Paleoliquefaction in Late Pleistocene alluvial sediments in the Hauraki and Hamilton basins
Kleyburg, M. A. (2015). Paleoliquefaction in Late Pleistocene alluvial sediments in the Hauraki and Hamilton basins (Thesis, Master of Science (Research) (MSc(Research))). University of Waikato, Hamilton, New Zealand. Retrieved from http://hdl.handle.net/10289/9512
Permanent Research Commons link: http://hdl.handle.net/10289/9512
Liquefaction susceptibility of the Late Pleistocene Hinuera Formation volcanogenic sediments is of interest to the engineering community as it is unclear whether materials of this age will still be prone to activation by cyclic stresses. Screening methods suggest a low susceptibility to liquefaction, yet instrumental tools such as Cone Penetrometer Tests (CPT) imply a much greater susceptibility. Recognising paleoliquefaction features in the geological record provides evidence of paleoseismicity. The Hinuera Formation was deposited by an active braided alluvial system of the ancestral Waikato River during the late Pleistocene, with the bulk of sediments located in the Hauraki Basin (deposited before c. 22,000 calendar years ago) and in the Hamilton Basin (deposited mainly c. 22,000 to c. 18,000 calendar years ago). Lithofacies in the Hinuera Formation include unconsolidated gravels, gravelly sands, sands, and silts, with interbedded peats. The deposits are complex structurally both laterally and vertically because of the migration of channels laterally during low-angle fan-building. Previous studies on post-sedimentary features of the Hinuera Formation identified uncommon secondary sedimentary structures that at the time were of uncertain origin, either from non-seismic or seismic triggers. Excavations into the Hinuera Formation at two sites showed definitive evidence of paleoliquefaction features: a sand quarry on Aspin Road (site 15) and Endeavour Primary School (site 16). These sites showed earthquake-induced injection structures (sand dikes) intruding through several lithological layers, including an organic layer. Through cross-cutting relationships, maximum age of occurrence for the injection structures are determined. A seismic event causing liquefaction occurred sometime after c. 20,749 ± 204 calendar years ago (95% probability range) at site 15 and after c. 19,964 ± 222 calendar years ago (95% probability range) at site 16. The instrumental CPT data provides a valid method of predicting liquefaction potential. However, the sedimentary materials are highly variable over short distances and so liquefaction is therefore localised; it is difficult to infer ground conditions more than a few metres from a CPT site without further ground-truth information. Sites of known paleoliquefaction features show Factor of Safety (FS) values of 0.25 to 0.5 for the critical layers, these are values that predict liquefaction. Liquefaction Potential Index (LPI) values that are calculated present high risks of liquefaction occurring and calculated liquefaction severity number (LSN) values show minor to moderate expression of liquefaction. Field observations and CPTu based predicted liquefaction are therefore in keeping. Key conditions observed at the two sites with identified paleoliquefaction features included the presence of silts associated with organic-rich materials. High levels of organic material reflect impeded or slow flowing drainage associated with overbank silt deposition, and thus are indicative of the elevated water tables required for liquefaction. The liquefaction structures observed are both at locations where the modern (pedological) soils (Te Rapa and Te Kowhai soil series) occurred in topographic depressions on the Hinuera Surface. Therefore this relationship enabled the development of a soil-landscape model using the modern soil pattern to tentatively predict, areas of higher susceptibility to liquefaction.
University of Waikato
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