Landslide Classification in Region and Recognition Signal

Landslide is complicated movement of slope. According to type of movement, it was classified in to seven types as fall, topple, translational slide, rotational slide, lateral spreading, flow and complex by (Varnes, 1978). The classification was updated (presented in table 6.1) and landslide types were classified in to 29 types depending movement and material (Oldrich, H., 2014). In the study area, the landslide classification is carrying base on Varnes’ 1978 classification system. However, most of landslides are classified based on remaining signals after movement, so in some case

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the classification was non-consensus from researchers.

Table. 6.1 A summary of Vanners’ 1978 classification system (Varnes, 1978)

In the study area, beside 5 typical classes as rock fall, debris rotational slide, translational rockslide, translational shallow debris slides, gully type slides and flows, which were classified, the following translational wedge type slides was added. Here after, typical landslide types in study area will be discussed for clear understand for classification.

6.2.1.1 Rock fall

A rock fall or rock-fall refers to quantities of rock falling freely from a cliff face.

A rock fall is a fragment of rock (a block) detached by sliding, toppling, or falling, that falls along a vertical or sub-vertical cliff, proceeds down slope by bouncing and flying along ballistic trajectories or by rolling on talus or debris slopes,” (Varnes, 1978). In the study area, rock-fall was found in some cases, where is in granitic rocks slide as rock falls of granite core stones formed at shallow layers effected by weathering.

However, the number of this phenomenon is small. At the site, after cutting natural slope for road construction, the remaining talus, which were step and some cases were nearly vertical created high potential energy for slope failure to continue development.

At most those locations, the dip of geological slope layer, which was strong weathering, was reverse with the dip of cutting slope surface of the road. As the result, rock fall occurred. Rocks falling from the cliff may dislodge other rocks and serve to create another mass wasting process. Pictures of Phenomenon of rock fall on HCMR – Quang Nam section is presented in Fig 6.1.

121 Fig. 6.1 Rockall on HCMR– Quang Nam section - Vietnam

6.2.1.2 Translational Shallow Debris Slides

Landslide in which the sliding surface is located within the soil mantle or weathered bedrock (typically to a depth from few centimeters to some meters) is called a shallow landslide. They usually include debris slides and failures of road cut-slopes.

The survey and study records showed that the most landslide phenomenon in study area had character of sallow landslide, largely related to heavy raining fall to the surface of the terrain, which was strong weathering. In the study area landslides occur in sedimentary rocks with bedding and schistosity, weakly converted crystalline schist, and granitic rocks. Landslides move as a translational slide when the beds are gently sloping, and otherwise as a wedge type slide at an intersection with a crack.

Fig. 6.2 (a) Debris Shallow landslide and (b), (c) Translational wedge type slides in HCMR -Hue section-Vietnam

Pictures of Translational type slides in HCMR -Hue section-Vietnam is presented in Fig 6.2.(a) (on left side). Typical slope in study area was discovered with silt and sand as its top soil and bedrock as its bottom soil. During an intense rainstorm, the bedrock will keep the rain trapped in the top soils of silt and sand. As the topsoil

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becomes saturated and heavy, it can start to slide over the bedrock and become a shallow landslide.

The slip surfaces usually have the same direction of geological slope layers or cutting slope of the road. Slip surface usually appeared at the position of interface geological or different weathering layers. In those cases, slip surface usually shallow and toe of surface of rupture usually ended up at middle or end of the cutting slope of the road talus. Accumulation of landslide was soil or debris greatly influenced by surface water and ground water, moved following stream over the road surface.

6.2.1.3 Translational Wedge Type Slides

Beside typical landslides as shallow, one more type of landslides was found as translational wedge type slides. The translational wedge type slides have characters of translational slide as mentioned. Beside it, moving mass slides has shape in a triangular form. Gully type slides and flows, and translational wedge type slides usually occurs in metamorphic rocks have fundamentally the same occurrence mechanism. These slides have a side scrap or a base that matches a weak surface such as schistosity, faults, and joints. In the study area, it was usually located in the rock with middle weathering.

