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

Geotextiles in Embankment DamsThe fibers are heat fused whichresults in excellent dimensionalstability in retaining their AOS. Wickdrains utilize this type of geotextileto filter and dewater fine clay andsilt.Knitted 0.6 The principal application is as apolyester “sock” wrapping around acorrugated perforated drainage pipe.Tensile stress causes large changes inopening size, This geotextile filter istypically used in shallow burialsituations with a sand backfill.Similar to the design of granular filters (Kleiner, 2005), geotextile filter design hasmatured over several decades to incorporate lessons learned from research andexperiences with poor performance (Christopher and Fischer, 1991), (Giroud, 2005).Past problems with clogging largely stem from a poor understanding of geotextilefiltration behavior and inadequate design criteria.In filtration, liquid flows across the plane of the geotextile while soil is retained.Similar to the design of a granular filter, the design of a geotextile filter requiresbalancing opposing criteria. The filter openings must be small enough to preventloss of significant amounts of the base soil (meet particle retention criteria) and theopenings must be large enough to effectively transmit seepage flows without blindingor clogging (meet permeability and clogging criteria).The proper design of a geotextile filter involves the identification of a fabric which isable to facilitate the establishment of a soil “filter cake” or “bridging network”against the geotextile (Watson and John, 1999), (Aydilek, 2006). The filter cake is atransition zone formed by modification of the base soil being protected by the filter.Upon initiation of flow, the particles in the soil that are adjacent to the geotextile aremobilized. The smallest sized particles are removed and pass through the geotextile,the medium and larger sized particles are retained in and on the geotextile. Agranular filter cake is built up and it acts to retain the remaining layers of base soil.In this ideal condition, neither excessive piping nor excessive blinding/cloggingoccurs. Since a variety of particle sizes are required for the filter cake to form,geotextiles may have difficulty in forming an effective filter for some soils. Highlydispersive clays, gap-graded and broadly-graded cohesionless soils have a tendencytowards blinding and clogging rather than forming a filter cake. Such soils can oftenbe filtered by a geotextile but design requires careful selection and laboratory testingmay be necessary.Current filter design is a multi-step process (Luettich, Giroud, and Bachus, 1992)which involves:98