Subsidence and Differential Settlement
Settlement is the most easily identified result of landfilling. Typically, settlement result in obvious consequences which can be planned for or corrected. Generally, 90% of the anticipated settlement will occur within the first five years after placement of solid waste in a sanitary landfill.
Subsidence
Subsidence (uniform settlement or sinking ) of the entire fill occurs slowly over time. Subsidence is caused by:
Weight of fill (related to height)
Decomposition of wastes resulting in less volume
Poor initial compaction
Differential Settlement
Non-uniform settlement of certain filled areas is called differential settlement. Resulting depressions may be large or small and occur randomly throughout the landfill with time. Problems created by differential settlement include water ponding, interrupted surface water flow, increased leachate production, increased landfill gas generation, and reduced vegetative growth. Differential settlement is caused by:
Traffic
Nonuniform compaction or no compaction
High organic waste placed next to inorganic or inert waste resulting in differing rates of decomposition
Settlement of materials or the creation of voids by decomposition.
Subsidence and Settlement Control
Methods to control subsidence and Settlement include:
Good compaction
Separate waste types and provide additional compaction effort as necessary
Crush bulky wastes on solid surfaces prior to placing in fill
Keep working area smooth and uniform
Buildup roadways with inert materials
Grade and shape surface areas to promote run-off
Fill depressions with clean fill (dirt) as they become evident and grade to promote run-off
During site operation (prior to site closure) large depressions may be filled with easily manageable wastes (household) to establish grade.
Determining If Differential Settlement Is Occurring
Recognition of Settlement is done by visually observing the fill for:
Cracks in cover (tight soils)
Visible depressions
Creation of high water lines
Standing water on filled and covered surfaces
Surveys by the landfill manager or the engineer when checking for compliance with design plans and specifications will also help identify both uniform and differential settlement.
Summary
Solid waste is not inert and in the presence of water, produces both leachate and landfill gas. In addition, settlement will occur as a result of decomposition (both differential and subsidence forms of settlement should be expected) and no manager can prevent any of the above occurrences. The manager, through knowledge and ability, must be prepared to minimize the potential impact of the resultant leachate, landfill gas and/or landfill settlement.