A Method For Evaluating Performances Of Composite Liners Considering Random Distributions Of Holes And Wrinkles

In order to keep leachate away from the surrounding environment,horizontal liner systems should be constructed at the bottom of sanitary landfills,and composite lines made up of Geomembranes and other materials are common types of lines.Previous researches form both domestic and overseas on the performance of composite liners neglected the random distributions of holes and wrinkles,which can not objectively reflect the actual conditions.Considering random distributions of holes,this study established a leakage evaluation method based on aerial photographs of wrinkle networks.Considering random distributions of holes,this study proposed to simplify wrinkles into polylines and use the statistical parameters of geometric parameters of wrinkles to describe the spacial distributions of wrinles in a wrinkle network,and then a leakage estication method was proposed for the initial design stage of composite lines based on the statistical rules of the spacial distributions of wrinkles.Furthermore,this study proposed to obatain the Darcy velocity in composite liners by incorporating the influence of the random distribution of holes and wrinkles and averaging the total leakage over the wetted aera of the liner when estimating the breakthrough time of the liner.Moreover,the study proposed a breakthrough time based failure probability estimation method.For illustration,failure probability estimation and sensitivity analysis have been conducted for 2m compacted clay liner.In addition,a centrifuge test device for modeling the long-term performance of vertical barrier wall was developed,and a 43.8h centrifuge test under 100g was conducted to model the performance of the a silt-bentonite wall and a losses-silt-bentonite wall after 50 years of service.The following conclusions have been made:(1)The leakage evaluation method based on aerial photographs of wrinkle networks uses the actual lengths of hydraulic connected wrinkles,which is capable of objectively reflecting the effect of various factors(e.g.,the hydraulic conductivity of the GCL,interface transmissivity between the GMB and the underlying GCL,the hydraulic gradient through the liner,and wrinkle width)on the hydraulic connectivity of the wrinkle network.This method considered the random distributions of holes,which was demonstrated by case study to have a strong influence on the evaluated leakages.When the hole frequency is relatively small,the evaluated leakage can be 30 times larger if random hole distributions are neglected.The application of the method in field scale has been verified,in an indirectly way,by back-analysising the monitoring leakage data of one landfill in China.This method is very convenient to apply.And with the rapid devepment of UAV aerial photography technology,the proposed evaluation method has a broad application future.(2)Statistical analysis on the rules of spacial distributions of the three wrinkle networks reported in literature showed that the X coordinates of the center points of wrinkles can be approximately modeled by uniform distribution,and the Y coordinates of the center points of wrinkles can be approximately modeled either by uniform distribution or by triangular distribution,and the lengths of wrinkles can be approximately modeled by exponential distribution and the orientations of wrinkles can be approximately modeled by normal distribution.The multi-dimensional normal random field with Markov correlation function can be employed to model the spacial variability of wrinkle density.The statistical parameters obtained by this study can provide renferences for similar sites to be built in the future.(3)Comparing to the method based on aerial photograph of wrinkle network,the method based on the statistical rules of the spacial distributions of wrinkles tend to be conservative.The method ignoring the effect of the spacial variability of wrinkle density needs less input parameters,and has no application limits as to the size of the site.However,it has a relatively poor accuracy;the method considering the effect of the spacial variability of wrinkle density is more accurate,however,more input parameters are needed and it can only be applied to larger sites.The method conquered the problem of how to evaluate the leakage through composite liners at design stage when the aerial photograph of the wrinkle network is unavailable,which shows great application future.(4)According to different ratio of wetted area to the aera of the whole site,the breakthrough time by assuming the Darcy velocity is obtained by averaging the leakage over the whole site can be 1.4-15 times larger than that by assuming the Darcy velocity is obtained by averaging the leakage over the wetted area.The relationship between the ratio of wetted area to the aera of the whole site and the nomorlized breakthrough time is nolinear.The effective diffusion coefficient of toluene,the retardation factor of Cd2+,the distribution coefficient of toluene,and longitudinal dispersivity were found to follow a lognormal distribution.The porosity,dry density,and effective diffusion coefficient of Cd2+ were found to follow a normal distribution.For a wide range of sevice life(SL)from 30 to 150 years and for both toluene and Cd2+,the uncertainties in adsorption parameters(e.g.,Kd and Rd),longitudinal dispersivity and hydraulic conductivity have the most significant influences on the failure probability of compacted clay liners.Efforts and resources should be directed to the proper quantification of uncertainties in adsorption parameters,longitudinal dispersivity,and hydraulic conductivity in engineering practice.(5)The centrifuge test device for modeling the long-term performance of vertical barrier wall enables the formation of model wall from the slurry by consolidation under supergravity.The Marriotte bottle and overflow aperture can be used to control the hydraulic heads of the upperstream and downstream of the wall at a relatively steady state.High precision level gauges help achieve real-time monitoring of the flux.Numerical modeling of the profiles of Pb2+ at different depths shows that the width of losses-amend wall reaching the breakthough standard which is assumed to 10%of the source concentration,decreased 43%compared to that of unamend soil-bentonite wall.