Effect of interface transmissivity and hydraulic conductivity on contaminant migration through composite liners with wrinkles or failed seams

The leakage and the peak chloride concentration in an aquifer for a single composite liner facility is modelled for (i) a hole in a geomembrane wrinkle and (ii) a failed seam. A method using a closed-form solution to calculate leakage together with a l½-dimensional (l½D) semi-analytic contaminant transport model is proposed, and the results compared with those obtained from two-dimensional (2D) finite element modelling (FEM). Leakage is shown to be highly dependent on the interaction between the interface transmissivity (θ) and hydraulic conductivity beneath the wrinkle (kb). Similar leakages arising from different combinations of transmissivity and hydraulic conductivity are shown to have significantly different impacts on an underlying aquifer. Contaminant transport modelling is needed to assess this effect for the likely range of uncertainty regarding interface transmissivity (θ) and hydraulic conductivity. The 2D FEM is conceptually more comprehensive; however, using conventional software only a very limited size of problem could be accurately modeled given the greatly different scales that must be modelled. In contrast, the semi-analytic 1½D approach readily allowed consideration of the highly variable scales, and gave results at the down-gradient edge sufficiently similar to the 2D approach.

Influence of Shrinkage and Viscosity of Flowable Composite Liners on Cervical Microleakage of Class II Restorations: A Micro-CT Analysis

SUMMARY This study determined the influence of shrinkage and viscosity of flowable composite liners on the cervical microleakage of Class II restorations using micro-CT. Seven composites of varying viscosities were selected and included five giomers (Shofu Beautifil II [BF], Flow Plus F00 and F03 [F00 and F03], Flow F02 and F10 [F02 and F10]) and 2 nano-filled composites (3M-ESPE Filtek Z350 [Z350] and Filtek Z350 Flowable [Z350F]). Polymerization shrinkage (n=7) was assessed with the Acuvol volumetric shrinkage analyzer while complex viscosity was determined with the advanced rheometric expansion system at 25°C. Standardized Class II restorations incorporating 1-mm horizontal layers of different flowable liners and 3-mm oblique layers of BF or Z350 were subjected to a silver nitrate test for 24 hours and examined using micro-CT. Microleakage was determined at 0.1-mm intervals from the buccal to lingual surfaces providing 30 sites per specimen and scored accordingly. Statistical analysis was performed with the one-way ANOVA, Kruskal-Wallis test, and Spearman's rho correlation at a significance level of p<0.05. Mean volumetric shrinkage ranged from 5.33±0.17% to 2.35±0.02% for F02 to Z350, respectively. The flowable materials had significantly higher shrinkage than did their sculptable counterparts (BF and Z350). Complex viscosities ranged from 9.65 to 4.20 (Z350 and F10, respectively) at a frequency of 10 rad/s and from 8.16 to 3.28 (Z350 and F03, respectively) for 100 rad/s. Giomer restorations had significantly less leakage than did those restored with nano-filled composites. No microleakage was observed with restorations lined with F02 or F10. The use of flowable liners reduced cervical microleakage of Class II restorations. Interfacial integrity of Class II restorations was significantly correlated with liner viscosity, filler volume, and shrinkage.

Field measurements of overlap reductions for two reinforced fabric-encased geosynthetic clay liners (GCLs)

Two geosynthetic clay liners (GCLs) reported to have experienced significant shrinkage at other locations are examined on both a 3H:1V south-facing slope and a relatively flat base on a silty sand. The GCLs were overlapped by 300 mm with 400 g/m of supplemental bentonite and covered by a black 1.5 mm high-density polyethylene (HDPE) geomembrane to form a composite liner that was left exposed in a full-scale field test embankment for a period of almost 5 years. It is shown that despite the relatively uniform exposure conditions, shrinkage is highly variable with a maximum shrinkage of GCL A being 165 mm on the slope and 415 mm on the base while GCL B shrunk by up to 75 mm on the side slope and only up to 25 mm on the base. The dominant role played by variable overlap stick and heterogeneity to the locations where the overlaps are re-wetted are discussed. Based on this study of shrinkage and a related study of downslope erosion at the same site, it is concluded that neither GCLs A nor B should be left in exposed composite liners when they can be subjected to thermal cycles that can lead to hydration and dehydration of the GCL.