Settlement characteristics of domestic waste

The composition of waste has changed and continues to change dramatically. These variations are seasonal and dependent upon the catchment area of the waste. The geotechnical properties of a lanndfill are determined by its waste composition amongst other factors such as initial waste placement density and age of the waste. With the mounting pressure from government and environmental groups to redevelop brownfield instead of greenfield sites, comes the increased probability that landfill sites will soon be targeted for redevelopment. Therefore it is important to know whether the site under consideration will have suitable foundation properties before construction takes place. It is also essential for developers to be able to identify how stable the site will be once construction has taken place, i.e. settlement of the landfill beneath the development. The important parameters influencing the settlement characteristics of a closed landfill site are: initial density, long term physical settlement due to overburden pressure, and settlement due to long term chemical and biological degradation. Initial density and long term physical settlements are dependant upon the waste composition and placement method. A higher initial density means that less physical settlement will occur, and so reduces the total overall settlement. Settlement due to long term chemical and biological degradation are influenced by the waste's composition, temperature, moisture content and the aerobic and anaerobic bacteria present. The identification of settlement due to chemical and biological degradation and that due to physical compaction is difficult to distinguish. However, the addition of inert waste results in an increase in the initial density of waste and may reduce the chemical and biological interactions and hence the overall long term settlement of landfills. In all modes of settlement the composition of the waste and its density are of the utmost importance. In long term physical settlement it determines the strength and geotechnical properties of the waste, in long term chemical degradation it provides the ions for exchange reactions, and in long term biological degradation it is the organic content that bacteria degrade to produce landfill gas, reducing the volume of waste in the landfill. Laboratory based experiments were conducted to study the settlement and stress distribution characteristics of domestic waste. A columnar cell was filled with domestic waste of known composition and density and subjected to stresses comparable to those found on a landfill site. The surface settlement and internal stresses of the waste column due to the applied stresses were recorded, and the settlement properties of domestic waste analysed.