| With accelerated industrialization and urbanization processes in China, an increasing number of industrial polluted sites are emerging rapidly, thus requiring polluted soil/water risk controls and remedial actions. To control the migration of the contaminants with groundwater flow, exploring an efficient contaminant containment vertical barrier technique is one of the urgent needs in geoenvironmental engineering. As its abundant reserves in China, calcium bentonite has wide prospects for engineering applications. A new vertical cutoff wall material amended technology based on phosphate dispersant is proposed in this dissertation. Studies on amendment mechanisms and engineering properties of phosphate dispersant amended vertical cutoff wall material is intended to provide scientific basis for promotion and application of soil-bentonite vertical cutoff wall for polluted site remediation.With financial supports provided by National Natural Science Foundation of China (Grant No.51278100), Key Project of National Natural Science Foundation of China (Grant No. 41330641),and Distinguished Young Scholars of Natural Science Foundation of Jiangsu Province (Grant No.BK2012022), the amended mechanism and engineering properties of the phosphate dispersant amended vertical cutoff wall materials were studied by combining laboratory tests and theoretical analysis. The main works and conclusions are incorporated as follows:(1) The variations in dispersity property of bentonite/backfill amended by three different phosphate dispersants were investigated. Effects of the type and dosage of the phosphate dispersants on dispersity of the calcium bentonite and backfills were evaluated by laboratory tests. The amendment efficiencies of three phosphate dispersants were compared. The results indicate that the dispersity of the calcium bentonite and backfills are significantly improved by the dispersants.Apparent viscosity, liquid limit, and settling volume of the backfills decreased speedily as dispersant dosage increased, and then those reduced trends become moderately when the dosage reached a certain value. The chemical resistance of the calcium bentonite backfill is enhanced significantly after amended with dispersant, and the sodium hexametaphosphate (SHMP) is found to be more effective than the sodium tripolyphosphate and sodium pyrophosphate.(2) The engineering properties of the bentonite-water slurries amended with SHMP were investigated. Workability experiments were conducted to explore effects of SHMP dosage and bentonite content on engineering properties of the slurries. The results indicate that workability of the slurry is obviously improved by the SHMP. The Marsh viscosity and density of the slurries increase, whereas the filtrate loss volume, filter cake thickness, and pH value decrease with increased SHMP dosage and bentonite content. There are good correlations among Marsh viscosity,filtrate loss volume, and filter cake thickness. The construction control parameters for the SHMP amended bentonite slurry are proposed, and the parameters optimization method is presented.(3) The engineering properties of the soil/SHMP-amended calcium bentonite backfills were investigated. Influences of SHMP dosage and bentonite content on the variation of hydraulic conductivity and consolidation parameters of the backfills were studied based on flexible-wall permeability tests and one dimension consolidation tests. The results indicate that the hydraulic conductivities of the amended backfills are one order of magnitude lower than those of the unamended one. Amending the backfill using SHMP is more effectual than increasing the bentonite content. Hydraulic conductivities of the amended backfills are lower than the commonly used hydraulic conductivity value for vertical cutoff walls, and comparable to the traditional soil/sodium bentonite backfills. The parameters of construction technology for the SHMP amended calcium bentonite backfill are recommended, and the parameters optimization process is proposed.(4) The mechanisms of amending calcium bentonite by SHMP were revealed. Variations in microstructure of the SHMP amended calcium bentonite were analyzed by SEM, XRD, FTIR tests,and Zeta potential measurement. The results indicate that the bentonite particles change from flocculated to dispersed after the amended by SHMP, and more negative potential particles and more dense soil structure are found after the amendment. The amended mechanisms include ion exchange, chemical adsorption, and steric stabilization effects. The increased diffused double layer,the decreased paticle size and pore size of the bentonite are responsed to higher free swell index,better dispersity, and lower permeability of the amended bentonite materials.(5) Chemical compatibility of the amended backfills to calcium, Ca(?), lead, Pb(?), and chrome, Cr(?), solutions or coal combustion residuer leachate impacted groundwater (CCR) was investigated. The effects of the metal species and concentration on hydraulic conductivity and consolidation properties of the amended backfills were studied via flexible-wall permeability tests and consolidation tests. The results indicate that there is insignificant increased in hydraulic conductivity as permeated liquid changes from clean tap water to Ca(@), Pb(?), Cr(?), or CCR solutions. The amended backfills possess preferable chemical compatibility compared with the traditional soil/sodium bentonite backfill. As a calcium bentonite backfill modifier, the SHMP possesses excellent social environmental and economic benefits.(6) The changes in adsorption property of the amended bentonite/backfill to Pb(II) and Cr(VI)were explored by a series of Batch adsorption tests, pH and electrical conductivity measurements,and zeta potential tests. The results indicate that the maxmum Pb(II) adsorption capacities increased by 45% and 72%, respectively, and those for Cr(VI) increased by 22% and 13%, respectively, after the bentonite/backfill are amended by 2% of SHMP. The amended bentonite/backfill shows higher adsorption capacity to the Pb(II) than to the Cr(VI). The mechanism for Cr(VI) adsorption is ionic exchange, while that for Pb(II) is combination of ionic exchange and precipitation.(7) Analytical evaluation model for containment performance of the SHMP amended calcium bentonite backfills was established. The superiority of the SHMP amended backfill was evaluated by regression analysis of the migration parameters as well as by investigation of the effects of different design factors on breakthrough time of the metal ions passing the cutoff walls. The results indicate that the coefficient of hydrodynamic dispersion for Ca(II) is found to be 2.29×10-10m2/s to 2.86×10-10 m2/s in this study, while those for Pb(II) and Cr(VI) are unavaliable due to insufficient testing time for valid breakthrough curves. Transportation of the metal ions within the cutoff walls is closely related to the metal species and concentration, amendment effect, wall thickness, and hydraulic gradient, etc. The proposed analytical model on cutoff wall thickness or service life determination provides useful reference for engineering design of the vertical cutoff walls. |
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