Weathered rockfill materials, characterized by a mixture of soil matrix and rock aggregates, are widely distributed in mountainous areas. These soils are frequently used for subgrade or riprap in engineering practice, and the mobilized shear strength is crucial for analyzing the displacement and stability of these geo-structures. A series of direct shear tests are performed on a gap-graded soil with a full range of coarse fraction. The behavior of gap-graded soils is analyzed, and a simple model is proposed for the evolution of mobilized stress ratio during direct shearing process based on mixture theory. The change of inter-aggregate configuration is incorporated by introducing a structure variable which increases with coarse fraction and decreases approximately linearly with the overall horizontal shear strain in double logarithmic plot. It reasonably reflects a gradually transformation from a matrix-sustained structure into an aggregate-sustained one with the increase of coarse fraction. The model has four parameters, and at least two direct shear tests need to be done for the calibration. Validation of the model is done by using the test data in this work and those from the literature.

风化的堆石料以土壤基质和岩石集料的混合物为特征，广泛分布在山区。这些土壤在工程实践中经常用于路基或石料，动员的抗剪强度对于分析这些地质结构的位移和稳定性至关重要。在具有完整粗度范围的间隙分级土壤上进行了一系列直接剪切试验。分析了间隙梯度土的特性，并基于混合理论，提出了一个简单的直接剪切过程中动应力比变化的模型。通过引入一个结构变量来整合骨料间构型的变化，该结构变量在双对数图中以随粗粒分数增加而随总体水平剪切应变近似线性降低。它可以合理地反映出随着粗粒分数的增加，从基质维持结构逐渐转变为骨料维持结构。该模型具有四个参数，并且至少需要进行两次直接剪切测试才能进行校准。通过使用本文中的测试数据以及来自文献的测试数据来完成模型的验证。