Clay content can influence gravel soil mass failure and debris flow initiation significantly. But quantification of the clay content influence in the published literature is lacking. This paper describes the experiments carried out to explore soil mass failure and debris flow initiation with different clay contents. The whole process of gravel soil mass failure and debris flow initiation can be grouped into five phases: (i) infiltration excess runoff, (ii) fracture extending, (iii) initial slip, (iv) large-scale slip, (v) debris-flow triggering. In the experiments, when the clay content of soil mass is lower than 2.5%, no soil failure and debris flow are triggered. Only the soil masses with moderate clay contents (5–10%) show the clear processes of the five phases that need the shortest rainfall duration among different clay contents. Soil masses with high clay content (> 10%) and low clay content (2.5–5%) need longer rainfall duration to cover the five phases for partially triggering soil failure and debris flows. Clay content is closely linked with the soil failure and debris flow initiation under rainfall. The key mechanism can be grouped into two reasons: (i) the clay particles absorb the moisture and dilate to fill the soil pores, and the content of the clay has a direct link with the filled pores, (ii) the gravel soil masses with different clay content differ in their structure. For the low clay content soil mass group (2.5–5%), the coarse grains of the gravel soil form large pores and saturated clay particles are too few to block most soil pores. As a result, pore water pressure of soil mass is released easily in the raining process. Therefore, inner friction holds strong and soil mass is hard to mobilize into slip and debris flows. On the other hand, for soil masses with high (> 10%) clay contents, soil failure and debris flow are triggered under the aid of surface runoff flux to reduce the cohesion of the soil. Whereas, only for the soil masses with moderate clay contents (5–10%), large-scale soil failure and debris flow can be triggered within the shortest duration. In this case, the debris flow is of the form of “soil mechanics,” which is triggered after soil masses slip and liquefaction. The others are of the form of “hydraulic mechanics.” The experiments provide valuable evidence for quantifying clay content impact on gravel soil failure and debris flow initiation. Because they were carried out in a flume, the research results are applicable to the soil failure and debris flow initiation in bedrock gully situations.

Clay content can influence gravel soil mass failure and debris flow initiation significantly. But quantification of the clay content influence in the published literature is lacking. This paper describes the experiments carried out to explore soil mass failure and debris flow initiation with different clay contents. The whole process of gravel soil mass failure and debris flow initiation can be grouped into five phases: (i) infiltration excess runoff, (ii) fracture extending, (iii) initial slip, (iv) large-scale slip, (v) debris-flow triggering. In the experiments, when the clay content of soil mass is lower than 2.5%, no soil failure and debris flow are triggered. Only the soil masses with moderate clay contents (5–10%) show the clear processes of the five phases that need the shortest rainfall duration among different clay contents. Soil masses with high clay content (> 10%) and low clay content (2.5–5%) need longer rainfall duration to cover the five phases for partially triggering soil failure and debris flows. Clay content is closely linked with the soil failure and debris flow initiation under rainfall. The key mechanism can be grouped into two reasons: (i) the clay particles absorb the moisture and dilate to fill the soil pores, and the content of the clay has a direct link with the filled pores, (ii) the gravel soil masses with different clay content differ in their structure. For the low clay content soil mass group (2.5–5%), the coarse grains of the gravel soil form large pores and saturated clay particles are too few to block most soil pores. As a result, pore water pressure of soil mass is released easily in the raining process. Therefore, inner friction holds strong and soil mass is hard to mobilize into slip and debris flows. On the other hand, for soil masses with high (> 10%) clay contents, soil failure and debris flow are triggered under the aid of surface runoff flux to reduce the cohesion of the soil. Whereas, only for the soil masses with moderate clay contents (5–10%), large-scale soil failure and debris flow can be triggered within the shortest duration. In this case, the debris flow is of the form of “soil mechanics,” which is triggered after soil masses slip and liquefaction. The others are of the form of “hydraulic mechanics.” The experiments provide valuable evidence for quantifying clay content impact on gravel soil failure and debris flow initiation. Because they were carried out in a flume, the research results are applicable to the soil failure and debris flow initiation in bedrock gully situations.