An experimental study of piping erosion is presented. Various artificial granular filter and base soil combinations are tested in a permeameter under variable confining pressures to determine the critical gradient where soil erodes through the filter. Previous research has concentrated on establishing a filter to soil grain size ratio criteria, typicallyD15f/D85s< 4, which separates safe from potentially unsafe filters. These works often ignored self-filtration zone formation phenomena and rarely documented the influence of variables such as confining pressure, filter thickness, and hydraulic gradient. To adequately control all variables that may influence piping erosion, a new permeameter was designed and careful attention was paid to sample preparation. Artificial glass beads were water pluviated to permit consistent repeatable uniform samples. By monitoring head, settlement, confining pressure, amount of eroded soil, and water outflow rate, the onset of piping can be determined. It is shown that the grain-size ratio is the most important parameter affecting piping erosion. A soil-filter system withD15f/D85s< 8 will not fail, whereas aD15f/D85s> 12 will not be able to retain base soil. For 8 <D15f/D85s< 12, piping will only occur if the hydraulic gradient exceeds a critical threshold. The critical gradient is lower if the head is rapidly increased, as a filtration zone is inhibited from forming. A very thin filter has a similar effect. Stability is somewhat inversely related to the confining pressure level for small grain-size ratios.Key words: filters, seepage forces, confining stress, piping erosion.