Abstract: 【Objective】Soil shear strength is an indicator that reflects the ability of soil confronting sliding, water erosion, and overturning. The rock fragments in purple soil can significantly affect its shear strength. To date, most studies on the effects of rock fragments on shear strength often stress the role of rock fragments with sizes ＞ 2 mm. The effects of rock fragments ＜ 2 mm in soil are often ignored. Thus, evaluating the mechanisms of ＜ 2 mm rock fragments on soil shear strength can promote sustainable development of soil.【Method】This paper used the purple soils of the sloping land developed from the mudstone as the research objects. The soil particle composition was measured using a pipette method, and the soil shear strength was measured via the strain control direct shear test under different moisture contents and vertical pressure. The ＜ 2 mm of rock fragments and their mechanisms on soil shear strength were studied by comparing the analysis.【Result】From the summit to the toeslope, the particle compositions of dryland soils from summit to footslope were highly significantly correlated with that of their parent rock, but the correlations between the particle compositions of paddy land soils from footslope to toeslope and their parent rock were not significant. Hence, the particle compositions of dryland soils were highly related to that of their parent rock. The content of soil particles ＜0.25 mm were about 99.84%-99.91%, in which the contents of particles presented as ＞0.25 mm rock fragments occupied a ratio of 0.96%-57.82%, particles presented as ＞0.25 mm aggregates occupied a ratio of 6.33%-19.66%. The content of ＜2 mm rock fragments reduced with the decreasing elevation, and the rock fragment content of dryland was much higher than that of paddy land; by contrast, the contents of aggregate and clay particles increased. When the soil water content was increased from 7% to 25%, the effects of soil water content on dryland soils cohesion were less than that of paddy soils. The internal friction angle decreased linearly, and the influence of soil water content on the internal friction angle increased with the decreasing elevation. Under the same vertical pressure, the impact of soil water content on shear strength increased with the decreasing elevation and rock fragments. 【Conclusion】 There are a large amount of ＜2 mm rock fragments in purple soils developed from mudstone. And these ＜2 mm rock fragments had changed the traditional understanding of the aggregates. In the present study, the aggregates can be subdivided into pre-aggregates and post-aggregates. The former represented the ＜2 mm rock fragments, and the latter was the water-stable aggregates. In addition, these ＜2 mm rock fragments contained in soils could decrease the effects of soil water content on the cohesion and internal friction angle, as well as the soil’s shear strength. The results could provide a theoretical basis for improving the theoretical system of soil particle aggregate state study, and also would have a certain value for the conservation and sustainable use of sloping land in mudstone-developed purple soil areas.

Key words: ＜2 mm rock fragment, soil particle composition, shear strength, purple soil, action mechanism