The operator of a modern landfill site in responding to the requirements for minimising leachate generation is likely to fill the site rapidly within small constrained areas to reduce rainfall ingress. When the operational cell is complete to restoration levels it is likely to be capped to prevent further ingress of rain water. This means that even the oldest waste at the bottom is therefore likely to be in only the early stages of degradation.
Significant settlement can then be expected due to the following mechanisms:-
i. The load on waste in the lower levels imposed by waste above it, particularly for deep sites, will be several times greater than that imposed during the initial compaction process using mechanical compactors. This will result in continued, graded compaction through the waste. This mechanism for settlement is likely to be predominant during filling and immediately following capping.
ii. The degradation process will break down waste into a denser material.
iii. The production of gas will mean a net mass loss of possibly 18% assuming 150m3/hr of landfill gas at 1.15kg/m3 is abstracted from each tonne of waste.
iv. Removal of leachate from lower levels of waste can also cause further settlement as pore pressures are reduced.
Settlement is therefore inevitable and must be catered for in the design of the restoration capping and in the design of both gas abstraction system. It is the job of the landfill design Engineer to assess the site “condition” and determine the potential for further settlement so that he can be satisfied a suitable design is proposed.
The CQA Engineer will verify the degree of settlement on-site through the contractor’s survey etc and ensure that the landfill design Engineer’s requirements are implemented during construction of the restoration capping.