ABSTRACT
This chapter reviews the Terzaghi method for filter design. Based on a gradation analysis of in-situ soils, it offers a filter design that is conservative in terms of safety, yet reasonably economical in terms of cost-benefit considerations. The most critical problem associated with seepage pressure is seepage erosion or piping. The objective of a filter design is to hold in-situ particles of foundation- support soil in place, but safely bleed off potentially dangerous levels of seepage pressure. The filter material must always be custom- designed to meet the prevailing soil conditions of the site. A characteristic feature of piping failure is its apparent sudden occurrence. As piping proceeds, the effective length of the seepage path beneath the dam is shortened and, at some point, a number of things happen very quickly: the hydraulic gradient becomes supercritical, the erosion process is suddenly accelerated, and the dam, deprived of its foundation support, collapses.
