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As one of the most effective methods for remediating VOCs contaminated site, air sparging technology is not suitable to low-permeability soil due to the poor remediation efficiency. To solve this problem, an improved approach aiming for mass transfer enhancement by establishing pressure gradient in soil is proposed in this study, and the remediation efficiency, removal mechanism, as well as the mass transfer characteristic are comprehensively investigated. Test results showed that, using the proposed approach significantly reduced the time for exhaust air contaminants reaching concentration equilibrium, and improved the contaminant removal zone and extent in soil, which were especially strengthened at sparging pressures higher than 40 kPa. The total contaminant removal rate was improved by introducing the proposed approach, with a maximum improved removal rate of 23.7% at 100 kPa sparging pressure. In mechanism analysis, the recorded changes in total pore pressure and average liquid saturation illustrated the pressure drop and discrete drainage phenomena, confirming the pressure gradient and air sub-channels formed in low-permeability soil. Finally, contaminant mass transfer characteristic was quantitatively analyzed using the lumped parameter model, in which the mass transfer coefficient and the air channel influencing fraction were enhanced almost fourfold and fivefold respectively by introducing the proposed approach. Compared to the conventional approach, the improved remediation efficiency using the proposed approach tackled the in-situ remediation challenge on low-permeability soil, and further expanded the application scope of air sparging technology on VOC contaminated site.

Solidification/stabilization technique has been widely adopted to remediate the heavy metal–contaminated sites. In the present work, the strength and leaching characteristics of the contaminated soils solidified/stabilized by cement/fly ash were systemically investigated. Electrical resistivity was also measured to establish empirical relationships for assessment of remediation efficacy. Tests results showed that the unconfined compressive strength increased and the leached ion concentration decreased with increasing curing time. In contrast, the unconfined compressive strength decreased and the leached ion concentration increased with increasing initial heavy metal ion concentration in the specimen. For the strength characteristic, the most notable detrimental effect was induced by Cr3+ and the least was induced by Pb2+. For the leaching characteristic, the trend was reversed. The electrical resistivity of the tested specimen increased significantly with increasing curing time and with decreasing initial ion concentration. The electrical resistivity of the Pb-contaminated specimen was higher than that of the Zn-contaminated specimen, which in turn was higher than that of the Cr-contaminated specimen. Empirical relationships between the strength, leaching characteristic, and electrical resistivity were established, which could be adopted to assess the remediation efficacy of heavy metal–contaminated soil solidified/stabilized by cement/fly ash.

To overcome the expense of superfine cement slurry and the unfriendly environmental influences of chemical binders, microbe-induced calcium precipitation (MICP) method with filler aggregates was used to simulate jointed rock repairment. The reinforcement quality was investigated in terms of filling material type, density, and particle size, indicating the significant enhancements in tensile strength when enlarging the filler dimension and improving its density. The samples containing coarse sands as a filler produced the best reinforcement in tensile strength, followed by fine and silty sand, respectively. The crystal morphologies differed by filling materials, potentially as a result of the CaCO3 precipitation intensity.

Red-bed soft rock in the drawdown area on bank slopes of landslide easily disintegrates upon exposure to water, and its properties experience comprehensive deterioration, which will cause bank slope instability. To better study disintegration mechanism of the red-bed soft rock, a series of laboratory tests were conducted in this paper to investigate the disintegration characteristics, durability and hydrogeochemical process of red-bed argillaceous siltstone under drying-wetting cyclic conditions. Experimental results showed that, with increasing number of drying-wetting cycles, red-bed argillaceous siltstone gradually disintegrated, from initial appearing the cracks on the surface of the samples to large particles gradually breaking up into small fragments. Significant changes in grain size distribution, and the durability index of the samples progressively decreased. Microstructural analysis showed that the size and distribution of pores and cracks in the sample surface significantly increased, such that the sample surface became disordered and complicated. Notable changed in concentrations of ions in the soaking solutions indicated continuous mineral dissolution and loss during the cyclic drying-wetting. Based on the results obtained from the experiment, it is concluded that the disintegration of samples undergoing drying-wetting cycles was the result of the synergistic action of water and temperature. To be specific, the dissolution of calcite, albite, gypsum, montmorillonite and kaolinite during the wetting procedure, which promotes the decrease in mineral content and increases in pores and cracks. The increases in temperature and the dehydration shrinkage of sample during the drying procedure accelerated the disintegration of the samples.

