Diversion Length Research Articles

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

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.

Assessment of the Design of an Experimental Cover with Capillary Barrier Effect Using 4 Years of Field Data

One important step in the design of inclined covers with capillary barrier effect (CCBE) is the determination of the water diversion length (DL). Numerical simulations can predict the DL more precisely than steady-state analytical solutions. Nevertheless, as simplified methods have always been part of engineering design, the application of analytical solutions with conservative boundary conditions, may allow engineers to make reasonable predictions, particularly during the pre-feasibility stage of a project. In this study, a CCBE was designed, constructed and instrumented at the Saint-Tite-des-Caps landfill, Quebec, Canada. This CCBE included a seepage control layer superimposing a sand-gravel capillary barrier. The seepage control layer was made up of deinking by-products (DBP), an industrial by-product that was previously disposed of as waste. The capillary barrier was designed using an adaptation of the Ross analytical solution and the scenario considered was that of steady-state flow during constant seepage flow applied uniformly at the top of the sand-gravel capillary barrier. Although these conditions appear simplistic, they were deemed reasonable because placement of the seepage control layer on the top of the capillary barrier led to very low suctions at the interface, thereby allowing uniform downward seepage rates, limited by the saturated hydraulic conductivity of the DBP. In this paper, a discussion about the behaviour of the cover system based on 4 years of field data from several instruments is presented. The challenge of using DBP, more precisely the settlement of the DBP layer and its impact on ksat, is also assessed. The DL was reassessed considering the new ksat. A discussion on the validity of employing analytical solutions to determine DL is also presented. This paper illustrates how certain variables affect the design of inclined CCBEs that include a highly compressible material as seepage control layer.