Crushable porous soils, such as volcanic pumice, are distributed worldwide and cause a variety of engineering problems, such as slope hazards. The mechanical properties of these soils are complicated by their high compressibility due to voids in the particles themselves and changes in the soil gradation due to particle crushing. They are usually classified as problematic soils and discussed separately from ordinary granular soils, and their behaviour is not systematically understood. In this study, isotropic and triaxial compression tests were conducted on artificial pumice in order to determine the relationship between the mechanical properties and the particle crushing of crushable porous granular materials. The results showed that the mechanical behaviour of artificial pumice, representative of such materials, can be explained using a particle crushing index, which is related to the degree of efficient packing. Furthermore, a new critical state surface equation was proposed. It is applicable to crushable porous granular materials and shows the potential for expressing the critical state or isotropic consolidation state of such materials as a single surface in a three-dimensional space consisting of three axes: the stress - void ratio - crushing index. The validity of this new equation was confirmed by applying it to natural pumice from previous research. (c) 2025 Production and hosting by Elsevier B.V. on behalf of The Japanese Geotechnical Society. This is an open access article under the CC BY- NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Volcanic pumice, with special characteristics such as crushable particles and high water retention, is distributed throughout Japan and serves as the source layer for slope hazards characterised by post-failure gentle slope flows and long-distance flows. The aim of this study is to determine the relationship between the crushing characteristics and the mechanical properties of porous pumice, which often contributes to such disasters. As the porous pumice material, Ta -d pumice, which caused numerous slope disasters during the 2018 Hokkaido Eastern Iburi Earthquake in Japan, was collected and subjected to a series of triaxial compression tests. The grain size distribution of the pumice before all the tests was adjusted to be uniform, and the amount of crushing was quantified by measuring the grain size distribution after the tests. The results suggest that the critical state and isotropic consolidation of porous pumice can be systematically expressed in a three-dimensional space with the axes of the void ratio, mean effective stress, and degree of particle crushing. Furthermore, a gentle slope disaster with an inclination of less than 21 degrees, that had occurred at the site from which the Ta -d pumice was collected, was discussed in terms of its flow potential, showing that the flow distance can be adequately explained. (c) 2023 Production and hosting by Elsevier B.V. on behalf of The Japanese Geotechnical Society. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Pumices with high pore voids of volcanic origin are distributed throughout Japan and are the causal layer of slope failures. In many cases of surface failures, it is difficult to assume that the resulting layers are fully saturated. The high water-holding capacity of the pumice suggests that they were deposited in an unsaturated state with a high degree of saturation. In this study, saturated triaxial compression tests and fully undrained unsaturated triaxial compression tests were conducted on artificially produced pumice and natural pumice while measuring the amount of crushing. This is to clarify the relationship between the crushing and mechanical properties of pumice with porous particles, which are often the cause of such disasters, and their behaviour under unsaturated conditions. The results showed that the pumice stones have an ultra-high pore structure. Moreover, pumice with porous particles reached a steady state under both saturated and highly saturated unsaturated conditions, and the amount of crushing increased under highly saturated unsaturated conditions.