, and elastic modulus of all these ScCO2 treated granite samples and one untreated granite sample were obtained by carrying out the wave velocity tests and uniaxial compressive tests. Wave velocity tests show that the P-wave velocities of all these ScCO2 treated granite samples are reduced compared to that of the untreated sample. Uniaxial compressive test show that the uniaxial comprehensive strength and elastic modulus are not affected. From the failure mode, it can be seen that the untreated granite more likely present brittle tensile failure, while the treated sample shows more likely shear failure. As the temperature increases, the failure mode becomes more and more close to shear failure. Experimental results show that the ScCO2 has very slight damage impact on granite that has no water or little water, causing a slight decrease in the brittleness, and a small increase on the plasticity. The P-wave velocity decreases slightly and the damage to the granite strength can be negligible. Therefore, the interaction of CO2-rock will not cause obvious effect on the mechanical properties of granite located in and nearby the CO2-EGS region.

The diffusion path of grout in porous media has a significant effect on the diffusion range and grouting effect. Based on the fractal characteristics and the seepage motion equation of Bingham fluid in porous media, the penetration grouting mechanism of Bingham fluid considering the diffusion path was revealed through theoretical analysis, and verified by the penetration grouting test carried out by the team in the previous studies. The effects of porosity of porous media, water-cement ratio of Bingham cement slurry, permeability coefficient of porous media, grouting pressure and groundwater pressure on diffusion radius are analyzed. In addition, a 3D numerical simulation program of penetration grouting mechanism of Bingham fluid considering the diffusion path was developed through Comsol Multiphysics platform. Then the penetration and diffusion morphology effect of Bingham cement grout in porous media was investigated by the developed simulation program. The results show that the theoretical calculation value of diffusion radius obtained by using the Bingham fluid penetration grouting mechanism considering the diffusion path is closer to the experimental value than that obtained by the spherical diffusion formula of Bingham fluid penetration grouting without considering the diffusion path. The results can provide theoretical support for practical grouting engineering.

In order to improve the strengthening effect of microbially induced carbonate precipitation (MICP) technology on calcareous sand in the marine environment, on the basis of previous studies, the multi-gradient artificial domestication culture test of Sporosarcina pasteurii in artificial seawater environment was designed and carried out. Combined with the mechanical test and micro-mesostructural analysis of MICP strengthened calcareous sand column, the domestication effect of Sporosarcina pasteurii was comprehensively evaluated. The results showed that: i) The bacterial liquid concentration after five-gradient domestication in seawater environment could reach more than 97% of that in freshwater environment, and the production of carbonate after the interaction with cementing fluid was increased to a certain extent compared with that in freshwater environment. ii) The domesticated Sporosarcina pasteurii had good temperature adaptability, and it had good MICP performance at 10 ℃ to 30 ℃. iii) Both carbonate production and unconfined compressive strength of calcareous sand columns strengthened in seawater environment were higher than those before domestication, especially the bacteria after five-gradient domestication, and the bacteria after domestication became smaller. The carbonate crystals (calcium carbonate and magnesium carbonate) generated in the seawater environment were smaller and denser, which could better fill the pores of calcareous sand particles and cement the adjacent calcareous sand particles, and had better MICP performance. The relevant research ideas and methods can provide reference for the research and application of MICP technology in the reinforcement of calcareous sand foundation in seawater environment.

Most previous studies focused on establishing the relationship between pore compression sensitivity and permeability at the macroscopic level. However, the effect of different pore-structures at multi-scales on the evolution of permeability characteristics is still not clear. Also, many mesoscopic studies have showed that the closure degree of pores at multi-scales are significantly different under the action of stress. Thus, the pore compression sensitivity at different scales is crucial to predict the evolution of permeability accurately. In this study, the hydro-mechanical experiments of sandstone were performed using a multi-field coupling NMR experimental system to obtain the variations of permeability and pore-size distribution under different stress conditions. The pore diameter is divided into three categories: large pore (>1 ?m), middle pore (0.1–1 ?m), and small pore (<0.1 ?m). The pore compressibility coefficients at different scales and transforming factors were calculated. A calculation formula for sandstone permeability considering pore compression sensitivity at different scales was also proposed. The results show that the compressibility of pores at different scales of sandstone is significantly different. The larger the pores are, the higher the compression sensitivity is. The proposed permeability formula considering the pore compression sensitivity at different scales are in good agreement with the experimental results.

