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Is there a salty solution to sensitive soil sliding in the Bay of Plenty, New Zealand?

Found globally, sensitive soils are widely regarded as a key cause of slope instability. These sensitive soils often contribute to the production of large retrogressive landslides that pose significant danger to a people and infrastructure. Within the Bay of Plenty region, sensitive soil based landslides are widespread and numerous Moum et al (1970) successfully trialled remediation of a sensitive soil landslide in Scandinavia by application of potassium chloride via leaching columns. The aim of this study is to determine whether salt treatment of sensitive soils formed in halloysite rich soils of the Bay of Plenty provides a viable means of improving soil behaviour. Soil collected from the base of the landslide at Bramley Drive, Omokoroa, was treated with 3 differing potassium based salts (KCl, KOH and K₂CO₃) to observe which would produce the greatest positive rheological effect. Results indicated that treatment with KCL and KOH had a negative rheological effect on the soil reducing the liquid limit of the soil from a baseline of 92.3 to 79.3 and 83.5 respectively. Only K₂CO₃ did not have a negative effect on the soil, maintaining the liquid limit at 92.6. Soil kept within soil cores was then soaked in K₂CO₃ solution for a period of three weeks before being subjected to tri-axial testing at three confining pressures of 205 kPa, 280 kPa and 355 kPa; untreated soil cores were tested at the same stress conditions Tri-axial testing of the soil showed a significant increase in the peak deviator stress measured when comparing the untreated and treated soils at its point of failure, with increases in peak deviator stress measured for the treated soils in the order of 227% at 205 kPa, 187% at 280 kPa and 124% increase at 355 kPa. Strain softening for treated soil was also measured to be less than that of the untreated soil at all confining pressures, a trait reflected in pore pressures measured at point of failure also. Stress path plots indicated that untreated samples underwent contraction rapidly as deviator stress increased, with no clear failure point observed as would be expected from an over consolidated clay. In contrast, stress paths for the treated samples showed the soils dilating before reaching point of failure and undergoing contraction following failure. Differences in friction angle and cohesion were also measured between treated and untreated samples, with untreated soil indicating friction angle and cohesion of Φ’=19.3o, C’=26.6 kPa. These results were consistent with those given in literature and fell within the range expected for halloysite rich clays (Mills, 2016; Moon, 2016). Both friction angle and coheion values increased in treated soils (Φ’=28.2 and C’=58.4). Based upon the results gathered during this study I infer that stabilisation of the sensitive layer of soil found within Omokoroa is possible, in particular when treated with K₂CO₃. It is my belief that this occurs due to the successful intercalation of the K₂CO₃ ion into the halloysite basal space, displacing the water found in the space originally, and causing an expansion of the crystal lattice. The expansion of the halloysite crystal lattices in turn increases the particle contact areas, which as a result causes a rise in the cohesion and friction angle as a result of increased energy required to overcome the grain to grain friction when stress is applied. Increased basal spacing in the halloysite crystals accommodates pore fluid and thus allows for reduced pore water pressure during stress application. Though further research is warranted, results produced from this study show that treatment increases the effective strength of the soil to a significant and noticeable degree, and provides the impetus to warrant further research into the subject in the near future, as the potential engineering impacts in being able to effectively stabilise soil rich in halloysite holds significant value both in New Zealand and on a global scale.
Type of thesis
Robertson, T. P. (2017). Is there a salty solution to sensitive soil sliding in the Bay of Plenty, New Zealand? (Thesis, Master of Science (MSc)). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/11823
The University of Waikato
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