Investigation of the Ability of Filters to Stop Erosion through Cracks in Dams

Since Terzaghi (1922) developed grain size criteria for granular soils in dam filters, many researchers have studied embankment dam filters. The major function of the filter is to prevent erosion and piping. In order to have this ability, filters must restrain the particles of the protected soil (the base soil) and allow water to pass freely out of the base soil. Sherard et al. (1984) modified these criteria for cohesive soils, and developed the concept of “critical filters,” that can prevent erosion even under the severe condition where the base soil is cracked, and where concentrated flow occurs through the crack. In addition to grain size criteria that ensure restraint of the base soil while allowing free passage of water, a filter must also be graded so that the filter itself will not crack. To ensure that filters will not support cracks, most current filter gradation criteria require that no more than 5% of the filter material should be finer than the #200 sieve, and that the fines within the filter should be non-plastic. However, it is not clear that this criterion is sufficient. At Ochoco Dam shown in Figure 1.1, a sinkhole developed in a filter that was designed to have a maximum of 3% passing the #200 sieve. This incident at Ochoco Dam gave rise to renewed interest in filter criteria, and resulted in sponsorship of the research described in this dissertation. This research was designed to investigate the crack-preventing and crack-stopping abilities of filters, and to develop criteria that can be relied upon to ensure that a filter will perform its essential function even when subjected to deformations that cause cracks in the adjacent core.