The quantitative evaluation of the settlement characteristics of landfills by aerobic degradation is an important basis for predicting and evaluating the stabilization of aerobic degradation. The settlement characteristics of man-made waste soil with typical household waste composition ratio in Wuhan were tested under different ventilation conditions, and the influence of aerobic ventilation frequency on settlement deformation was analyzed. The results showed that aerobic ventilation significantly increased the settlement rate of municipal solid waste under anaerobic condition when the settlement rate was stable. The settlement efficiency of the upper, middle and lower layers of waste soil at high ventilation frequency was increased by 145%, 150% and 100% respectively, compared with the settlement efficiency at low ventilation frequency. Also, an anaerobic-aerobic combined settlement model (ANACS model) was established, and the reliability of the model was verified by comparing against the settlement values obtained from experimental monitoring. Outcomes from this study provide a theoretical basis for the evaluation of the settlement stability of waste soil in aerobic ventilation process.

Analytical solutions of contaminant transport in a compacted clay liner (CCL) under non-isothermal condition are derived through the generalized integral transform technique. The proposed analytical solutions account for the coupling effects of molecular diffusion, advection, sorption and thermal diffusion. In addition, the variations of CCL permeability coefficient, CCL distribution coefficient and CCL effective diffusion coefficient with temperature are also considered in the analytical solutions. The proposed analytical solutions are successfully validated against the experimental results of thermal diffusion tests, the analytical solutions available in previous studies and COMSOL numerical results, respectively. Then the effects of non-isothermal condition, permeability coefficient, effective diffusion coefficient and distribution coefficient of CCL on the transport of benzene in CCL system were studied through the verified analytical solutions. The results indicate that the non-isothermal condition and the variations of permeability coefficient, effective diffusion coefficient and distribution coefficient of CCL with temperature all have significant effects on benzene transport in the CCL system. The mass flux and breakthrough time of contaminant can be substantially underestimated without considering the effect of non-isothermal condition. The previous analytical solutions that neglect the variations of CCL permeability coefficient and CCL effective diffusion coefficient with temperature can substantially underestimate the benzene outflow rate, whereas neglecting the variation of CCL distribution coefficient with temperature can substantially overestimate the cost time of benzene to penetrate the liner system and reach steady-state. The proposed analytical solutions that consider the effects of thermal diffusion as well as the variations of CCL permeability coefficient, CCL distribution coefficient and CCL effective diffusion coefficient with temperature are more suitable for the engineering practice. It can be adopted to improve the design scheme and the service performance evaluation of CCL system.

The compaction degree is one of the main factors affecting the geo-environmental properties of arsenic (As) and antimony (Sb) co-contaminated soil stabilized by ferric salts. The effect of compaction degree on the geo-environmental properties of As and Sb co-contaminated soil stabilized by a ferric salt-based stabilizer (PFSC, polymerized ferrous sulfate-Ca(OH)2) was investigated, including unconfined compressive strength (UCS), leached concentrations of As and Sb, and hydraulic conductivity . The varied characteristics of the micro pores and the element valence in the stabilized soil with compaction degree were clarified by adopting industrial CT scanning and X-ray photoelectron spectroscopy (XPS) in the study. The leached concentration of As decreased first and then increased with the increase of compaction degree, and reached the lowest as the compaction degree was 93%. The leached concentration of Sb decreased with the increase of compaction degree, while remained constant until the compaction degree was larger than 85%. When the compaction degree increased from 75% to 96%, the UCS of the stabilized soil increased from 4.26 kPa to 43.78 kPa. As the compaction degree increased from 80% to 96%, the of the stabilized soil decreased from 1.33×10–7 m/s to 2.81×10–9 m/s. In addition, it can be observed from the industrial CT results that the porosity of stabilized soil decreased from 7.54% to 5.30% with the increase of compaction degree, hence leading to the more compactness structures of the soil. The XPS analysis of the As, Sb and Fe indicated that increasing the compaction degree of stabilized soil promoted the transformation of As(V), Sb(V), and Fe(III) to As(III), Sb(III), and Fe(II), respectively. The study mainly focused on revealing the effects of compaction degree on the geo-environmental properties of As and Sb co-contaminated soil stabilized by PFSC, which will provide a theoretical basis for the engineering application and the optimization for the operation parameters of PFSC-stabilized As and Sb co-contaminated soil.

