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The dispersion of residual graywacke clays from near Wellington was investigated as part of the feasibility study for a water supply dam. The clay had a low salt content, but a high percentage of sodium. The clay was nondispersive and erosion resistant in the pinhole test.
A clay derived from weathering of tertiary rocks at a location near Auckland had similar salt contents, but was eroded in the pinhole test, indicating dispersivity.
Tunnel erosion also occurs on hillsides in the area in old landslips. However, this same clay has formed a waterfall in an adjacent stream for five years. This unusual behavior may possibly be explained by the stream having higher salt contents than the clay after floods, causing reverse diffusion of ions, and forming a protective skin on the clay.
clays, dams, soil structure, dispersions, residual soils
Senior engineer, Beca Carter Hollings & Ferner Ltd., Consulting Engineers and Surveyors, Auckland,