Geogrid reinforcement in harsh environments: their role in landfill slope veneer stability design and related performance aspects under high temperatures
Geocomposite liners of landfill barriers may experience harsh environmental conditions such as excessive loading and/or extreme high temperatures over their design life. Landfill covers and barriers are often installed on steep slopes. Smooth geomembranes have been used for decades in such structures while textured geomembranes are often chosen to allow for greater interaction between the individual geosynthetic materials and/or between the geosynthetic and the cover soil. In the first case, due to the often low available interface friction angles, very weak inter-surfaces should be considered in the veneer stability analyses. Whereas in the second case attention must be paid to the tensile load acting on the geomembrane. In fact, as some studies have demonstrated, even small strains can cause cracks in the geomembrane.
The use of appropriate geogrid reinforcement can play a decisive role to limit the tensile load in the geomembrane and to guarantee the global stability of the entire system. The serviceability of the geogrid has to be assured during the entire design life of the structure and the geogrid may also be exposed to very high temperatures.
In this paper, a comparative study of two different geocomposite liners including a smooth geomembrane and a textured geomembrane is conducted and the role of the geogrid in both systems is assessed. When assessing the performance of geosynthetic reinforcement the long term temperature regime also has to be taken into consideration. Therefore, an extensive laboratory test campaign has been performed to investigate the performance of polyester (PET) and polyvinyl alcohol (PVA) geogrids under normal temperatures and high temperatures up to 70°C. First results have shown that PVA geogrids perform significantly better under high long term temperatures compared to PET geogrids.
When a (HDPE) gmb is under tensile stress and/or shear stresses at the same time as oxidation the dynamics of degradation change and as indicated by Peggs and Rowe, geomembrane elements in a barrier system which is under constant load (tension) will deteriorate at an accelerated pace particularly when exposed to elevated temperatures. It can therefore be safely assumed that decoupling the barrier system from imposed loads and decreasing tension in the geomembrane will increase the expected service life.
There are concerns (Peggs) about double textured gmbs on side slopes where there is a higher shear resistance on the upper surface than on the bottom surface, which results in the liner becoming a load bearing member of the lining and/or cover system due to an induced shear stress. This relationship is an oxymoron because the liner is designed to be without stress but at the same time the texturing is provided to hold neighboring surfaces/soil layers. When slides do occur on slopes and gmbs rupture/tear it is often assumed that the reason is the movement of the soil. It is also possible that the gmb may experience stress cracking due to the induced shear stresses which in turn initiates soil movement. It is the opinion of Peggs that the use of smooth gmbs on the slopes will have a positive impact on the service life of the gmb and cover soils would be better served with a form of veneer stability.
A comparative study between a smooth and textured gmb lining system on a slope has considered the benefit of geosynthetic reinforcement and shown that for the smooth gmb the use of the geogrid reinforcement not only stabilizes the system but also reduces the tensile load carried by the gmb. The use of a textured gmb permits higher slope inclinations but, if the actual critical interface friction angle is lower than the design value (for example due to installation damage or smoothing of asperities), the textured gmb will be subjected to additional tensile load and will carry the majority of the tension load. If a reinforcement geogrid is placed in this system the resulting tensile load carried by the textured gmb is reduced by half.
For the inclusion of geosynthetic reinforcement in elevated temperatures, it is the reduction factors A1 (creep reduction factor) and A4 (environmental reduction factor) which are most affected by temperature. The most influential factor is A4 (environmental effects) rather than A1 (creep), nevertheless, the creep factor remains influenced by change in temperature. As a designer of veneer reinforcement cover systems it is important to consider the effects of potential elevated temperatures on the long term behavior of the reinforcement. It is equally important that the correct environmental situation is modelled, because the majority of the research on elevated temperatures assumes a fully saturated environment (i.e. a worst case scenario). Presently, it is difficult to accurately model the influence of temperature on hydrolysis for 'semi-saturated' environments. This is especially relevant and dependent upon the choice of polymer used in the reinforcement. If consistently elevated temperatures are likely to be present in the cover system then it may be prudent to adopt a more resilient polymer (e.g. PVA). The remaining option is to significantly increase the relevant reduction factors for creep (A1) and (if a fully saturated environment) environmental effects (A4), which could lead to an unduly expensive reinforcement because of the requirement for a very high short term tensile strength (i.e. because this strength will be factored down/reduced significantly due to the high reduction factors for A1 and A4).
The results of ongoing testing of PVA are showing positive results relating to the resistance under high temperatures. The final results and interpretation will be available before the end of 2015.
The recent research by Kasozi et al provides an interesting summary of the relationship between atmospheric temperature and soil temperature, and it is considered beneficial to obtain more accurate readings of the project in-situ soil temperature and moisture content regime to accurately predict the project conditions, which in turn will lead to projects that are designed more accurately and better value engineered. Long life battery powered monitoring units are now available which can monitor temperature and moisture parameters for several years and such monitoring is recommended for veneer reinforcement projects to build up a database of actual conditions in relation to temperature and moisture which in turn will help to develop more accurate assessments of the reduction factors of geosynthetic reinforcement in relation to elevated temperatures.