Taking the lateritic soil collected from Miaoling town, Ezhou city, Hubei province as the study object, the free iron oxide in the undisturbed clay was removed by chemical selective dissolution combined with leaching as the treatment method in gradient, and the relationship between the drenching time of dithionite-citrate-bicarbonate (DCB) solution and the iron removal rate was achieved, and the effects of different leaching times on the physical, mechanical properties and the microscopic pore structure of the lateritic soil were analyzed. The experimental results show that the leaching time of DCB solution has a strong correlation with the iron removal rate, and the iron removal rate first increases rapidly and then gradually stabilizes with the time. Since the free iron oxide primarily plays the role of cementation between lateritic soil, the drenching time has a great influence on the physical and mechanical properties of lateritic soil. With the increase of drenching time, the contents of clay particles and colloidal particles gradually increase, the heat resistance increases marginally, and the unconfined compressive strength decreases significantly, while the overall trend of unconfined compressive strength decreases sharply in the early stage and tends to be stable in the later stage. Results of nuclear magnetic resonance (NMR), differential thermal analysis (DTA) and scanning electron microscopy (SEM) reveal that as the leaching time increases, the internal pores of the Miaoling lateritic soil enlarge, the free water diminishes, the bound water increases, the microscopic morphology of the agglomerate structure is destroyed, the cemented material is significantly reduced, and the structural form gradually converts from a compact granule stacking structure to an aggregate-loose granular structure.

In order to evaluate the antifouling performance of the composite cut-off wall composed of geomembrane and soil- bentonite to the organic pollutants, a one-dimensional transient diffusion model was established to describe the diffusion behavior of pollutants through the composite cut-off wall when the pollutants were degraded in the source region. The solution of the analytical model was calculated by the Laplace transform and the Talbot numerical inversion. When the effects of pollutant diffusion and degradation were considered under the 3rd type inlet boundary condition, the 100-year breakthrough concentration of the type I (geomembrane/soil-bentonite) and type II (soil-bentonite/geomembrane/soil-bentonite) composite cut-off walls decreased by 59% and 53% than that under the 1st type inlet boundary condition, respectively. Since the permeability coefficient of geomembrane was assumed to be higher than 10–12 m/s, the convection of pollutants in the composite cut-off wall cannot be ignored. Thus, the type II composite cut-off wall enabled the soil-bentonite to play a dominant role in isolating the organic pollutants better. By reducing the permeability coefficient of geomembrane from 10–10 m/s to 10–16 m/s, the breakthrough time of type I composite cut-off wall was increased from 26 years to 188 years, and that of type II composite cut-off wall was increased from 32 years to 81 years, correspondingly. The antifouling performance of the composite cut-off wall can be effectively increased by adopting the methods, such as adjusting the negative head loss inside and outside the wall by a pumping well, and increasing the degradation capacity of pollutants in the source region.

As the anchoring foundation of the tension leg platform (TLP), suction caisson foundation is not only subjected to the vertical pullout load but also subjected to the cyclic load. However, there are few studies on the mechanism of the cyclic characteristics of soft clay under unloading condition. In this study, the cyclic triaxial tests under axial unloading were carried out to obtain the cyclic cumulative deformation and strain softening characteristics of soft clay. The results show that the degree of strain accumulation and softening coefficient were low under the condition of low static deviator stress and dynamic deviator stress. With the increase of dynamic deviator stress, the cumulative cyclic deformation gradually increased and rapidly developed in the early stage and tended to be stable in the late stage. Meanwhile, the softening of soil gradually increased. The cyclic cumulative deformation of soil increased with the increase of dynamic deviator stress, and the cyclic cumulative strain curve showed attenuation-stability creeping characteristics. Based on the tests, a formula for softening coefficient related to static deviator stress and dynamic deviator stress was introduced to describe the test results of soft clay under different stress levels. Considering the influence of static deviator stress, dynamic deviator stress and softening coefficient, an equivalent cyclic creep model of soft clay was established. On this basis, a three-dimensional equivalent cyclic creep model was built, and a finite element subroutine was developed. The finite element method for cyclic uplift capacity of caisson was established to analyze the cyclic cumulative deformation of soil. It was verified by comparing with the test results.

