Abstract: The stability of colluvial slopes in high-altitude mountain areas, three rivers sources regions, northwestern Yunnan is one of the main problems in road construction. Model test is one of the most effective means for investigating the factors for the slope stability. Based on the theory of similarity and the definition of safety factor of the slope, a range of tools are used to transform the model test materials to reach the strength similarity requirements. A series of model tests were carried out, which reproduced the real conditions for the failure of colluvial slope. The model test results can give references for geotechnical designers and researchers. The slope angle when the slope fails increases by about 42%-100%, the cohesion increases by about 50%-120%, the internal friction angle increases by about 6%-8% when model test material changes from loose state to more dense state. The results show that when material density increases the strength increases and the slope stability increases; thus slope angle also significantly increases when slope fails. When water content of the material increases and the material is nearly saturated under rainfull, the cohesion decreases by about 70% and the internal friction angle decreases by about 7%. It is indicated that the material softening by infiltration is the most fundamental factor for the stability of colluvial slope.

Key words: colluvial slope, model test, dense state, softening, strength degradation