Engineering geology and stability of a partially-infilled valley at Lynn Reserve, Bayview, Auckland

Abstract

Following a sudden collapse sinkhole event in June 2019 at Lynn Reserve, Auckland, urgent remedial works and geological investigations were orchestrated by the local unitary council. The site is characterized as an anthropogenically-infilled valley. The main unknown variable relates to the nature of the construction fill present at the site, and how it allowed the sinkhole and extensive voiding to proliferate, despite its invariably well-compacted and engineered nature. Forms of piping erosion processes are discussed in detail, as are inherent soil engineering geological properties that are relevant to the site. UAV, GPR, soil characterization, in-situ shear vane and DCP, PSD, Atterberg Limit, Emersion Dispersion, and XRD tests were all performed in an attempt to understand these properties of soil in greater detail. Results suggest the mechanism of piping erosion occurred in a staggered manner due to a lack of rainfall. The mechanism consisted of two piping erosion processes; broadly classified as internal erosion and tunnelling/jugging erosion. While the soil was well-compacted, it consisted of expansive and dispersive clays, which likely contributed to a weakening of the construction fill mass over time, due to wet-dry cycling behaviour associated with infrequent, low-magnitude rainfall events.