Magat Dams: Design of internal filters and drains

Abstract

This paper describes the design and testing for the filter and drain zones for the Magat Dams. These four rockfill structures are located in the Philippines and range in height from 25 to 115 m. Construction was completed in late 1982. The project area is in a seismically active region and, as a result, the dam design had to be inherently conservative. This resulted in a highly zoned cross section and the provision of several wide filter and drain zones. The design of the filters and drains were checked by a series of large scale tests involving a 580 mm diam. permeameter and a large flume. Owing to the risk, however slight, of the dam cores being cracked during a major earthquake, the tests were carried out using both intact core material and core material into which a crack had been introduced. The results of the testing indicate that: (a) the filter gradations chosen are satisfactory, (b) it was prudent at Magat to introduce two filter zones between the dam core and the chimney drain, (c) conventional filter criteria are overly conservative when applied to the Magat core material, and (d) conventional criteria do not apply to the case of a cracked core. In this case the protective filter should contain a small amount of noncohesive fines. This is in accordance with developing practice. Keywords: filter design, testing, dam design, seismic effects, cracking.