Study of the permeability of aged sludge is important for the drainage reduction of sludge in landfills. Eight kinds of sludge under different aging conditions were taken as samples in the present work. Compression drainage tests were carried out, and the content and type of soluble organic matter in the supernatant were determined. The aggregate size distribution and Zeta potential were also measured. The interactions between the physicochemical properties of aged sludge were analyzed by conducting a Pearson correlation. The results show that the permeability of aged sludge increases with the decrease of soluble organic matter content in the supernatant, the increase of the relative degree of humification and the absolute value of filtrate Zeta potential. Simultaneously, when the aggregate tends to be stable and the particle size increases, the organic matter wrapped in the aggregate is not easy to release into the supernatant, and the permeability of aged sludge increases. In addition, due to the aging environment variations, the change in total organic content of aged sludge lacks of consistency with the change in organic matter content in the supernatant. Judging permeability based on the organic matter content of aged sludge would cause errors. Finally, the influence models of specific resistance of aged sludge were established, which were the reference for improving the permeability of aged sludge in the pre-treatment process.

The repeated freeze-thaw cycles with seasonal alternations have an obvious effect on soil structure. To reduce the sensitivity of saline soil to temperature and then use it in engineering, a combined treatment method is proposed, where lime, fly ash and modified polyvinyl alcohol (MPA) are used as solidified materials. Unconfined compressive strength (UCS) and microstructure characterization tests are firstly used to evaluate the solidified effect and obtain the parameters scope of solidified materials. Then the tests of shear strength (cohesion and internal friction angle) and the orthogonal experiment are used to analyze the influence of each factor, and then obtained the optimal combination of solidified parameters. The results indicate that the combination of lime, fly ash and MPA can improve the strength of saline soil. After combined treatment, the UCS is 1 130.25 kPa, which is 5.18 times than that of saline soil (218 kPa). The strength of combined solidified saline soil meets the requirements of engineering specification (JTG 3430－2020). The stable value of UCS of combined solidified saline soil under freeze-thaw cycles is 700 kPa. The fluctuation is about 5% after three freeze-thaw cycles. The cohesion and the internal friction angle of combined solidified saline under the most appropriate ratio can be 208.2 kPa and 38.56°, respectively after three freezing-thawing cycles. The sensitivity ranking of the factors is as follows: curing time, lime content, MPA content, dry density, salt content, and times of freeze-thaw cycles. With an increase in lime, fly ash and MPA content, the strength of combined solidified saline soil increases and then tends to become stable. The optimization of solidification parameters can effectively weaken the influence of freeze-thaw on coastal saline soil. Based on tests results of compressive strength and shear strength, it can be concluded that the optimal combination of solidified parameters is 14% of lime, 30% of fly ash, 1% of MPA, 28 days of curing time, and a dry density of 1.65 g/cm3.

To accelerate the stabilization of degradation in landfills, leachate recirculation is often used in practical engineering, and recirculation using a vertical well is one of the more effective ways. With the injection of liquid, the temperature in the waste will change due to convection. In this study, the variation of waste temperature during the field test was first simulated based on the water injection test in the Wuxi landfill. A seepage flow model considering the variations in permeability and porosity with depth as well as a thermal convection-thermal conduction model considering the effect of seepage flow were then established, which were used to calculate the seepage and thermal convection-conduction processes of the landfill in numerical simulation. Comparison of theoretical and numerical results shows that the established models can well simulate the variations of leachate level and waste temperature during the water injection. The results are as follows: The leachate level is relatively lagged in the position more than 6 m away from the injection well, at least 0.15 days behind, and the farther the distance, the longer the lag time. Below the leachate level, the waste temperature around the injection well is lower than the initial temperature. However, the waste temperature 3.6 m away from the injection well in the radius direction is not all below the initial temperature. The temperature will be higher than the initial temperature within 2 m above the new and old waste junction, where the temperature difference can reach 3 ℃. The waste degraded time significantly affects temperature distribution under single well water injection.

