Research on Formation Mechanism of the Debris Flow

Research on Formation Mechanism of the Debris Flow on Slope Induced by Rainfall YANG Weimin 1, ∗, WU Shuren 1, ZHANG Yongshuang 1, SHI Jusong 1, XIANG Lingzhi 2 7100042008 趙 � 勛 7100042014 周廷韋

Introduction Impact factors for slope flow formation Kinematics and dynamic characteristics of slope flow Formation and evolution of the debris flow on slope Study of typical debris flow on slope Conclusions 2

Introduction Debris flow on slope (Slope flow) A one-off slip-flow accumulation process occurring in a very short time, in which the sliding-mass rapidly converts its potential energy into kinetic energy. It is a flow type that between the mass movement and sediment-laden flow 3

Impact factors for slope flow formation According to the force and movement patterns, Li Shude has classified it into landslide-type debris flow and collapse-type debris flow. There is no uniform terminology, but the overall trend is to elect the terminology of “slope flow” or “landslide type debris flow”. 4

Slope flow can be induced by 1. Rainfall 2. Earthquake 3. Wind 4. Human engineering activities 5. Other factors Major inducing factor is the rainfall especially the rainstorm 5

The formation of rainfall-induced debris flow on the slope must have specific geological conditions 1. Topography gradient 2. Vegetation 3. Micro-topography 4. Hydrological conditions 5. Other factors Geological conditions are the deciding factor of the slope flow 6

The research on slope flow in the country showed that in most cases, slope flow formed with the slope gradient between 30º and 50º. Place The gradient of slope flow Jiang jia ravine 30º ~ 40º Beibei in Chongqing 25º ~ 37º Ningshan region in the south of Shaanxi 40º ~ 50º Southern California 26º ~ 45º San Nateo County (1982) 14º ~ 45º 7

Slope flow occurred mainly in the slope areas with bushes or low shrubs, and in some plots with exposed residual slide rock, but the natural slope flow is rare in forest coverage region. Vegetation Depth of roots Grass Less than 0. 15 m Shrubs roots Less than 0. 4 m Most failure depth of slope flow is 0. 5 to 1. 5 m. 8

Kinematics and dynamic characteristics of slope flow The rainfall-induced slope flow has its specific kinematics and dynamic characteristics. 9

Failure mode of the overland flow Rotational slide Translational slide Earth flows 10

Move direction and distance of slope flow Generally, the slope flow always moves downward along the largest tilt direction. The distance of the movement is mainly influenced by the characteristics of clastic material, the way of slope flow movement, the topography conditions of the running path and other factors. 11

Dynamic conditions for slope flow formation � 12

Formation and evolution of the debris flow on slope Formation mechanism of the rainfall-induced slope flow Formation and evolution process of slope flow 14

Formation mechanism of the rainfallinduced slope flow Brand et al. Sassa Li Shude Hu Xiewen Hu mingjian and Wang Nian 15

Formation and evolution process of slope flow 1. Rainwater infiltration and saturation stage 16

2. Local shear failure phase 17

3. The throughout extending stage of the slide 18

4. The initiation of landslide on slope 19

5. Disintegration and mud making stage of the slide 20

6. Slope flow movement and accumulation 21

Study of typical debris flow on slope Ningshan County in southern Shaanxi The main disaster to the County and the surrounding area is the debris flow on slope, and a small amount of gully debris flow. 22

Background of the environmental geology with slope flow in Ningshan County 23

Characters of debris flow on slope in Ningshan 24

Formation mechanism of debris flow on slope in Ningshan 1. The composition of the slope rock-soil mass is the material basis of the occurrence of the slope flow Grain size curve of residual soil on the slope at the Behind Hill, Ningshan County 25

Indicate that with the increase in specificmoisture content, the cohesion and internal friction angle of the residual slope soil drop rapidly. 26

The limit equilibrium method is used to calculate the stability of the slope in the east of Ningshan County. The results are shown in Table 1. 27

Rainfall is the major inducing factor of slope flow. 28

Slurry-debris flow 1. Cracks and collapse occur in the sliding soil mass. 2. The detritus (blocks) and mud (clay) mixed with water rush down along the slope. 3. The mountain is high, the slopes are steep, the content of solid clastic material is high, the content of clay composition is high and the mud-making effect is induced 29

Conclusions 1. Slope flow is mainly a translational sliding type of failure; its formation is influenced by specific geological conditions, topography, vegetation, hydrological conditions and other factors. 2. Most slope flow occurs along with the slope inclination, but its direction may also intersect the slope with a small angle; generally, the flow distance of slope wash is not far, often piles up at the foot of the slope; it is the embryonic form of development of gully debris flow. 30

3. Dynamic conditions of slope flow is the rainwater infiltration after rainfall, the decrease in matrix suction of slope rock-soil mass, and increase in shear stress; when local soil shear stress is greater than its shear strength, the instability, deformation and damage of the slope mass would occur. 4. Rainfall is the main trigger factor of the debris flow on slope. Rain causes the residual slope clay rock to be saturated and softened; when water content of soil is over 28%– 30%, the relation curves of cohesion and internal friction angle versus water content show clear inflection, or when saturation is more than 75%, its cohesion and internal friction angle are dramatically reduced. 31

4. By continuous rainfall (or rainstorm), some local residual slope soil starts sliding down as a whole, that is becomes flowing soil; then it collapses along the slope, and under the action of slope current, mudmaking occurs, and the soil flow rapidly changes into a mud stream. This is the major cause of large area debris flow happened simultaneously in Ningshan, and strong rain was relatively delayed. 32

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