Breaching of an earth dam in the Western Cape by piping

Abstract

A 12m high earth dam breached during its first filling in 1986, by formation of a large erosion tunnel through the wall under its highest section, adjacent to a concrete encased outlet pipe. It is concluded that breaching was due to piping erosion, and reasonably speculated that cracking and/or hydraulic fracture of fill adjacent to the outlet instigated a concentrated seepage path through the entire width of the dam, along which piping subsequently developed and led to the breach. The results of finite element analysis are presented, which demonstrate the vulnerability of the fill adjacent to the outlet to cracking and/or hydraulic fracture. The process of piping is defined in the paper, and the mechanism by which it develops and causes breaching of dams is described in general terms, to provide a framework for understanding the evidence which remained after the dam was breached. Various other factors which may have contributed to the failure are also reviewed. Design measures which might have prevented breaching are described briefly. The purpose of this paper is to contribute to a better understanding of piping, and in particular, the stress and strain regime necessary in the soil to instigate piping.