The parameters for heat generation of municipal solid waste (MSW) are the basis for studying the temperature variation and utilization of heat resource in landfills. Inverse estimation of parameters for heat generation due to degradation in landfills was proposed based on the peak heat generation model obtained from laboratory tests, heat conduction theory and the Levenberg- Marquardt (LM) algorithm. One-dimensional heat conduction model was established according to practical working situation of landfills, and analytical solution was then obtained. Base on the measured temperature data of a certain depth, the parameters for heat generation were obtained by inversion using the LM algorithm. The temperature at any depth of the landfill can be calculated with the parameters. The peak heat generation rate, the time corresponding to a peak heat generation rate and total heat generation of the landfill can be calculated using the heat generation parameters. Taking the Taohuashan Landfill in Wuxi, China and the Michigan Landfill in the United States as examples, the parameters of heat generation due to degradation were obtained by inversion. The temperature of the measuring point outside the inversion point was calculated by using the degradation heat production parameters obtained from the inversion, and compared with the measured values to verify the rationality and applicability of the inversion model. It is found that the laboratory test has a larger peak heat generation and an earlier peak time than the practical landfill engineering. Compared with the Michigan Landfill, the Wuxi Taohuashan Landfill has a higher heat production rate in the early stage, and the time to reach the peak heat generation rate is shorter.

Compaction by deep vertical vibration increases not only the soil stiffness but also the horizontal effective stress. Based on the first practice of deep vibratory compaction in the treatment of collapsible loess foundation in northwest China, this paper discusses the influence of deep vibration compaction on the change of horizontal stress of collapsible loess and reveals the mechanism of compaction of collapsible loess by vibration. The results obtained from the field test showed that the cone stress and the sleeve resistance of the soil increased by more than 2 times within the treatment depth (–8 m), and the friction ratio increased by 20%–50%. The value of the sleeve resistance ratio was relatively large above –3 m depth, and corresponding to a higher horizontal stress change ( ), indicating an increase of horizontal stress after compaction. At the depth from –3 m to –7 m, the sleeve resistance ratio was maintained at about 4, and the corresponding was between 3 and 4. The value of the soil below –7 m gradually decreased and became close to 1. The horizontal stress change was greatly affected by the sleeve resistance ratio but was less affected by the effective friction angle. The increase of horizontal stress caused the “pre-consolidation” effect of the soil, which was manifested in the microscopic view that loess changed from a granular structure to a mosaic structure. The large pores dominated by trellis pores disappeared or evolved into small pores such as mosaic pores, and the soil layer gradually became dense.

With the rapid development of global industrialization, the pollution of hexavalent chromium (Cr(Ⅵ)) in soil and groundwater has become increasingly serious. Field investigation and laboratory tests were carried out for the soil polluted by the chromium slag of a ferroalloy plant. The adsorption, infiltration and dispersion experiments were conducted to study the adsorption characteristics and migration mechanism of Cr(Ⅵ) in silty clay. A three-dimensional kinetic mathematical model of Cr(Ⅵ) migration considering convection-dispersion-adsorption was established. The migration and distribution characteristics of Cr(Ⅵ) in groundwater with the pollution source located upstream or downstream of the contaminated site were obtained using the numerical approach. Meanwhile, the effects of dispersity (?) and distribution coefficient ( ) on the spatial and temporal distribution of Cr(Ⅵ) were revealed. The experimental results show that the Langmuir isotherm model well fits the adsorption data of silty clay. The maximum adsorption capacity of silty clay for Cr(Ⅵ) was 466.6 mg/kg. The hydraulic conductivity of silty clay under the infiltration of distilled water and 160 mg/L Cr(Ⅵ) solution was 6.5×10–7–6.7×10–7 cm/s, while it increased to 4.4×10–6 cm/s under infiltration of Cr(Ⅵ) solution with a concentration of 1 000 mg/L. The hydrodynamic dispersion coefficient (D) of silty clay was 1.4×10–4 m2/d. The value of the retardation factor ( ) was found to be 4.2–10. The results of the numerical simulation indicated that when the downstream was contaminated by Cr(Ⅵ), there was still a risk of pollution in the upstream even if molecular diffusion was not considered. The degree of pollution depended on the dispersity of the aquifer. Considering the adsorption of Cr(Ⅵ) by the aquifer, the higher the soil distribution coefficient, the smaller was the distribution range of the Cr(Ⅵ) pollution plume. Therefore, the transformation processes such as Cr(Ⅵ) adsorption should be focused on when predicting the distribution of Cr(Ⅵ) in contaminated sites.

