The Nature of Pumiceous Soils Developed on the Matahina Formation Ignimbrites, Bay of Plenty, New Zealand

Abstract

Pumiceous soils are problematic from an engineering point of view and are widespread in the central part of the North Island of New Zealand. Due to sedimentary redistribution they are found mixed with hard grained materials on river plains and along major rivers in the region. They also form many of the in-situ soils encountered in other parts if the region. They are thus frequently encountered in engineering projects. A better understanding of their geotechnical behaviour is thus important. A number of authoritative experimental and field studies have been conducted in the last two decades on pumiceous soils. Due to the fact that most industrial activity occurs on pumice soils of a sedimentary nature most research has focused on these soils. Very little data is available from studies of the in-situ residual pumiceous soils developed over the source volcanic deposits. In order to add to the existing knowledge of their geotechnical properties a study was undertaken on the pumiceous soils developed on the ignimbrites of the Matahina Formation in the Eastern Bay of Plenty. For this purpose, undisturbed samples were collected from sites south of Whakatane in the Bay of Plenty. The results of this investigation confirm characteristics previously demonstrated for pumiceous soils. Laboratory testing data shows the soils to have peak shear strengths of up to 165 kPa and these compare well to the in-situ vane shear strength values. The pumiceous soils have higher friction angles than hard grained (quartz) sands (Pender et al., 2006) and have been shown in the literature to display higher liquefaction resistance (Orense et al., 2020). It is also shown that most traditional investigation techniques tend to overestimate instability issues (Orense et al., 2020). This is mostly due to the crushability of the soils. It is suggested that the volcanically derived soils may have some added complexities that are not found with sedimentary deposited pumice soils. The study also highlights a lack of experimental or field data for in-situ volcanically derived pumiceous soils thus presents valuable insights into the geotechnical properties of these materials.