Debris PF01367338 flow is formed with sliding sand and flowed water. Figure 5 is the top view of the failure process of debris flow.Figure 5The failure process of debris flow (top view).Through the above analysis, it is found that the failure in slopes starts with shear deformation at the slope toe and takes the form of retrogressive toe sliding failure.3.2. Displacement FieldFull-field displacement can be acquired by digital methods [20]; in this paper it was analyzed by digital photogrammetry for deformation measurement (DPDM). The DPDM technique has been proven to be a powerful tool for observing the process and local deformation of granular soil [21]. Local area was chosen to analyze displacement of debris flows slope as indicated in Figure 6.Figure 6The analysis area of displacement field.

Figure 7 shows the displacement field in different colors of debris flows, and the colors representing displacements are shown under each figure. As Figure 7(a) indicates, two potential sliding surfaces are found in the 5.0~7.0cm deep. Soil above potential sliding surfaces can be considered as sliding mass which has a downward trend. As rainfall continues, the shape of sliding mass keeps no change, but the displacement of the upper slope has greater increase, and the largest displacement in Figure 7(b) is 9.1mm. However, the displacement of slope outside sliding surfaces still remains in 0mm. With the working of seepage water and losing the support of sandy slope toe, the stability of slide mass is distorted as shown in the Figure 7(c). At this stage, sliding mass is divided into small parts and slide down with greater displacement.

When the old slide mass disappeared, new sliding surfaces are found in Figure 7(d), which means that the failure cycle will be repeated.Figure 7The deformation field of debris flow.The evolution process of displacement field analysis was confirmed with the flume model test, and the failure of granular debris flow is slopes collapsing in form of slide mass with greater displacement.3.3. Failure ModeIn order to study the relationship of grain size distributions and failure mode of debris flow, seven sand samples were prepared in this paper by mixing medium and fine sand. Fine sand contents in a sample are 0% (medium sand), 10% (C-10), 20% (C-20), 30% (C-30), 40% (C-40), 50% (C-50), and 100% (fine sand), respectively.

And the grain size distributions of different sand samples are shown in Figure 2.Pictures were taken during the tests to record the progress AV-951 of the failure of each slope. The results are shown in Figure 8. The failure in slopes with minor fine particle contents as less 10% (Figures 8(a) and 8(b)) starts with cracks at crest and takes the form of retrogressive toe sliding failure. The slide surface is circular, and the sliding part is small and travels only a short distance.