## 1 Introduction  
Gravel-soil mixture is an important medium of concern in the field of geotechnical engineering [1], and several studies in the literature have been conducted to clarify the engineering characteristics of gravel materials including investigations on the effect of gravel content on precompression stress [2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13]. However, the mechanical properties and deformation mechanism of gravelly soils are complex because of inhomogeneity, high porosity, and under-compaction [14]. As a result, some findings from previous studies have shown contradicting results. For example, a study by [PERSON] and [PERSON] (2003) [2] revealed that precompression stress increases as gravel content increases; however, [PERSON] et al. (2009) [3] described a contradicting effect with an increase in gravel content in multiple regressions. In the past, investigations on gravel-soil mixtures with grain sizes exceeding 75 mm could not be routinely performed, due to size limitations of equipment [12; 15]. For this reason, previously studied calibration values for mixtures not exceeding 2 mm were utilized to obtain the initial density and material parameters for larger-sized gravel-soil mixtures. [PERSON] and [PERSON] (1951) [16], [PERSON] (1957) [17], and [PERSON] (1991) [18] are among those that proposed equations for modifying the compaction properties of mixed soils containing coarse aggregates. For use in dealing with the stability of earth and rock structures, [PERSON] (1993) [19] summarized other equations available. Among the many correction equations, the one proposed by [PERSON] and [PERSON] is widely used in Japan. However, since the physical meaning of this equation is not clear and does not apply to materials with gravel content exceeding 40%, the use of this equation is not appropriate in the case of mixtures with high gravel content. In 2010, [PERSON] et al. [20] were able to perform large-scale one-dimensional (1D) compression tests on gravel-soil mixtures with particles larger than 2 mm. From the results of the experiments, they were able to propose an expression that applies to mixtures with gravel content exceeding 40% to quantitatively evaluate the influence of gravel content through a material parameter.  
In analyzing large-scale embankment projects, the utilization of an appropriate method in interpreting the behavior of soil-gravel mixtures through quantitative evaluation of compression tests is necessary. However, the application of such methods in numerical analysis is equally as important, and there are only a limited number of studies in the literature related to the simulation of embankments composed of gravel-sand mixtures. In order to evaluate the performance of embankments through numerical analysis, parameters that are representative of the behavior of embankment materials must be utilized. The behavioral characteristics greatly depend on the material constants such as compression index (\(\lambda\)) and swelling index (\(\kappa\)), which may be influenced by the interaction between the two materials. In this study, the results of large-scale one-dimensional compaction tests on mixed decomposed granite soil (D-G-S) are analyzed, and a material constant estimation method based on the ideal mixing theory is presented to quantitatively evaluate the effect of the gravel content on the compression properties of the gravel mixture. Numerical simulations using the properties of D-G-S obtained from experiments are then performed to confirm the applicability of the method used in this study.