Diffusion is an important phenomenon that governs contaminant transport through compacted clay liner at the bottom of landfill. The diffusion process is influenced by many factors, i.e., type, valence and concentration of ions. Therefore, an accurate diffusion model is of great importance for the design of liner system of landfill. The simplified Guntelberg activity coefficient was introduced to indicate concentration difference between ideal solution and actual solution. The driving force of diffusion usually considered combined action of chemical and diffusion potential. The semipermeable membrane effect of clay was considered. A diffusion model describing simultaneous movement of multi-ions through clay barrier was proposed. Moreover, the multi-physical field simulation software COMSOL Multiphysics was employed for conducting simulations for the proposed diffusion model. The results indicate that the factors such as solution non-ideality, diffusion potential, and semipermeable membrane effect of clay play an important role in the diffusion process of multiple ions. A remarkable retarded behavior was captured, when the solution non-ideality or semipermeable membrane effect was considered in the proposed model. Further, an accelerating effect on cations was observed taking diffusion potential into consideration. The multi-ion diffusion process under combined action is not a simple superposition of single influence mechanism, but rather, each mechanism restricts each other and affects the multi-ion diffusion behavior.

The hybrid buffer material added with the auxiliary aggregate not only retains the material’s sealing and anti-seepage ability, but also overcomes the defect of low thermal conductivity and inferior construction performance of pure bentonite blocks. As an alternative for the buffer material of high-level radioactive waste repository, it is becoming a new research hotspot. Based on the previous research data and the permeation theory of unsaturated porous media, a 3D calculation model containing one tunnel and a single wellbore is established using COMSOL Multiphysics, which considering the wetting expansion of bentonite and the real-time change of material physical parameters (density, saturation, thermal conductivity, etc.). By simulating the THM coupling process of the hybrid buffer material (bentonite-sand mixture) in the barrier system for 100 years, the time evolution and spatial distribution of each physical quantity are analyzed, and the influences of wetting expansion and sand mixing rate on the evolution process of the barrier system are discussed. The temperature and saturation of the material in the system are related to the distance from the vitrified HLW and the rock wall. The stress in the tunnel and wellbore is mainly compressive stress, and the deformation tends to compress first and then expand. The wetting expansion of bentonite-based materials has little effect on temperature evolution, but it will slightly accelerate the saturation process and cause significant time evolution and regional distribution differences in the stress and strain of the material. The stress in the area close to the rock wall in the wellbore and roadway rises quickly, and significant vertical displacement occurs at the junction of the roadway floor and the wellbore. Increasing the mixing rate can reduce the surface temperature of the tank effectively, enhance the heat transfer capacity of the barrier system, reduce the maximum historical stress of the buffer material, and control the vertical displacement on the axis of the borehole. On the other side, it will also weaken the anti- seepage capability of the system.

A numerical simulation study on the rainfall infiltration characteristics of unsaturated fractured soil was performed based on COMSOL Multiphysics software. By discretizing the fracture and the matrix into finite elements, a discrete fracture-porous medium model was established to fully simulate the fracture flow, matrix flow and fracture-matrix flow exchange in the soil. The upper boundary of the fractured soil was simulated by using the concept of “air element”. This method can describe not only the phenomenon of preferential infiltration for rainwater along the fractures at the beginning of rainfall, but also the phenomenon of rainwater flowing away along the surface when the rainfall is greater than the infiltration of the fractured soil. By simulating the low-permeability fractured soil at a depth of 2 m below the ground surface, the influences of the geometric characteristics of the fracture, the hydraulic properties of the matrix, the previous moisture condition and the rainfall intensity on the rainfall infiltration process of the unsaturated fractured soil were investigated. The results show two main seepage processes in the unsaturated fractured soil: firstly, water flows preferentially along the fracture; secondly, water is continuously imbibed into the matrix from the fracture, and the matrix imbibition inhibits the development of preferential flow in the fracture. Compared with the geometric characteristics of the fracture, the hydraulic properties of the matrix have a greater influence on the seepage flow in the unsaturated fractured soil. Increasing the saturated permeability coefficient of the matrix may change the seepage flow dominated by the fracture flow into that dominated by the matrix flow. The unsaturated properties of the matrix as well as the initial water content of the fractured soil change the soil moisture storage capacity, thereby accelerating or delaying the time for rainwater to infiltrate into a certain depth. The rainfall intensity has an influence on both the infiltration rate and infiltration amount in the soil. When it exceeds the infiltration capacity of the fractured soil, the excess rainwater flows away along the surface, and the infiltration rate across the section tends to stabilize with time.