Lysimetry has been acknowledged in the literature as the most effective means of assessing deep percolation rates through landfill final covers. Designing lysimeters requires several tools, including thorough laboratory testing and numerical modelling. This paper focuses on a fundamental boundary condition, namely breakthrough suction, and how it affects lysimeter design. This study was triggered by a multi-year database of suction and seepage data collected from a large-scale lysimeter constructed at the St-Nicephore landfill, Quebec, Canada. Tensiometers were installed at several levels near and at the interface between the cover material and the drainage layer inside the lysimeter, and outflow was measured using tipping counters. Based on field results and an analysis thereof, different geometry scenarios were simulated with HYDRUS-2D, and it was observed that in lysimeters designed using the linear method, breakthrough suction at the lower boundary can result in a greater than 90% percolation performance. With climate change, lysimeter percolation rate estimates may also be affected by rainfall events of varying magnitudes. Lastly, we were able to assess the existing design procedures and suggest a simple and conservative approach.
Keywords: Lysimeter design; breakthrough suction; final covers; landfill; large-scale lysimeter.