Potassium acetate treatment as a potential means for stabilising extra sensitive, halloysite-rich soils in the Bay of Plenty, New Zealand — A novel approach to an old issue

Abstract

Landslides are a rapid, destructive natural hazard that pose a significant risk to human life and infrastructure. Globally and in New Zealand, landslides are among the most significant hazards. Within New Zealand, it is estimated that the annual costs associated with landslides are between NZ$250300 million, with over 600 fatalities recorded. While the types of landslides in New Zealand vary due to changeable geology and triggering conditions, one area that is particularly heavily affected is the Western Bay of Plenty region, specifically the Ōmokoroa peninsula, where numerous landslides have occurred. A key contributor to this instability is the presence of a series of weathered primary and reworked rhyolitic tephra deposits dominated by halloysite, known as the Pahoia Tephra, that exhibit a sensitive response. Based on successful tests undertaken in the Northern Hemisphere using potassium chloride salt wells, as well as a pilot study conducted on the same halloysitic materials, the research reported here aimed to treat a halloysite-rich sensitive soil with potassium acetate to determine if the soil could be regarded as strengthened. Atterberg limit testing of the soil showed the untreated Pahoia Tephra to have a liquid limit higher than the natural moisture content (61%), meeting the requirements for an extra-sensitive soil, as was determined by field shear vane tests. Treatment with the potassium acetate increased the liquid limit (58%65%) and reduced the liquidity index to below 1 (2.1 untreated, 0.57 treated). Treatment of the soil did produce a drastic increase in shear strength within the soil, with effective cohesion increasing from 4.2 kPa to 40.9 kPa following 12 months of treatment, while effective friction angle was slightly reduced from 29.8o to 25.3o. Stress paths of the soil show a large shift in behaviour, with samples no longer dilating during the pre-failure stage. To explain these changes, various mineralogical and chemical analyses were undertaken. X-ray diffraction analysis showed no expansion of the clay and lack of intercalation (all halloysite showing 10.1 Å). The Fourier transform infrared testing yielded a similar result, with no notable shifts observed in the 3600 cm-1 wavenumber peaks. With this said, new peaks corresponding to an interaction between potassium acetate and the silica sheet present within halloysite were noted. Scanning electron microscope analyses showed, following 12 months of treatment of the soil, a shrinking of the clay spheroids over the duration of treatment. From these findings, it was theorised that, while the potassium acetate did not intercalate into the halloysite spheroids themselves, the potassium acetate interacted with the external silica sheet so that it curled and rolled, resulting in a smaller spheroid, with acetate entering into the ditrigonal space within the silica sheet and displacing the H2O (‘water’) present. This resulted in a twofold action, with the second action leading to the creation of stronger short-chain van der Waals forces between clay spheroids, thereby increasing cohesion within the soil. While the initial aim of this research was achieved, a large number of avenues have been opened that certainly warrant further investigation.