Small strain stiffness, microstructure and other characteristics of an allophanic volcanic ash

Small strain mechanical properties are a prerequisite for seismic analysis and design. Corresponding studies on volcanic soils are by themselves limited while research on some volcanic ashes which intrinsically have high fines content is even rarer. Presence of clay minerals like allophane and imogolite confers uncharacteristic properties to the soils which somehow seems underappreciated. This is evidenced, for instance, by the near absence of literature related to the 2016 Kumamoto Earthquake explicitly mentioning the influence of these particular clay minerals. Against this background, this study aimed at partially filling the gap with respect to the small strain shear modulus, G_max of an allophanic volcanic ash and underline the influence of allophane and imogolite on some of the soil properties. Remoulded black volcanic ash, kuroboku was sampled from two sites in the Kyushu area in the south of Japan. As part of the experimental work, a series of index properties, engineering properties, electron microscope (Scanning Electron Microscope (SEM) and Field Emission Scanning Electron Microscope (FE-SEM)) examinations and bender element (BE) tests was performed on these soils. The paper elaborates on the peculiar characteristics of the allophanic volcanic ash, including presence and effect of water of crystallization and influence of electrostatic bonding on grain size distribution and liquefaction resistance. Based on the SEM and FE-SEM images, the microstructure of the kuroboku soils was assessed in terms of its various components, the contact relation and the pore space. Observations from the microscopy analysis like presence of diatoms and intra-elemental pores were key to understanding the slight variation in the G_max among the tested soils. An attempt was also made at situating the G_max of the kuroboku soil vis à vis other types of soils. Empirical relationships for estimating the G_max of kuroboku separately and with other volcanic soils were proposed. Findings from this research could rekindle an interest into further understanding the particular characteristics of allophanic materials as well as encourage more research on their dynamic properties, which is essential considering that these materials are mostly found in seismically active regions.