Pahoia Tephra Sequence: strength, sensitivity, and stabilisation

Large, retrogressive landslides with long runouts have been attributed to the sensitive nature of quick- clay soils. In the Northern hemisphere remediation attempts have been made with chloride salt solutions. The use of potassium chloride salt has been shown to significantly reduce sensitivity in situ and to strengthen quick clays dominated by illite to a point where they can be regarded as stable. In Bay of Plenty, New Zealand, the Pahoia Tephra Sequence (PTS) has been identified as a sensitive soil layer, dominated by the 1:1 clay halloysite. This paper examines previously published data, together with fresh samples from the base of a recent large landslide using consolidated undrained triaxial strength testing. Initial results indicate a highly sensitive, weak layer at the base of the PTS, near the interface with the underlying Kidnappers Ignimbrite. The possibility of in situ stabilisation was investigated at a laboratory scale through submersion of samples in 2 molL-1 potassium acetate solution. Submersion resulted in increased shear strength and an overall reduction in sensitivity. These results are attributed to interaction between the potassium ions with the dominant halloysite clays, suggesting that there may be a method to universally increase shear strength in the sensitive soil in situ.