TECHNICAL ARTICLE
Landslides in landfills
By A.O. Landva1 and S.J.E. Dickinson2
- TerrAtlantic Engineering Limited
515 Beaverbrook Court, Fredericton, NB E3B 1X6 Canada
Phone 506-460-8660, Fax 506-460-8679, Email arvid@terratlantic.nb.ca
- TerrAtlantic Engineering Limited
515 Beaverbrook Court, Fredericton, NB E3B 1X6 Canada
Phone 506-460-8660, Fax 506-460-8679, Email simon@terratlantic.nb.ca
ABSTRACT
In general, MSW is a non-textbook material, being highly fibrous, heterogeneous, erratic, highly compressible, and displaying significant long-term settlement caused by plastic creep and decomposition. Its strength is significantly affected by its fibrosity and its degree of decomposition. Neither undisturbed sampling nor field testing of shear strength is practical. Laboratory testing of more or less undecomposed MSW can be carried out meaningfully only in large-scale equipment developed specifically for geotechnical tests such as consolidation, simple shear, ring shear and tension tests. The effects of decomposition cannot, however, be accounted for by such tests, unless the test specimens represent truly decomposed MSW. The present study suggests that a weakened zone may often develop along the base of MSW landfills as a result of advanced decomposition there. If the material has been tested in its more or less undecomposed state, which seems always to be the case, it is likely that the measured strength is not relevant, being too high and thus leading to unrealistically high factors of safety.
1 INTRODUCTION
The geotechnical behaviour of municipal solid waste (MSW) has attracted increased attention over the past two or three decades as a result of several catastrophic landfill failures worldwide. For example, a waste slide in the Philippines in 2000 claimed more than 220 lives, with an estimated 200-800 people missing. In 1996, a failure of a landfill in La Coruna, Spain, led to more than 100,000 tonnes of MSW sliding nearly a kilometre, coming to rest at a short distance from the Atlantic Ocean and narrowly avoiding what could have been one of the greatest environmental disasters in history. Although widely separated geographically, these events had much in common. Both landfills were large and their ages (approximately 30 years) were comparable. The age issue is particularly interesting since, whereas one would expect the foundation soils to gain strength with time due to consolidation beneath the waste, it appears that the opposite is the case with MSW material, i.e. the material in the basal failure zones seems to become weaker with time as it continues to decompose.
Most of the as-placed constituents in waste fill have a high to extremely high aspect ratio (greatest dimension ÷ least dimension), i.e. the “degree of fibrosity” is generally high. The undecomposed MSW therefore behaves much like a reinforced material, having discontinuous “fiber” reinforcement generally oriented in a direction parallel to the bedding of the material as placed.
Since MSW material is subject to decomposition, its geotechnical properties will change with time. The amount of time involved will vary considerably, depending on the nature of the MSW constituents, from a few years to perhaps several hundred or even one thousand years. Weakening of the landfill will also result if (i) the MSW material is mixed with compost or other weak materials, (ii) if it is subject to smouldering fires, not uncommon in landfills, or (iii) if it has been subject to scavenging and thus removal of reinforcing elements such as construction debris.
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