The complicated geological conditions, including the Fault Sun, in East No. 2 mining sub-area of the Longdong coal mine will influence the stability of strata during mining, leading to serious geological hazards. To circumvent this issue, a similarity simulation experiment was designed and performed in this study, in which the failure characteristics and evolution of displacement and stress within the strata were investigated, and the optimum width of a waterproof coal pillar was determined. The results showed that, as the working face progressed, the coal seam roof gradually deformed, from initial caving of the immediate roof to complete movement and curved subsidence of the entire roof. Significant changes in displacement and stress within the coal seam roof were recorded, and these increased during continuous mining activity. Displacement and stress difference on either side of the fault gradually increased and reached remarkable values with increase in mining distance. On the basis of the experiment results, water inrush is believed to be caused by the interaction between mining and the fault, and, as calculated from parameters collected in field investigations, a waterproof coal pillar of 50 m width should be established to prevent Fault Sun activation, thereby reducing the risk of water inrush from neighbouring aquifers.

Lime, fly ash, and alkaline residue are used widely as effective binders to solidify/stabilize heavy metal-contaminated expansive soil. Carbonation, however, may influence the effectiveness of solidification/stabilization (S/S) by decomposing hydration products and decreasing pH, which would seriously damage the long-term durability of stabilized soils. This study focused on the variation of leaching characteristics of solidified/stabilized lead-contaminated expansive soils before and after accelerated carbonation under the leachant of pH 3 and 5 by the semidynamic leaching test. After semidynamic leaching, leaching indexes such as the effective diffusion coefficient (De), leachability index (Lx), and slope (rc) were used to reveal the ion leaching mechanism. The results indicated that the amount of Pb2+ and Ca2+ leached out under different pH conditions increased after carbonation, which confirmed that carbonate on solidified/stabilized lead (Pb) had a negative impact. Additionally, the De values of Pb2+ and Ca2+ varied in the range of 1.16E − 10 cm2/s to 1.71E − 07 cm2/s, which demonstrated that ion migration was low. The contaminated soil solidified by lime and AR could be used in “controlled utilization” as Lx was higher than 9, and the leaching process was controlled by a dissolution reaction according to the analysis of rc. Moreover, the strong acidic leachant (pH = 3) resulted in more ions leaching out and lower pH in leachate compared with a mildly acidic leachant. Finally, with literature and experimental results, we found that the main reason for the increase of lead ion filtration of the carbonation reduced the pH value of the matrix and made the hydration products denatured and decomposed.

Soil contaminated by potentially toxic trace elements (PTEs) may cause serious deteriorations to the environment and human health. With the aims of PTEs removal and resource reuse in solidification/stabilization technology, the soda residue was adopted as the adsorbent to mix with the soil in this study. The adsorption capacity of the mixture on Pb(II) was investigated by performing a series of adsorption tests with considerations of changes in soda residue content, pH value, initial Pb(II) concentration and temperature. While the adsorption mechanism was revealed by analyzing the adsorption isotherm models including Langmuir, Freundlich and Dubinin–Radushkevich models. Results showed that, the higher pH value, soda residue content and initial Pb concentration will improve the adsorption capacity of soda residue on Pb ions. In contrast, the enhanced temperature will reduce the corresponding adsorption capacity. The Langmuir adsorption isotherm model is much better than the Freundlich model at describing the adsorption behavior, indicating the monolayer adsorption with uniform distribution of active sites on the surface of mixture particle. The calculated maximum adsorption amount of the tested specimen is 34 mg/g, which is significantly higher than those of other clay materials, implying that soda residue has a notable potential for adsorption of Pb(II) at contaminated sites. Analysis using the Dubinin–Radushkevich adsorption isotherm model shows that the adsorption mechanism is strongly controlled by the chemical effects. The strong sensibility of adsorption capacity to environmental condition indicates that other cementitious materials should be explored and mixed with highly-content soda residue to enhance the durability of solidified/stabilized PTEs contaminated soil.

