Geotextiles in Embankment Dams - Association of State Dam Safety ...

Chapter 5—Current Status of Design Procedurestheir intended function. Finally, as with all designs, engineers have to concede thatproblems can develop. To the extent practicable, the design should considerpotential failure modes and provide measures to guard against such failures shouldthe geotextile or any other feature of the dam perform below design requirements.Specifically, that documentation needs to expressly state and document whereappropriate:• The function that a geotextile is to perform; the reader is referred to Chapter 3where the subject is discussed at length.• The timeframe over which it needs to perform satisfactorily,• The mechanisms that degrade the ability of the geotextile to perform asplanned,• The consequences of the geotextile performing at levels short of the designcapacity, and• Provision of complimentary design features as necessary to achieve the desiredlevel of reliability.Finally, where practicable, one should provide means to monitor the performance ofthe dam to provide data that infers how features are performing, i.e. trust but verify.The concept of reduction factors has been developed as a means of ensuring that ageotextile will perform as expected (Koerner, 2005b). Rather than the conventionalmethod of applying a factor of safety, the laboratory test values are identified asultimate values which are reduced to an allowable level (of stress, etc.) that can besafely applied to the material in service. The reduction factors decrease the ultimategeotextile property value to a safe level. Because of uncertainties in design, thefactors are conservative and can result in allowable values that are up to 16 times lessthan the ultimate value determined by laboratory testing (Koerner, 2005c). Thisissue is discussed further in Section 5.5.3.5.1.1 Geotextile service lifeWhile it would be desirable to have geotextiles last indefinitely, they have a finite lifeas do other man-made materials used in embankment dams such as concrete, plasticpipes and metal. Field data is available for geotextiles placed in 1970 in Valcros Dam(France) as a filter wrap on drains and as an underlayment for riprap. Mechanicaland hydraulic testing of exhumed samples of the non-woven geotextiles showedlosses from the original properties “were generally nominal with maximum91