Filter Design: The Why, When, and Where

A critical component of modern embankment dam design includes the use of protective filters and drains. The filter and drain in the embankment defends against internal erosion and slope instability by reducing pore pressures in the embankment and in some cases in the foundation near the embankment contact. A typical filter design process involves selecting (1) a filter geometry that conforms to the core geometry and addresses the site specific foundation conditions and (2) finding a suitable filter gradation using available sources that meets particle retention and drainage requirements. This process has evolved significantly over the years. Not only have filter criteria for selecting appropriate grain size distributions evolved, but the types and locations of filters (toe, blanket, and chimney) and number of zones have changed as well. This paper provides a review of these changes and why the changes were made. The evolution of filter design is discussed chronologically through a review of dam construction projects and research that altered filter criteria over the years. The various changes in typical geometry, type, and location of filters in embankment dams are discussed through a review of over 140 cross sections of Corps of Engineer’s dams. The influence of a near failure at another agency’s dam is also captured. Results of this study indicate that the level of filter protection for any given dam is dependent on the era in which it was built and the region in which it was constructed.