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Q: Do you have any case history information where a geocomposite was used as an underdrain below flat-plate, upstream armoring on an embankment dam? Specifically, I am looking for any long-term performance information (good or bad) and how much of that system (if any) may have had to be replaced due to poor performance issues (i.e., clogging, biofouling, crushing, etc.).
A: This is a very challenging application for a drainage geocomposite. Geosynthetics have been used beneath erosion control structures since the 1950s (Barrett 1985). If precast concrete blocks are used as armor, a layer of sand is often specified with the geosynthetic separator and drain to help as a pore water dissipater. Please note that you are dealing with reversing flow as well and rapid drawdown conditions. This challenging design is discussed nicely in the following paper:
Artieres, O., Delmas, Ph., and Lugmayr, R. (2000). “Examples of river bank and coastal protection by a new two-layer filtration system.” Proc., 2nd European Geosynthetics Conf. (EuroGeo 2), Italy, 613–618.
I would also recommend reading
Barrett, R. K. (1985). “Geotextiles in earth reinforcement.” Geotechnical Fabrics Report, 3(2), 15–19.
In talking with U.S. Bureau of Reclamation (USBR) engineers, there appears to be no indication of poor performance to date (based on the performance of the toe drain) with respect to the following projects:
Red Willow Dam, location: McCook, Neb. (This application was for a partial reconstruction of the dam, with the geocomposite located in the downstream section of the dam.)
L-8 Reservoir, location: western Palm Beach County, Fla. (There is a good paper on the use of a geocomposite with this project: “Use of a geocomposite drain against rapid-drawdown loading for roller-compacted concrete (RCC) armored earth dam” by A. T. Ozer and L. G. Bromwell, June 2011. published by Geosynthetics International Oct 2011 Vol 18 Issue 5 pp 207-220.)
C-51 Reservoir, location: western Palm Beach County, Fla.