The mass of waste and the subsoil are subjected to deformations due to the progressive dumping of waste on the site and the consequent reactions of waste and subsoil. These deformations are known as “displacements”. The displacements can be horizontal or vertical. The vertical displacements are commonly called “settlements”. Due to the different landfill geometry and waste characteristics, the rate of settlements can differ from an area to another. This kind of settlement is referred as differential settlement. The quantification of the landfill displacements is essential for the design and the management of a landfill. More in detail, a basic knowledge of the displacements mechanisms (and the corresponding magnitude) is essential for the following aspects:
- Proper design of the bottom liner and the capping liner system
- Understanding of the potential failure mechanisms
- Evaluation of the effect of the displacements (mainly horizontal and uplift movements) on the surrounding structures
- Evaluation of a possible site requalification
Pollux Consulting evaluates the displacements by performing a stress-strain analysis with the renowned software package Plaxis 2D/3D.
Mechanisms of deformation
In organic landfills, 2 main phenomena contribute to the deformation of the waste:
- The biodegradation of the organic fraction of the waste
- The mechanical compression provoked by the landfill mass build up
In all the other types of landfills, the first phenomenon is absent and therefore only the mechanical compression occurs.
The mechanical compression can be further divided in:
The instantaneous compression : this is a short duration stress dependent deformation (typically a couple of days). The deformation is exclusively provoked by the waste overloading.
Even though this mechanism is not time dependent, the deformations can be plotted against the waste filling time.
The primary compression : this is a stress and time dependent deformation. The deformation of the material is a function of the capacity of the soil to dissipate the pore water pressure (material hydraulic conductivity) and so to convert the total stress in effective stress.
- The secondary compression : this is a purely time dependent deformation at constant stress (creep mechanism). The deformation takes place due to the compaction and redistribution of the voids in deformable materials. There are 2 ways of thinking with regard to this mechanism: the first one assumes that the secondary compression runs “in parallel” with the primary compression; the second assumes that the secondary compression starts only after the primary compression.
In organic landfills, the biodegradation is typically englobed in the secondary compression. This means that the parameters of the creep models are calibrated for this “source” of deformation. It is important to underline that biodegradation doesn’t cause a mechanical compression of the waste. Organic waste matter is converted into landfill gas and escapes but there is no intrinsic variation of waste density ; waste lying on top just settles to fill in the voids.