The deterioration of cement-based materials due to chloride attack is a well-known phenomenon attracting extensive attention worldwide. However, the effects of chloride attack on the strength and long-term stability of stabilized/solidified heavy metal contaminated soils have not been studied in detail. In the present work, systematic tests were developed to investigate the effects of chloride attack on the strength and leaching characteristics of heavy metal contaminated soils treated with cement and fly ash. The test results suggested that increasing the NaCl concentration from 0% to 15% had a detrimental influence on the unconfined compressive strength (UCS) of solidified/stabilized contaminated soil. The UCS of the specimens decreased during the initial 7 days of soaking and then increased at longer soaking periods. In leaching tests, the concentration of Pb2+ in the leachate decreased with the increasing NaCl concentration, while that of Zn2+ and Cr3+ increased. The leached Pb2+ concentration reached its maximum level after 7 days of soaking and then decreased significantly at longer soaking times. In contrast, the leached Zn2+ and Cr3+ concentrations decreased significantly during short-term immersion. The leachability of the three contaminants followed the order: Zn2+ > Cr3+ > Pb2+. The mineralogical analysis performed by X-ray diffraction revealed that high concentrations of heavy metal contaminants significantly inhibit the development of hydration under such an aggressive chloride environment. The microstructure analysis performed by scanning electron microscopy indicated that Friedel's salt was formed during chloride attack, resulting in larger porosity and a looser structure of the solidified soils. More compact and uniform structures were observed for the specimens soaked for 28 and 90 days than that soaked for 7 days.

Re-utilization of industrial wastes or by-products in construction, including dredged river sediments (DRS), iron tailing slag (ITS) and calcium carbide slag (CCS) etc., can reduce the construction cost, decrease the storage requirement and also be of great benefit to the environment. In this research, the aforementioned wastes (i.e., DRS, ITS and CCS) were synthetically recycled and used as the backfilling materials in the mine goaf to solve the problem of subsidence. Systematic laboratory experiments were conducted to ensure that these recycled materials can fulfill the requirement on the slump value and unconfined compressive strength after 7-day’s curing. The results show that despite that Ordinary Portland cement (OPC) suppresses the slump value, CCS elevates the flowability, and then the maximal flowability is achieved when the mass ratio of DRS to ITS was 70:30 with a cement content of 16.7%. ITS addition into DRS upgrades the strength of the mixtures of DRS and ITS stabilized by OPC and CCS for the skeleton effect and water content reduction. The optimal proportion between DRS, ITS, OPC and CCS in this case is 60:40:16:4 in mass, with a slump value of about 160, and unconfined compression strength after 7-day’s curing about 2.8 MPa. At last, a simple expression to predict the strength of the backfilling materials was proposed referring the gel-space theory and the concept of volumetric solid content, where just two variables (i.e., the binder content and the volumetric solid content) can characterize this complex matrix.

The deposition of large amounts of industrial solid waste titanium gypsum via weathering causes wastage of land resources and environmental pollution. The resource utilization of titanium gypsum is mainly limited by its low reaction rate. This paper presents a simple physical modification to improve the performance of titanium gypsum for use in the stabilization of expansive soils. The engineering properties and mechanisms of stabilized soils were investigated by combining the evaluation of stabilized soil properties including physical properties, deformation characteristics, mechanical capacity, microstructure, and mineral evolution on a macroscopic and microscopic scale. The results showed that the coarse particles of the soil increased and the plasticity index, swelling potential shrinkage, and compression index decreased while the strength characteristics of unconfined compressive strength, cohesion and internal friction angle of the stabilized soil were significantly improved. Furthermore, a curing time of 7 d was recommended for stabilizing the expansive soil. Microcosmic experiment results revealed the densification process of the internal structure and reflected the ion exchange, coagulation, and agglomeration of the stabilized soil. Moreover, stabilized soil at 25% titanium gypsum admixture can satisfy the requirements of the Chinese standard for subgrades below Grade II. This study suggests that titanium gypsum has broad application prospects as a stabilizer for expansive soils.

