Influence of composition and degradation on the shear strength of municipal solid waste

This study aimed to evaluate the shear strength of municipal solid waste (MSW) of different landfilling ages exhumed from disposal sites in a subtropical humid environment. Wastes which had been landfilled from ages of 2 up to 25 years were characterized using physical, chemical and biochemical tests and were tested in a large scale direct shear device. The results indicate that the tested wastes older than five years had reached similar decomposition stages, but showed different compositions in terms of soft plastics, incompressible material and reinforcing elements. Different composition was also noticed between less degraded and more degraded samples. In the former, the soil-like materials, that is the particles smaller than 19 mm, are essentially reinforcing components while in the later it is formed mainly by incompressible components. Although MSW composition did not vary significantly throughout the years, some difference in the originally landfilled waste could account for the observed variations. However, they are mainly the result of exhuming and preparation methods, whose influence is discussed in the paper, as well as the waste degradation state. The reinforcing components, rather than the soft plastics content, correlated well with cohesion intercept increase, both for the less and more degraded waste samples. The results also indicate that as MSW degrades the waste material evolves from an initially highly cohesive material to one that loses cohesion yet gains in shear strength angle over time.