Testing the cyclic strain softening behaviour of an ageing puddled clay core dam

Abstract

Seismic resilience of the 150-year-old Ross Creek puddled clay core water supply dam in Dunedin has recently been enhanced through a programme of refurbishment. The very low permeability moderately plastic soils comprising the embankment dam are susceptible to cyclic strain softening behaviour that had been analysed using published empirical cyclic strain performance relationships based on material classifications and static testing results. The availability of several undisturbed embankment soil tube samples obtained during the installation of piezometer instrumentation has provided the opportunity to undertake complementary specific cyclic strain testing at the University of Canterbury geotechnical laboratory. This included specialist cyclic triaxial (CTX) and cyclic simple shear (CSS) testing. The paper covers the challenges experienced when undertaking undrained cyclic strain testing on soil of this nature, including the preparation and saturation of the very low permeability samples. When compared to values obtained from the traditional empirical design methods, the test results showed a high degree of variability and sensitivity to both the degree of saturation that could be achieved, the respective clay/silt/sand fractions, and the test method (i.e. CTX cf CSS). CTX test results adjusted for anisotropic field consolidation conditions matching the CSS test method appeared to be quite conservative compared to the single CSS test result. However, we found that average results were broadly consistent with the design values previously adopted. Monotonic shear strength testing undertaken when excess pore pressures were still present following cyclic testing revealed considerable softening, but average undrained strengths were similar to previous TXCU results.