Composite Filters of Geotextiles and Natural Materials for Critical Applications in Dam Engineering

Structures of a geotechnical nature have been constructed for thousands of years and a brief review of this infrastructure over time shows that the use of filters and drains is a relatively recent development in a wide range of applications. The use of granular filter design criteria for sands and gravels has been common practice for a century whereas granular filter criteria for cohesive silts and clays evolved in the 1980s. It was at about this time that geotextiles as filters rapidly gained favor as use in many civil, environmental and mining applications. However their use as primary filters in critical applications such as the internal filters of embankment dams was not generally accepted, due to longevity concerns and construction considerations. A laboratory research programme and subsequent field investigation into the performance of both geotextile and granular filters is reported. This experience exposes the advantages and disadvantages of each filter material type. A world wide review of dam failures confirms the critical importance of preventing internal erosion or piping in critical applications especially during the first decade of service. Furthermore the use of filters and drains in critical applications such as in dams and in mono-waste disposal facilities requires filter systems to perform under diverse conditions of hydraulic gradient; wetting and drying; stress and strain; with varying quality of the permeating fluid. These conditions have an effect on the short and long term performance of filters and drains. This paper proposes the simultaneous employment of benefits from geotextile and from granular filter materials in composite filters in critical applications, such as the internal drainage of embankment dams and mono-waste disposal facilities. In such applications the geotextile serves as an adjunct to the granular filter and the granular filter supplements the geotextile filter performance, with resultant reduced risk of failure.