Predicting the diversion length of capillary barriers using steady state and transient state numerical modeling: case study of the Saint-Tite-des-Caps landfill final cover

Abstract

Covers with capillary barrier effect (CCBE) have already been proposed to meet regulatory requirements for landfill final covers. Modeling of CCBE can be a relatively complex and time-consuming task. Simpler, albeit conservative, design tools — such as steady state numerical analyses — can, in certain cases, be justified and have a positive impact in practice. In this study, numerical simulations were performed of the experimental CCBE constructed on the Saint-Tite-des-Caps landfill (Quebec). The CCBE consists of a capillary barrier, composed of sand and gravel, on top of which a layer of deinking by-products (DBP) was installed as a protective layer (also to control seepage). The addition of a protective layer over the infiltration control layer (such as a capillary barrier) is required in most jurisdictions. In many European countries, such as Germany and the Netherlands, a thick “recultivation” layer is required. The results of numerical simulations were compared with the in situ behaviour of the Saint-Tite CCBE as well as with analytical solutions. The effectiveness of the capillary barrier was assessed by quantifying the diversion length (DL), which reflects the lateral drainage capacity of the CCBE, i.e., the capacity to drain water laterally. Collection of the water that has drained laterally prevents seepage into the waste mass. This study shows that when the seepage rate reaching the top layer of the capillary barrier is controlled, it is possible to predict the worst-case scenario in terms of seepage (and therefore predict the shortest DL) using steady state numerical simulations. These simpler-to-perform numerical simulations could be adopted in practice, at least in a pre-feasibility study for cases with a similar profile as the one at the Saint-Tite-des-Caps experimental CCBE.