In geological surveys of coastal regions, data interpretation and site identification are usually affected by underground water (including salinity). This study aimed to investigate the effect of pore-water salinity on soil identification via in situ CPTs. Combinations of triaxial shearing, cone penetration, and vane shear tests were also performed at a laboratory scale on Lianyungang marine clay with different pore-water salinities. The results suggested that the effective friction angles and shear strength increased with the pore-water salinity. The in situ CPTs showed that the normalized tip resistances increased with an increase in pore-water salinity, whereas no significant correlation between the normalized sleeve fraction and salinities was found. The use of the conventional soil identification method (soil-behavior-type chart, i.e., SBT chart) may be effective in very low-salinity conditions despite that it does not take into account the pore-water salinity effect. The salinity investigation of Lianyungang marine clay suggests that an inhomogeneous salinity distribution significantly influences soil identification when with rich clay mineral of montmorillonite and high pore-water salinity. From the theoretical and practical analyses, an improved soil identification method considering pore-water salinity modification was proposed to avoid the misclassification of in situ soil.

A series of incremental loading oedometer tests was conducted on pure clays and sand–clay mixtures with various sand/clay ratios and clay mineral compositions. The void indexes Ivand IvHwere introduced to evaluate their intrinsic compressibility. Test results revealed that Ivwas more suitable for depicting the compression behavior of pure clays than IvH; whereas, for the compressibility of sand–clay mixtures, the normalized compression line by using Ivwas obviously different from that of pure clays and traditional soils due to the presence of sand particles. Therefore, a four-phase analysis framework of sand–clay mixtures was introduced to unify the intrinsic compression behavior of soils with and without sands. It was found that the updated clay void index Ivcparameter was effective to unify the soil’s compression behavior by excluding the influence of sand particles in clays before the sand skeleton formation. After the formation of the sand skeleton, the cluster particles change the stress distribution of mixtures, leading to less stress being applied on the clay matrix and thus the bifurcation of the intrinsic compression behavior.

Adopting more efficient and sustainable remediation materials is of great importance for the development of solidification and stabilization (S/S) technology. Among them, soda residue could be considered as a desirable binder due to its strong adsorption for heavy metals. For understanding of the performance of Cr-contaminated soils treated by cement/soda residue, the strength, leaching and microstructural characteristics, and the long-term effectiveness under wetting-drying cycles were comprehensively investigated in this study. The results showed that the unconfined compressive strength (UCS) increased and the leached Cr3+ concentration decreased with curing time, binder content, and binder ratio. Increasing the soda residue from C6S14 to C6S24 could improve soil strength and reduce leachability of Cr3+, while a reverse trend was presented with increasing initial Cr3+ concentration. With subsequent wetting-drying cycles, the UCS further increased and then decreased; inversely, the leached Cr3+ decreased, followed by an increase of Cr-contaminated soils. For the specimens of C6S14 and C6S24, the maximum UCS of 6.04 MPa and 6.48 MPa was reached; correspondingly, the minimum leached Cr3+ concentration of 2.78 mg/L and 1.93 mg/L was reached after 3 wetting-drying cycles, respectively. Microstructure analysis results found that reaction products like calcium silicate hydrate (C-S-H) and ettringite (AFt) increasingly occupied the soil pore space and caused a denser soil structure after 3 wetting-drying cycles, which indicated the long-term effectiveness of contaminated soils treated by cement/soda residue.

International audience; In the past 20 years, more than ten highways are built in the Lianyungang region, Jiangsu province, China. Among them, two highways (Coastal Highway A, close to the coastline, and Lianyan Highway B, approximately 25 km from the coastline) are crucial to the local highway network. However, the settlements of these two highways are significantly different, despite the same building method and location in a region with similar sediment provenance and sedimentary environment. A higher settlement rate was observed for the highway constructed far from the coastline. To clarify the mechanism, a series of field and laboratory tests based on regional geological survey were conducted in this study. Test results showed that the sensitivity difference of the soil induced by the pore water salinity leaching is the main reason for those observations. With pore water salinity leaching, the water content and plastic limit of Lianyungang marine soft clays remained almost unchanged, whereas the liquid limit and plasticity index of the soft clay near highway B gradually decreased with an increase in the liquidity index. The increase in the liquidity index subsequently decreased the remoulded strength of the soils far from the coastline. Simultaneously, the natural strength remained approximately the same due to leaching, and this led to an increase in the sensitivity, suggesting a strong structure. During the filling construction, the construction load and deviatoric stress under the embankment may have destroyed the clay structure. Therefore, a large settlement and high settlement rate resulted for the highway constructed far from the coastline.

