Applications For Modelling In Landfill Design

Numerical and analytical models are used to address some issues in the design of landfills: the evaluation of hydraulic conductivity of aquitards; the interactions between the engineered and the natural hydrogeological systems; controlling leachate mounding; and the contaminant impact assessment under uniform, non-uniform and transient flow conditions. Design charts and equations (or correction factors) are presented. Simple analytical flow and transport models are also developed. These simple models are found to be efficient and economical to use in the design of landfills; and, they give comparable results with sophisticated numerical models.;The ratio method used in the interpretation of pumping tests to evaluate the bulk vertical hydraulic conductivity of an aquitard has some limitations (or errors). To overcome these limitations, the response of a piezometer (installed in an aquitard) to a pumping test in an adjacent aquifer has been studied using a finite-element technique based on Biot theory. Anisotropy of the hydraulic conductivity of the aquitard is also considered. Correction factors are presented to correct the errors due to time lag and the length effect associated with the simple piezometers used in pumping tests. The effects of the borehole diameter and the presence of a sand pack are also studied.;Design charts and equations are developed to design the layout of leachate extraction (purge, pump) wells which may be used, as a backup to the leachate collection system, to control leachate mounding in landfills, if there has been a failure of the leachate collection system or if there is no leachate collection system. Anisotropy of the hydraulic conductivity of the waste, the leakage through the liner and the plugging of wellbore are considered. Correction factors are presented to account for the plugging of the wellbore. The effects of having a higher leachate level in the well are also investigated.;Simple analytical flow models are developed for two-layer and multi-layer aquitard-aquifer systems. To solve the equations and the iterative procedure involved in these models, two computer programs are developed. These flow models are considered suitable for sensitivity studies to assist in the preliminary design of landfills, initial calibration of hydrogeological models and to prepare velocity input for transport models. The interactions between the engineered system and the natural hydrogeological system of landfills are studied using these models. The potential for shadow effect is also examined for two hypothetical landfills.;The application of simple transport models, such as POLLUTE, in modelling landfills which are engineered with drainage layers is verified. An approximate method is developed to model non-uniform flow field using POLLUTE. The use of POLLUTE to model transient flow is also examined. The verifications are done by calibrating POLLUTE against the equivalent finite-element (LTG) models