Consolidation-breakthrough-bender Elements Study On The Evolution Mechanisms Of The Long-Term Performance For Vertical Barriers

Soil and groundwater pollution are a global challenge.In China,1.01 million square kilometers of sites exceed environmental quality standard for soils(GB15618-1995).Europe had identified 80,000 contaminated sites by 2007.In United States,it was estimated that about 294,000 contaminated sites require treatment.Unregulated municipal solid waste landfills are a major source of pollution,tremendous groundwater and seepage area around the landfill are contaminated by the leaked landfill leachate.Soil-bentonite vertical barriers are widely applied to reduce the risk of landfill contamination.However,the poor chemical compatibility of soil-bentonite and the diffusion of pollutants are the most serious problem for long-term service of soil-bentonite.In order to solve the above problems,it is very important to explore the mechanism behind the lack of chemical compatibility and establish a novel on-site monitoring method for soil-bentonite barrier.The shear wave is nondestructive to barrier structure,efficient and low cost,which has the potential to monitor the service performance of the soil-bentonite barrier.To make a robust conclusion on the mechanism of chemical compatibility and the establishment of a shear wave velocity monitoring method,plenty of soil column tests in this issue are needed.However,there are few experimental devices can conduct consolidation,breakthrough and bender element test on a same soil specimen.To solve the above problems,a Rowe cell type Consolidation-Breakthrough-Bender Elements column(CBB column)was developed.Ottawa sand and Glass beads-salt mixtures were then applied to conduct the verification test for CBB column.At last,consolidation-ammonia nitrogen breakthrough-bender elements coupled test was carried out on soil-bentonite through CBB column.The following findings were obtained:(1)The consolidation-internal erosion-bender elements test for glass beads-salt mixtures were carried out through CBB column.The results indicated that:shear wave velocity decreased mostly in the first pore volume.The reduction of particles contact number and growth of void ratio were the main reasons for a sharp decrease of shear wave velocity after internal erosion.For mixture with low salt content,particles rotated and force arches were formed from the beginning,the force arches structure was stable during the whole internal erosion process.K0 and pore volume increased after internal erosion.For mixture with high salt content,particles were interlocked and K0 decreased at the early stage.But the particle interlocking broken and force arches were formed with more salt dissolved,so the K0 further increased.After internal erosion ended,the anisotropy of both Mixture-1 and Mixture-2 decreased,and a "honeycomb fabric"appeared.Angularity of salt particles were dissolved preferentially.The mixture undergoes four stages in the seepage process:break of cementation,passivation of salt particle,decrement of salt particles diameter,and stabilization of the shear wave velocity.Shear wave velocity reduced most significantly at the first two stage and stabilizes at a time when the salt had not been completely dissolved.(2)The swelling index of Gaomiaozi and Wyoming bentonite in COD,ammonia nitrogen and lead solution were measured.The results suggested that:the high concentration of ammonia nitrogen in landfill leachate greatly reduced the swelling capacity of Gaomiaozi and Wyoming bentonite.Yet,due to the concentration of lead ion from leachate,ammonia nitrogen and lead ion from the contaminated groundwater was low,which was lower than the critical ion strength(221.5mM for monovalent cation,7.45mM for divalent cation)required to reduce the expansion index of bentonite,the swelling index of bentonite didn't reduce in the above three solutions.(3)The hydraulic conductivity,effluent concentration and shear wave velocity of the soil-bentonite under 30kPa K0-consolidation and ammonia nitrogen breakthrough were measured.The adsorption process of ammonia nitrogen by Gaomiaozi bentonite was studied on the aspect of chemical reaction and micro level.The results suggested that:after 0.5 pore volume of ammonia nitrogen was injected,the ammonium ions in the soil pore water gradually diffused into the double layer of bentonite and made it shrinkle.Meanwhile,the preferential flow channel was gradually generated at the place where the pollutant had flowed,which leaded a sharp increment of the soil hydraulic conductivity.The dramatic increase in hydraulic conductivity further accelerated the migration of ammonia nitrogen,making the concentration of ammonia nitrogen in the pore water increased significantly.When the ammonia nitrogen concentration in the pore water rose to a certain level,the bentonite flocculated,and fine particles transported through the widened dominant flow channel and flowed out,which together caused the increament of shear wave velocity after the sample was breakthroughed and a decrease of the fine particle content after the experiment ended.After four pore volumes,the soil-bentonite specimen was breakthroughed.The hydraulic conductivity of soil-bentonite specimen had increased by two orders of magnitude after eight pore volumes.The results of EDS test showed that during the adsorption of ammonia nitrogen,the sodium,potassium and magnesium ions of the bentonite were replaced by ammonium ions.The ion hydration radius of ammonium ions is slightly smaller than that of sodium ions.The larger the hydration radius of monovalent cations,the lower the hydraulic conductivity of bentonite is.Coupled with that the loss fine particles,so the hydraulic conductivity of soil-bentonite couldn't recover to its initial value after flushing.