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/).

The concept of critical state has been a cornerstone of modern critical state soil mechanics. It remains inconclusive how grain crushing affects critical state behavior in crushable granular sand. A multiscale computational approach is employed in this study to simulate the shearing behavior of crushable granular sand at critical state. Grain crushing is rigorously considered by preserving the co-evolutions of grain size and shape in the simulation of the shearing process. Systematic simulations on specimens with varying initial states and loading paths show unique characteristics of critical state for crushable granular sand in terms of critical state stress ratio, void ratio, breakage index, and shape descriptors which are independent of stress path and initial conditions. To further understand the deformation mechanisms of crushable sand at critical state, the volumetric strain is decomposed into three components due respectively to grain size reduction, the interlocking of irregular shaped grains generated by crushing, and inter-particle friction. Competing mechanisms among the three strain components are quantitatively analyzed and discussed. Initial void ratio and stress levels are found to play a prominent role in shaping the critical state deformation of crushable sand and such impact may be gauged through the fraction of grains that experience crushing.