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 25years 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 19mm, 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.

Characterizing a Brazilian sanitary landfill using geophysical seismic techniques.

Two different geophysical techniques, namely crosshole and multichannel analysis of surface waves - MASW, were applied to investigate the mechanical response of Municipal Solid Waste buried under humid, subtropical climate. Direct investigations revealed that the buried waste was composed mainly of soil-like material (51%) and plastics (31%) with moisture content average values of 43% near the surface and 53% after around 11m depth. Unit weight varied between 9kN/m(3) and 15kN/m(3). Seismic investigation of the landfill yielded shear wave velocities (VS) estimated from the crosshole tests ranging from 92 to 214m/s, while compression wave velocities (VP) ranged from 197 to 451m/s. Both velocities were influenced by vertical confining stress and thus tended to increase with depth. VS calculated from MASW tests were lower than the ones calculated from the crosshole tests, probably due to the different frequencies used in the tests. The results of both methods tended to configure a lower bound to the values reported in the technical literature in general, as expected for low compaction waste with small amounts of cover soil. Although VS did not show abrupt changes with depth, VP profile distribution combined with direct investigations results, such as temperature, in-place unit weight and moisture content, suggest that the waste body could be divided into two strata. The lower one is poorly drained and shows higher moisture content, as a consequence of the operational techniques used in the first years, while the upper stratum is probably related to a better drained waste stratum, resulting from the improvement of operational standards and increase in drainage facilities throughout the years.