Mechanical Behavior Of Soil-bentonite Cutoff Walls And Its Impact On Contaminant Control

Underground soil and water pollution is a serious problem in China today. Vertical cutoff wall is an effect way to control underground contamination. Soil-bentonite cutoff walls can be widely used in underground contamination control for the low permeability, good chemical compatibility and small project cost. The stress in soil-bentonite cutoff wall affects its hydraulic conductivity greatly, and the long-term performance of soil-bentonite slurry trench cutoff walls is highly dependent on the hydraulic conductivity. Consequently, a reliable estimation of hydraulic conductivity of backfill in the field requires proper calculation of effective stresses. The primary design criterion of soil-bentonite cutoff walls in China is the achievement of a low permeability which is depend on experience of hydraulic engineering, it failures to consider diffusion, mechanical dispersion and adsorption of contaminant, and will lead to an insecurity design.In response to the status, a key scientific question of what is the mechanical mechanism of soil-bentonite cutoff walls during construction and its impact on contaminant transport was proposed. A combination of laboratory test, theoretical analysis, field test and monitoring were taken to solve the key question. The study had three levels which were study of material properties, key scientific question, and engineering application respectively. Then a design method based on the whole service life of vertical cutoff wall was proposed, a large-scale field test of soil-bentonite cutoff wall was carried out in Jingjiang solid waste landfill, Jiangsu Province. A series of significant conclusions were drawn accordingly.(1) The swelling index (59.0 mL/2g), content of montmorillonite (71.4%) and specific surface area (80.2 m2/g) of Wyoming bentonite are much higher than most domestic bentonite. The relationship between slump and moisture content of wall materials obtained from slump test can provide guidance in preparation of soil-bentonite on-site. The hydraulic conductivity of soil-bentonite can reach the requirement of less than 1.0×10-9 m/s when bentonite content is greater than 4% and consolidation pressure is larger than 10 kPa.(2) The hydraulic conductivity of bentonite cake was estimated using the API fluid loss test under the pressure of 20 kPa to 80 kPa, it ranges from 4.0×10-10 m/s to 1.3×10-9 m/s, and is 1 to 2 magnitude larger than the permeability coefficient that obtained at the pressure of 69 kPa to 690 kPa by foreign scholars. Factor analysis showed that wall permeability is controlled by the backfill when the backfill permeability is low (less than or equal to 1.0×10-9 m/s) and by the filter cake when the backfill permeability is high (larger than or equal to 10×10-7 m/s).(3) A model which considers arching effect and lateral squeezing effect was proposed to predict the steady-state horizontal and vertical effective stresses in the backfill after consolidation. The model revealed mechanical mechanism of soil-bentonite cutoff walls and its impact on wall permeability. A limit equilibrium method for assessing stability of slurry trenches within an inclined ground surface and a horizontal slice method for stability analysis of slurry trenches which allows consideration of soil stratification are proposed. Keeping a certain height of slurry surface and unit slurry weight are very important to the stability of slurry trench.(4) Column test method of determining contaminant transport parameters with shorter time and fewer samples was presented. Hydraulic conductivity, wall thickness, head difference and retardation factor of wall material are the key factors that affect the breakthrough time of cutoff walls. The requirements of structure and head difference were proposed to meet a service life of 30 years to 50 years on the situation of typical landfill leachate.(5) A design method of contaminant control cutoff walls was proposed. Data from monitoring instruments in the large-scale field test in Jingjiang showed that the pressure in site was similar with the pressure calculated by the model of this paper. Optimization measures on wall structure were formed to enhance shallow part of cutoff walls.