Vertical Barriers for Land Contamination Containment: A Review

Soil pollution is one of the major threats to the environment and jeopardizes the provision of key soil ecosystem services. Vertical barriers, including slurry trench walls and walls constructed with soil mix technology, have been employed for decades to control groundwater flow and subsurface contaminant transport. This paper comprehensively reviewed and assessed the typical materials and mechanical and permeability properties of soil–bentonite, cement–bentonite and soil mix barriers, with the values of mix design and engineering properties summarized and compared. In addition, the damage and durability of barrier materials under mechanical, chemical, and environmental stresses were discussed. A number of landmark remediation projects were documented to demonstrate the effectiveness of the use of barrier systems. Recent research about crack-resistant and self-healing barrier materials incorporating polymers and minerals at Cambridge University and performance monitoring techniques were analyzed. Future work should focus on two main areas: the use of geophysical methods for non-destructive monitoring and the optimization of resilient barrier materials.

SHMP-Amended Ca-Bentonite/Sand Backfill Barrier for Containment of Lead Contamination in Groundwater

This study investigated the feasibility of using sodium hexametaphosphate (SHMP)- amended calcium (Ca) bentonite in backfills for slurry trench cutoff walls for the containment of lead (Pb) contamination in groundwater. Backfills composed of 80 wt% sand and 20 wt% either Ca-bentonite or SHMP-amended Ca-bentonite were tested for hydraulic conductivity and sorption properties by conducting laboratory flexible-wall hydraulic conductivity tests and batch isothermal sorption experiments, respectively. The results showed that the SHMP amendment causes a one order of magnitude decrease in hydraulic conductivity of the backfill using tap water (1.9 to 3.0 × 10−10 m/s). Testing using 1000 mg/L Pb solution resulted insignificant variation in hydraulic conductivity of the amended backfill. Moreover, SHMP-amendment induced favorable conditions for increased sorption capacity of the backfill, with 1.5 times higher retardation factor relative to the unamended backfill. The Pb transport modeling through an hypothetical 1-m-thick slurry wall composed of amended backfill revealed 12 to 24 times of longer breakthrough time for Pb migration as compared to results obtained for the same thickness slurry wall with unamended backfill, which is attributed to decrease in seepage velocity combined with increase in retardation factor of the backfill with SHMP amendment. Overall, SHMP is shown to be a promising Ca-bentontie modifier for use in backfill for slurry trench cutoff wall for effective containment of Pb-contaminated groundwater.

Stability Analysis of a Slurry Trench in Cohesive-Frictional Soils

The slurry trench has become increasingly common in underground engineering and the stability of a slurry trench has been an important design issue. Although many studies have focused on the overall stability of a slurry trench, few of that are related to its local stability. Based on the limit analysis, both two dimensional and three dimensional rotational failure mechanisms for the local failure of a slurry trench in a sandwiched weak layer are proposed, and the upper solutions of 2D and 3D safety factors for local failure mechanisms are derived to evaluate the stability of a slurry trench. Moreover, a numerical analysis combined with the strength reduction technique is performed to investigate the local stability and the local failure process of a slurry trench. The proposed analytical method is verified through the comparison with the results of FLAC3D. Finally, a parametric study on the influences of geometric and geologic parameters on the local stability of the slurry trench are investigated. The results show that the investigation on the local stability of a slurry trench is effective and reasonable, which can provide a reference for the engineers in the practical engineering.