Service Life Of Vertical Cutoff Walls For MSW Landfills And Determination Of Its Thickness

Pollution caused by leaking of leachate from simple dump is more and more be concerned. Comprehensive treatment is commonly used measures in remediation actions of simple dumps. Vertical cutoff wall is an effective barrier for pollution control and is widely used in remediation practice. However, research for the service life of vertical cutoff wall in landfill is rarely reported.Firstly, in this thesis, the migration of leachate in natural soils strata of the dump and influencing factors were simulated in order to acquire the distribution of contamination plume in the soil atrata on the basis of previous field and laboratory studies. And long-term simulation was also carried out to predict the migration of contamination into the soil strata Then, taking COD as pollutant, two-dimensional simulation model was set up based on former analysis and the effect of factors, including leachate head, width, depth, permeability of wall, geological condition, on the service life of plastic concrete vertical cutoff walls was analyzed. Eventually, simplified design formula for width of plastic concrete vertical cutoff walls was presented on the basis of Ogata-Banks' analytical solution and some assumptions. The main conclusion is as follows:(1) Results of migration of contamination into the natural soil strata of Huainan landfill show that advection is the predominant mechanism controlling the migration of contamination into the soil strata under this dump with a high leachate head. This indicates that lowering the leachate head and weakening groundwater seepage are the most economical and effective measures for pollution control and treatment. It implies that vertical cutoff walls could be used as barrier in remediation action because of perfect performance of vertical cutoff walls in controlling groundwater seepage.(2) Results of analysis for service life of keyed-in plastic concrete cutoff wall show that the effect of factors, including leachate head, permeability, width, on performance of plastic concrete vertical cutoff walls was significant. The less of cross-head, lower permeability of cutoff wall and wider of width, the longer service life of cutoff wall. If the hydraulic permeability of wall reaches as low as 1×10-/ cm/s and is keyed into aquitard layer, the groundwater flow field can be approximated to one-dimensional horizontal seepage. And diffusion has equivalent effect on wall with advection, which indicates that the diffusion can not be neglected in determining width of cutoff wall. It is suggested that service life of cutoff wall can be prolonged by controlling cross-head, strengthening the shallow part width and lowering permeability of the wall.(3) Results of analysis for service life of hanging vertical cutoff wall show that the geology condition and depth of wall have significant effect on performance of cutoff wall besides the leachate head, permeability and width of wall. The results also show that contamination can migrate to downstream soil strata through the bottom of cutoff wall. It implies that the service life of hanging cutoff wall cannot be prolonged by lowering the permeability of wall. Conversely, the depth of wall should be increased to prolong the service life of wall and it should be determined by two-dimensional advection-diffusion analysis of evaluating the effect of depth on performance of wall.(4) Conclusions of analysis for service life of keyed-in vertical cutoff wall show that it may be unsafe for pollution control if only advection is considered in determining the width of wall by commonly used formula The simplified design formula for width of plastic concrete vertical cutoff walls presented in this thesis, which is on the basis of Ogata analytical solution for one-dimensional advection-diffusion transport, takes into consideration both diffusion and advection and makes the cutoff wall more safety and reasonable in practice.