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Q:At the moment there are two issues that create uncertainty with the use of PET fabrics and geogrids in the Netherlands. First, for many years we built pile-supported geosynthetic reinforced embankments (so-called piled mattresses to reduce settlements under embankments on soft soils). The fill is supported by a geosynthetic mat, which rests on top of the pile (caps). The fill on top of the PET geosynthetics is mostly made of crushed concrete and stone granulate. This fill can have very high pH values due to the crushed concrete part. This high pH value could have a negative impact on the long-term strength of the PET due to hydrolysis. Do you have a recommendation for this issue? The second point is about the reduction in strength of geogrids that are used in reinforced walls. In our case, the front of such a wall is made of concrete panels. The space between the panels of some concrete units is wide and left open. In this space there is no UV protection, which means that the strength of the geogrid will reduce in time. The geogrid is a PET geogrid with coating. Do you have any information from GSI’s mechanically stabilized earth (MSE) wall database that indicates if and how quickly reduction of strength due to UV could develop in such a case?
A: Nice to hear from you. I would recommend the following reference, Soleimanbeigi, A., Tanyu, B. F. Aydilek, A. H. and Florio, P. (2019), “Evaluation of recycled concrete aggregate as backfill for geosynthetic-reinforced MSE walls,” Geosynthetics International 26(4):1-50, DOI:10.1680/jgein.19.00025. The story is not good when the pH gets near 11, particularly when you are dealing with poor-quality (uncoated, low molecular weight) polyester.
Regarding your second question, we have not seen this failure mechanism (near face UV degradation) in any of our 403 MSE failures that we have investigated for the GSI database, which dates back to 1987. If it is a possible failure mechanism, however, it is new to us and we have not deemed it a trigger in any of our case histories. This might be an area for future investigation.