Formulation of PG-FA-L composite modifier for repairing expansive soil based on the statistical mixed design method.

The generation of large amounts of solid waste has led to exploration of solid waste-modified expansive soils; however, the effect of a single solid waste-modified expansive soil is not ideal. This study proposes a composite modification of expansive soils using a PG-FA-L system. Statistical analysis showed that the properties of the cured soil were significantly improved. PG and FA increased soil strength after a certain threshold, and L increased it at all stages. The presence of PG accelerated the volcanic ash reaction. Both PG and FA have a small effect on the swelling of the soil, whereas lime improves it significantly, but has a negative effect after a certain threshold. The 28-day unconfined compressive strength and deformation characteristics were used to derive the relevant regions for roadbed fill requirements and determine the optimum dosage.

Air sparging remediation of VOCs contaminated low-permeability soil based on pressure gradient control.

Air sparging (AS) is deemed unacceptable for remediating VOCs contaminated soil with low-permeability. To improve air flow and contaminant removal in sparging process, an original approach, termed as pressure gradient-enhanced air sparging (PGEAS) approach, is proposed by controlling pressure gradient in soil. Then the remediation efficiency, mass transfer characteristics, and remediation mechanism are investigated. Results showed that, the PGEAS approach accelerates gaseous contaminant exhaust, reduces residue contamination in soil, and promotes total contaminant removal, finally results in an improved remediation efficiency compared to the conventional approach. Controlled by sparging pressure and flow distance, the pressure gradient is created in soil, and a critical value needs to be exceeded to enhance the VOCs removal and mass transfer characteristics. The measured results of pore pressure and liquid saturation confirm a notable pressure gradient and drainage behavior in soil, which indicate the massive air subchannel formation during air sparging. At a two-dimensional scale, discrete distributions of contaminant concentrations in exhaust air and soil are presented, the removal extent and area are both enhanced using the PGEAS approach with a pressure gradient higher than the critical value. The reached conclusions are of great importance to contaminant removal in heterogeneous stratigraphy at sites.

Mass transfer enhancement of air sparging on VOCs contaminated low-permeability soil by establishing pressure gradient.

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.

Assessment of strength and leaching characteristics of heavy metal-contaminated soils solidified/stabilized by cement/fly ash.

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.