Technical Note 2:
Solar Energy Facilities on Unstable Substrates
Technical problem needing to be solved : The highly-variable, random and non-predictable nature of landfill settlement and subsidence rates found across and within disused landfill sites is due to the inherent heterogeneity of landfill composition deposited at varying depths, for varying times and for the different mixtures of the waste materials. This can literally result in potentially different substrate settlement rates (and thus changes in solar panel orientation) being experienced by each of many 1,000’s of solar panels on a given landfill site. The occurrence of on-going settlement of substrate is totally unavoidable, even after decades since cessation of landfill activities. NB: Soil settlement is usually measured in geological time, not years and decades. Thus, one is faced with dramatically-complex geotechnics on disused landfill sites, namely, high-dimensional mathematical space.
This phenomenon is complicated because of the involvement of a multitude of factors that eventually affect surface settlement rates and surface deformation (Mechanical instability, subsidence, cracking, erosion) that can each vary over time as a function of each of the following interrelated phenomena:
Figure 1: Cross-section of stability and deformation concerns in landfill.
NB: No. 9: Differential Settlement
1. Heterogeneity of waste material, including often considerable variance within and across the landfill mass as a function of the time and duration of deposition of different waste mixtures.
Figure 2: Major categories of landfill likely to undergo variable settlement rates across a given landfill site durig a SEF project. In turn, these are influenced by the gravimetric and volumetric water content of landfill waste.
Figure 3 : Schematic representation of typical time-settlement data for landfill under vertical stress. The dominant mechanisms governing the secondary settlement of waste materials are mechanical, ravelling, physicochemical change, biochemical decay, and interaction among these mechanisms.
2. Water infiltration: Stability problems are often the consequence of excess water percolation into landfills. The excess water results in saturation of the waste mass changing the unit weight of the waste body creating an excess pore water pressure. Similarly, the water saturation of void volumes may cause a build-up of gas pressure and alter gas flow rates.
3. Leachate (Leaching of organic and inorganic materials by water): Internal leachate pressures; the standard practice of leachate recirculation; movement of leachate through the landfill fill; and leachate exudation from landfill mass.
4. Engineering of landfill prior to waste deposition activities is designed to contain and manage waste body, this may include impermeable geomembrane barriers underneath landfill installed prior waste deposition and / or a geomembrane cover designed to prevent water infiltration after site closure; or no such pre- and post- engineering apart from mechanical compaction of surface layers. The extent of depollution and decontamination works post-closure can also impact this scenario.
5. Form of landfill mass and the effects of different heights of the waste column on settlement and subsidence due to both vertical and lateral compression, e.g. flat, undulating or knoll-shaped hillocks and thus different depths of refuse between the periphery and the central zone, for example.
6. Risk of flooding, particularly for culverts and valleys nearby a water course. This can result in an exaggerated impact of flooding and minor seismic activity on unstable landfill substrates. Such landfill sites are regularly exploited for landfill, e. g. commonly encountered in major population centres across Africa.
7. Pluviometry and resultant erosion due to stormwater run-off on external slopes (greater lower-down slope); and /or on the downside of individual solar panels or ballast systems (concrete, enclosed rubble or soil) employed on capped landfill to replace panel supports that would otherwise penetrate through cover geomembrane.
8. Decomposition of organic matter (aerobic and anaerobic), the secondary phase of which can last for up to 30 years.
9. Subterranean water flows
10. Duration of waste deposition at landfill site
11. Time elapsed since closure of landfill site.
12. Subterranean gas accumulations occurring as air spaces or interstitially, such as, predominantly methane and carbon dioxide and to a lesser extent nitrogen, oxygen, and other species. The presence of these gases alter settlement rates, as does their escape from landfill.
13. Movement of dissolved materials by concentration gradient and osmosis.
14. Movement of liquids caused by differential heads.
15. Differential settlements caused by consolidation of materials into voids.
16. Temperature and pH which directly influence levels of methane production which in turn influence settlement rates.
17. Subsurface soil properties, i.e. geology beneath landfill and / or underlying land topology, e.g. an open-ended naturally-occurring ravine.
18. Tensile properties of burnt or unburnt waste materials which vary widely with respect to waste composition and the interlocking of various fibrous particles, i.e. when shear stresses are mobilised.
19. Compressibility of various waste materials.
20. Shear Strength of landfill is difficult determine due to the heterogeneity of disposed wastes, the difficulty in obtaining and testing representative samples, time-varying properties, and strain incompatibility between the waste mass and the and underlying materials.
21. Slope stability as a function of slope geometry (slope inclination, height, intermediate embankment, etc.); geometry of various layers of the slope ; mechanical characteristics of constitutive materials of various layers of the slope; the base; and conditions of pore water pressure and/or dynamic stresses in the slope.
22. Gas Permeability and flow path within waste mass (Diffusion between fissures and the surrounding waste matrix) and its variance with respect to the depth of the associated waste column.
23. Moisture content with respect to micro- and macro- porosity and the extent to which these are saturated or otherwise (The concept of double porosity) and their impact on hydromechanics within the landfill mass and their variance with respect to the depth of the associated waste column.
24. Solid density and porosity of individual waste components.
25. Extent of waste degradation by either or both aerobic and anaerobic means, which in turn are much influenced by air being pumped into landfill (‘aeroisation’) and / or leachate recirculation.
26. Physical compression due to bending, pressing reorientation and crushing; ravelling settlement due to migration of particles; consolidation phenomenon and viscous behaviour; decomposition settlement due to organic components; and collapse of components due to corrosion, oxidation and degradation of inorganic components
27. Variation in particle size of waste components: Small particles will tend to move more easily into voids than their larger counterparts, causing additional settlement.
28. Loading conditions: Effective stress path, i.e., drained and un-drained; and type of loading, i.e., magnitude, rate (static, dynamic), and time history (monotonic, cyclic).
29. Physicochemical changes, such as, corrosion, oxidation, and combustion.
30. Landfill barriers are a necessity in engineered landfill, but they in turn also interact with the adjacent waste mass to modifying stress conditions.
31. Strength and stiffness parameters (shear modulus, shear strength) impact settlement rates as a function of the waste column height.
32. Compressibility of waste, including anisotropic variability.
33. Gas explosions, vibrations and liquefaction effects and their impact as catastrophic changes to surface topology, e.g. craters and landslides.
When the above are measured in isolation or estimated mathematically alone or together, at no point are conclusions able to be made concerning landfill surface deformation or their potential impact on solar panel tilt. Thus, the use of micro-inverters to conduct continuous performance analytics on individual solar panels is an appropriate technical solution in the face of unpredictable landfill geotechnical outcomes and resulting landfill surface modifications. In the absence of such, it can be assumed that the majority, if not all, solar panels installed (without microinverters) in “string series” will undergo suboptimal energy production during the duration of a given SEF project situated on landfill. Projected energy production rates are rendered unreliable and border on the fanciful using traditional methodologies for the estimation of solar energy production over time, i.e. in the absence of high-granularity panel monitoring. Micro-inverters in conjunction with LanneSolaire’s monitoring solution will obviate investor uncertainty and provide a means to deliver reliably against predicted levels of energy production by a focus on effective Operations and Maintenance of SEF’s situated on landfill.
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