Landfill Settlement and Predicting the Effect of Surcharging to Increase Landfill Void Part 2

This article is a review of the paper PREDICTING LONG-TERM SETTLEMENT OF LANDFILLS K. S. WATTS*, J. A. CHARLES AND H. D. SKINNER* *Building Research Establishment Ltd, Watford,at The Waste Conference, September 2006,  Stratford upon Avon, UK.

Watts and Charles (1990, 1999) have described the mechanisms of settlement in detail. These can be simply categorised as:

- Immediate compression of partially saturated waste - An application of load will produce an immediate compression in a partially saturated fill. This condition arises both during the initial filling operation and with the addition of further waste to an existing deposit.

- Creep compression of partially saturated waste - There is an approximate linear relationship between creep compression due to physical creep processes and the logarithm of the time that has elapsed since the time the fill was placed or since later surcharge was placed.

- Primary consolidation of saturated waste - The application of load to saturated fill (below the leachate level) may generate positive pore pressures. After pore pressures have dissipated, long-term settlement will continue due to physical effects similar to creep compression in a partially saturated fill.

- Bioconsolidation - A major long-term reduction in volume is likely to occur due to biodegradation of refuse fill. The settlement which is due to biodegradation will be much larger than physical processes and is likely to be affected by changes in the biochemical environment within the fill.

Thus, the overall long-term settlement of the refuse will be due to the cumulative effect of physical compression and biodegradation, but the latter process is likely to be the dominant cause of long-term settlement of any waste fill with a high organic content.

The magnitude and rate of settlement of refuse will be a function of complex interactions between physical and biodegradation processes and will depend on original composition, density and moisture content of the waste, the leachate level and time since placement.

The research described and measurements presented relate to sites where there are relatively high leachate levels. Whilst the behaviour of any landfill is likely to comply with the general model, the magnitude of movements may differ significantly and will rely on data specific to other situations. A higher proportion of unsaturated waste would be expected to result in larger mechanical and biological settlement components.

Actual field data is relatively easy to acquire and can be used to create both generic and specific models of behaviour. A major advantage of the methodology is that field monitoring may be commenced after the completion of a landfill and retrospectively fitted to the generic model.

It is emphasised, that settlement data obtained from the time of completion and information (dates) on the commencement, completion and capping of waste at any point in a landfill is vital to the process of accurately predicting future movements. The model used represented all lifetime settlement, rather than that due to the state of degradation alone.