Use of recycled concrete for a multi-layer landfill cover without geomembrane under humid climate : numerical modelling

Abstract

A final cover system is a crucial component for limiting negative environmental impacts of a closed landfill. Previously, a novel three-layer landfill cover system has been proposed to serve under all-weather conditions without the use of geomembrane. Existing studies have verified the effectiveness of the cover system constructed with recycled concrete aggregates (RCAs) under heavy rainfall conditions in both laboratory and field. However, no systematic investigation has been carried out to optimize the particle size combination of RCAs for the cover system. Therefore, two numerical parametric studies were conducted in this thesis. The main objective is to assist engineers in designing a multi-layer landfill cover system without geomembrane under humid climate conditions by using an easily controlled soil parameter D₁₀.

In the first parametric study, water infiltration into the three-layer landfill cover system under a 100-year return period rainfall was simulated at an inclination angle of 3°. The cover system consisted of a fine-grained RCA (FRC) layer, a coarse-grained RCA (CRC) layer, and a clayey silt (CS) layer from top to bottom. Different particle sizes, as characterized by the D₁₀ value, were considered in each layer, but the particle size induced changes on the water characteristic curve were ignored for the same type of material. The computed results show that particle sizes of the top FRC and middle CRC layer of the three-layer cover system are less important in controlling the amount of percolation under the extreme rainfall event. This implies particle size selection of these two layers may be based on the availability of materials (e.g., using sand and gravel). In contrast, particle size of the bottom CS layer has a determining influence on the performance of the cover system. By increasing the D₁₀ of bottom layer from 0.002 mm to 0.02 mm, about 29 mm of percolation is resulted, which is slightly lower than the percolation limit suggested by the USEPA. This indicates the three-layer cover system can still minimize water percolation under a 100-year return period of rainfall without geomembrane, provided that the bottom layer should have a D₁₀ value smaller than 0.02 mm (e.g., silt).

Based on the above findings, the three-layer cover system was reduced to a two-layer design in attempt to simplify construction procedures and reduce the usage of fine RCAs. This is done by combining the top FRC and middle CRC layer into one single recycled concrete (RC) layer. In the second parametric study, water infiltration into the proposed two-layer cover system was simulated under the same condition as before. The influence of particle size on the cover performance were investigated. Results from the study indicate that a large particle size (i.e., D₁₀ > 3.4 mm), such as coarse gravel, should be used in the RC layer to improve the performance of the cover. In addition, a D₁₀ value of 0.006 mm could be used in the bottom CS layer to further reduce the amount of percolation. The computed results have suggested that the proposed optimum design for the two-layer landfill cover can perform reasonably under the 100-year return period rainfall without geomembrane. However, to account for addition rainfall events in reality, a D₁₀ smaller than 0.02 mm (e.g., silt) is recommended for the bottom CS layer.

Date of Award	2022
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Charles Wang Wai NG (Supervisor)