Investigation of volcanic deposits using a combined penetrometer-moisture probe: Application in Izu-Oshima Volcano, Japan

A combined penetrometer-moisture probe (CPMP) was used to determine the structures of layers of volcanic deposits with differing permeability on the volcanic hillslopes of Izu-Oshima Island, Japan. Here, heavy rainfall in 2013 caused shallow landslides due to the hydrogeomorphological structure of the soil mantle. In these volcanic areas that are prone to landslides, accurate determination of the detailed structure of deposit layers with differing permeability is crucial as it would enable us to predict the depth at which pore water pressure could increase and where slip surfaces could be formed. Profiles of volumetric water content, θ, measured using the CPMP, successfully detected strata of deposits consisting of pure and alternating layers of loess, ash, and scoria regardless of the measurement date, point, and depth. Those different types of deposits clearly showed the different order of values of saturated hydraulic conductivity, Ks. We also demonstrated that Ks profiles of interlayered deposits could be determined using an identical relationship between θ, measured by the CPMP, and Ks for all types of deposit on the slope within a particular range of soil moisture, excluding extremely wet or dry conditions. This is because the variations in θ values for each deposit type according to the quantity of antecedent precipitation were small, except in saturated or nearsaturated conditions. However, for determining strata deposit types, CPMP profiles of penetration resistance, Nc, may be considered a supplementary index.