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Description
In Japanese port facilities, rubble mound foundations supporting gravity-type structures such as breakwater caissons are designed using Mohr–Coulomb strength parameters—apparent cohesion (c) and friction angle (φ)—that have been established from large-scale triaxial compression tests on specimens of 300 mm diameter or larger. However, such tests demand specialized equipment and considerable cost, limiting the number of tests that can be practically conducted. If the same strength parameters could be reliably obtained from small-scale specimens prepared with parallel, scaled-down grain size distributions, the quality assessment of rock materials for port construction would become substantially more accessible and economical. This study presents a systematic experimental program of consolidated drained (CD) triaxial compression tests at two specimen scales: large-scale (D = 300 mm, H = 600 mm) with a gradation named Grading X (maximum particle size 40 mm), and small-scale (D = 50 mm, H = 100 mm) with six parallel gradations (Gradings A through F) whose dmax/D ratios were systematically varied. Each gradation was prepared as a parallel curve on the grain size distribution chart, preserving the shape of the prototype grading while adjusting the maximum particle size. The materials tested are mixtures of dense stone (DS) and porous stone (PS) at mass-based PS contents of 0%, 50% and 100%, in order to examine whether the scale correspondence holds not only for dense rockfill but also for porous, highly crushable materials. All specimens were prepared at comparable initial dry densities and tested under confining pressures of 50–200 kPa. The results indicate that the secant friction angle φ₀ from both specimen sizes shows a reasonably consistent trend when plotted against dmax/D, suggesting that parallel gradation scaling may offer a viable approach for estimating large-scale strength parameters from small-scale tests. This tendency was observed across the range of PS contents examined, including fully porous specimens, suggesting that applying parallel gradation scaling to small-scale triaxial tests may provide a more efficient and accessible approach to evaluating the strength of non-standard rock materials used in Japanese port facilities.