Effect of aging on the physical properties of landfill cover layers

Physical properties of soil material are essential criteria for the suitability of material to be used as cover layers or water retaining (ET) layers of landfills. Important parameters, such as available water capacity and saturated hydraulic conductivity, are usually derived from easily measurable properties (such as soil texture) with the help of tables, or are measured on artificially compacted samples in the laboratory. Both methods do not consider structural changes taking place mainly in the first years after installation. Key factors for the development of the soil structure are freeze-thaw cycles, swelling and shrinkage due to moistening and drying, and the influence of root growth. The investigation was carried out with dredged material (river sediments) which was planned to be used for a landfill cover layer. Freeze-thaw cycles were simulated for a few days each in a laboratory freezer; swelling and shrinkage was simulated by alternating between water saturation and complete drying in a drying oven. The vegetation experiment was carried out in the open on a site filled with 20 cm dredged material. The effects of the environmental factors result in a modification of the pore system. All variants showed a significant increase in air capacity and a significant decrease of the available water capacity at constant total pore volume. With respect to the suitability of the material for landfill cover layers, the results imply that that the legally specified minimum values for available water capacity should be rather increased due to a possible decrease over time. However, the average decline of the available water capacity of 6%v/v with time due to aging, and the assumed penetration depths of the aging processes in the upper third of the cover layer, would result in a rather small increase of a few decimeters in layer thickness necessary to achieve the water storage targets. More important seems the increase in air capacity due to aging processes, which is of considerable importance for the growth of plants especially in the upper part of the cover layer. The risk of too high soil density associated with too low air capacity for optimum plant growth, thus, is somewhat reduced due to the increasing air capacity with aging