Both high-calcium fly ash and soda residue are the industrial by-products generated during the combustion of coal for energy production and sodium carbonate production, respectively. Previous studies suggested that both high-calcium fly ash and soda residue can be used to treat heavy metal contaminated soils. Therefore, a series of laboratory experiments were performed to investigate the strength and leaching properties of Pb-contaminated soils solidified/stabilized by high calcium fly ash and soda residue. The results presented that adding high-calcium fly ash to the contaminated soils can significantly improve the soil strength and reduce the Pb2+ amount released out. The specimens treated with soda residue got a higher strength, but a relatively lower chemical stability than that of fly ash. SEM results confirmed that Ca(OH)2 and CSH was the main products that improved the engineering properties of the stabilized contaminated soils.

The improved engineering properties of the expansive soil by mixing with various additives will be changed during the long-term variation of the meteorological and hydrological conditions. In the present work, a series of tests are performed to investigate the evolution of the unconfined compression strength and the Atterberg limits under drying-wetting cycling conditions for specimens treated by iron tailing sands and calcium carbide slag. Typical results of the unconfined compressive strength can be divided into three stages. The unconfined compressive strength increases initially and then decreases to reach a stable state with continuous drying-wetting process. The calcium carbide slag content (αCCS) of 10% can be determined for the minimum effect of the drying-wetting cycle on the strength of the treated specimen. An exponential relationship is established to describe the evolution of the unconfined compressive strength with the drying-wetting cycle. The liquid limit and plastic index of the specimen increase initially followed by a decreasing trend, while a reverse trend was observed for that of the plastic limit during the drying-wetting process. The minimum effect of the drying-wetting cycle on the Atterberg limits can be presented for the specimen with αCCS of 10% as well.

Solidification/stabilization (S/S) has been considered as one of the most effective techniques for remediation of the heavy metal-contaminated sites. Among various binders adopted in S/S, alkaline residue (AR) could be considered as a new binder to treat heavy metal-contaminated soil due to its strong adsorptive capacity for heavy metal ions. So in this paper, the strength, leaching, and microstructure characteristics of the solidified/stabilized Pb-contaminated soil by using alkaline residue are systematically investigated. Test results present that the unconfined compressive strength (UCS) of the treated soil will increase, while the leached Pb2+ concentration will decrease, with the increase of the alkaline residue content in the specimen. The UCS increases significantly with the curing time increasing during the initial 28 days, after which the UCS of the specimen becomes stable. The leached Pb2+ concentration decreases significantly at the initial 28 days followed by a stable trend with curing time increasing. The UCS decreases and the leached Pb2+ concentration increases with the increase of the initial Pb2+ concentration in the specimen. The microstructural analysis performed by scanning electron microscope (SEM) showed that the increase of the alkaline residue content and curing time will result in more hydration products and densified microstructure, which could effectively improve the engineering properties of the specimen.

Gas flow behavior in saturated cohesive soils having low permeability is a major concern in most engineering practices. The effect of stress, including that caused by gas pressure and confining pressure, on gas permeability must be investigated in detail. In the present study, gas permeability tests with cyclic gas pressure, as well as different gas and confining pressures, were conducted. Results showed that a sudden increase in the gas flow rate appeared when the gas pressure approached the confining pressure. This increase in gas flow was possibly caused by a gas leakage along the lateral boundary, which was not the true gas breakthrough. The real gas breakthrough linearly increased with the increase of intrinsic permeability at a log-log scale. In gas permeability tests, the cyclic gas pressure induced a significant hysteresis: the gas flow rate during decreasing stages of gas pressure was higher than that during increasing stages. In addition, increasing confining pressure weakened this phenomenon. Under a constant confining pressure, gas permeability increased significantly with the gas pressure increased. However, little variation was aroused by increasing the confining pressure under a constant gas pressure. In conjunction with the pore size distribution determined in mercury intrusion porosimetry test, it can be concluded that, gas flow behavior in saturated cohesive soils with low permeability was mainly controlled by the capillary effect, even though the mechanical effect was presented during the gas migration process.