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The variability in strata of foundation soil in marine environments makes it tedious to design foundations for offshore structures. Hence, it is essential to investigate and evaluate the strength properties of this type of soil. This study investigates the variability of the soil strata (which is quantified by the index of the mixing degree between coarse and fine particles) and its influence on the stability of the soil by mixing coarse and fine particles at varying proportions. A series of discrete element method triaxial shear tests were conducted on binary geotechnical mixtures with a varying proportion of coarse content (25%, 50% and 75%) and different mixing degrees (ranging from 0.0 to 1.0). The macroscopic results show that the peak shear strength increases with an increase in mixing degree, and the increase is more obvious with increasing coarse content, while the critical shear strength is independent of the mixing degree. The main evaluation of the number, mean normal force and distribution of the coarse–fine (cf) contact helps to clarify the meso-mechanisms that result in the variations in peak shear strength and critical shear strength with mixing degree. The increase in the peak strength may primarily be due to the increased number and globalized distribution of coarse–fine contact. However, the decreased contact force of coarse–fine contact counterbalances the strength gain due to the increased number and globalized distribution, which maintain the stability of the critical strength.

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Binary geotechnical mixtures are usually studied as homogeneous materials; however, it is difficult for coarse and fine particles to mix evenly during the formation process of binary geotechnical mixtures. Whether the mixing degree index (Md), which is one of the factors that can affect the local and overall properties of granular materials, will affect the strength of binary geotechnical mixtures has become a topic of interest. At the same time, the evaluation of the mixing degree of binary geotechnical mixture by the existing indexes is obviously insufficient. In this context, an improved index is proposed and well verified and applied. Then, a series of triaxial shear tests are performed as both physical experiments and numerical simulations. The samples are prepared from particle packings with different coarse particle contents and Md values. The reliability of the numerical simulations is verified by physical experiments. The macroscopic mechanical results of the physical experiments and numerical simulations show that the shear strength of the binary geotechnical mixture increases with increasing Md. The microscopic mechanism results of the numerical simulations show that the anisotropy of the normal contact forces a n dominates the shear strength and that the increase in shear strength is mainly caused by the increase in a n and normal contact anisotropy a c . In addition, a detailed analysis of the three contact subnetworks (coarse-coarse, coarsefine and fine-fine contact subnetworks) indicates that the increase in the shear strength of the binary geotechnical mixture may primarily be due to the increased number and globalized distribution of coarse-fine contact. This conclusion is further proven by meticulous analysis of the force and fabric anisotropies of the three subnetworks.