| Summary: | Coarse-grained soil is extensively utilized in soil and rock dam engineering as well as in projects addressing weak foundations. The permeability characteristics of coarse-grained soil are critical to the safety of engineering designs. Due to indoor instrument size limitations, only a scaled-down version of the original coarse-grained soil can be tested. However, reducing the original grain size alters the pore size between soil particles and the connectivity of pore channels, leading to variations in the permeability coefficient between the scaled-down and original grain-sized soils. This variation is crucial for accurately determining the permeability characteristics of the original grain-sized soil. Therefore, accurately predicting the permeability coefficient of the original grain-sized soil through tests on scaled-down samples has become essential in evaluating the permeability characteristics of coarse-grained soil at engineering sites. Utilizing the continuous grading equation, 16 different grading samples are designed, using the maximum particle diameter <italic>d</italic><sub>max</sub> and grading area <italic>S</italic> as variables. The permeability of each sample is quantitatively analyzed using a self-made large constant head permeameter, and the variations in the permeability coefficient of scaled-down grading are examined along with a method for converting the permeability coefficient between coarse-grained soils with varying gradations. The results indicated that when the grading area <italic>S</italic> remains constant, the permeability coefficient <italic>k</italic> increases with the increase in the maximum particle diameter dmax. This trend is more pronounced when the grading area is smaller. Conversely, when the maximum particle diameter <italic>d</italic><sub>max</sub> remains constant, the permeability coefficient <italic>k</italic> decreases with the increase in the grading area <italic>S</italic>. The impacts of dmax and <italic>S</italic> on <italic>k</italic> can be quantitatively described using specific functions. A permeability coefficient conversion model between different gradations of coarse-grained soil is established based on the relationship among <italic>d</italic><sub>max</sub>, <italic>S</italic>, and <italic>k</italic>. According to the literature, the prediction errors of the permeability coefficient values for different grades of coarse-grained soil obtained using this model are within 6% of the measured values. Additionally, the original permeability coefficient values are successfully estimated using the scaled-down sample test results, with an error of only 2.5%, verifying the applicability of this permeability coefficient calculation model to coarse-grained soil.
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