Investigating hydraulic fracture at a puddle clay core dam

Abstract

More than 20 years after the construction of Greenbooth dam, internal erosion of the puddle clay core close to the abutments led to localised settlement. Following remedial grouting, the dam appears to have behaved satisfactorily with relatively little settlement of the crest. No comparable problem has occurred in the central section of the dam, confirming that the steep abutments were a major factor in the internal erosion incident. However, in 2001 a borehole investigation to evaluate seismic stability revealed a connection between the reservoir and a borehole in the middle of the core. As a consequence of this finding, investigations have been carried out to assess the vulnerability of the puddle clay core in the central section of the dam to hydraulic fracture, which is often cited as a mechanism that leads to leakage and internal erosion in clay cores of embankment dams. Earth pressure cells were installed in the core and subsequently critical pressure tests were performed in standpipe piezometers. Laboratory tests have been carried out to assess the erodibility of the puddle clay and the filter properties of the downstream fill. The results of the investigations are presented and the limitations and advantages of the various forms of field and laboratory testing are compared.