Pictures of Translational wedge type slides in HCMR -Hue section-Vietnam is presented in Fig 6.2 (b), (c); (Right side)

6.2.1.4 Translational Rock Slides

Translational rockslides have the sliding surface mostly deeply located below the maximum rooting depth of trees (typically to depths greater than ten meters). They usually occurred in relation with the geological structure. This type of landslides are potentially occur in a tectonic active region. They tend form along a plane of weakness such as a fault or bedding plane. Some landslide cases, the depth of slip surface is located over hundred meter on deep, with the size is over several square kilometers and usually is considered as deep-seated landslide. In many cases, the organic layers, such as crushed coal bed, black mudstone, black shale, and black schist, act as a slip plane in sedimentary rocks with developed bedding. Some landslides in areas distributed by limestone occur with limestone as a cap rock. A slip plane is formed in the bedding plane of shale as the underlying layer of limestone and the schistosity plane of weakly converted crystalline schist. All landslides have occurred on a large scale as rockslides.

123 Fig. 6.3 Deep seated Landslide on Hai Van pass - Da Nang –Vietnam

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Fig. 6.4-a Photo of Translational rock slide Northwest ward of Thach My town Fig. 6.4-b Topography of Translational rock slide on Khom9-Kham Duc

A deep seated Landslide original form changes to many minor debris flow inside its landslide body. B means block slide; D means debris flow

In the study area, a number of topographic area of deep-seated landslide distributed. Among of them, the huge landslide distributed at the west point of Kham Duc town, Northwestward of Thach My town and Hai Van pass. The huge landslide distributed at the west point of Khom 9 - Kham Duc My cause by the relationship with

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the characteristic of ultra-basic rock. Topography map of this Translational rockslide is presented in Fig. 6.4-b. Regarding landslide Topography map, a deep seated landslide original form changes in to many minor debris flows inside its landslide body. B means block slide; D means debris flow. The cause of the one near the Thach My town on HCMR was relation of existence of coal layer between rock layers. The photo of translational rockslide North West ward of Thach My town is presented in Fig. 6.4-a.

Hai Van landslide is special remarkable huge landslide that the real cause is unknown up to now. The land mass of the Hai Van mountains consist with granite. In the granitic area, deeply sheeted landslide is seldom discovered. Pictures of deeply seated Landslide on Hai Van pass - Da Nang - Vietnam is presented in Fig. 6.3.

6.2.1.5 Rotational Debrisslide

Rotational slides occur when a slump block, composed of sediment or rock, slides along a concave-upward slip surface with rotation about an axis parallel to the slope. Rotational movement causes the original surface of the block slipping down, breaking in to many parts with scraps. The top of the slump is rotated backward. In the study area rotational landslides was discovered in deep weathering sedimentary rocks with bedding and schistosity, weakly converted crystalline schist, and granitic rocks. A small number of slip surface with middle to large radius, which was predicted as old landslides, located under the road embankment.

Fig. 6.5-a Photo of landslide body of a rotational landslide in HCMR (Quang Nam section)

Fig. 6.5-b Photo of main scarp of a rotational landslide in HCMR (Quang Binh section)

The sliding movement of the deflection and accumulation zone of deep landslide created cracks and in some cases put up pavement or facilities of road over and they is continues to move with slow velocity depending on the water absorption of the sliding block. Fig. a presented main scarp of a rotational landslide and Fig.

6.5-125 b presented the body a rotational landslide and in HCMR (Quang Binh section).

6.2.1.6 Gully Type Slides and Debris Flows

Debris flows are geological phenomena in which water-laden masses of soil and fragmented rock rush down mountainsides, funnel into stream channels, entrain objects in their paths, and form thick, muddy deposits on valley floors. Slope material that becomes saturated with water may develop into a debris flow.

Fig. 6.6 Debris Flow and gully on HCMR-Hue Section-Vietnam

In study area, gully type slides and debris flows, by which an arc-shaped sliding mass originating at the upper area of a slope slides down the slope surface, and the morphology resembles an erosional gully. The development of the small gullies is the initial manifestation of the phenomenon due to the movement of soil particles or fragments of weathering surface, which has resource from broken type of geology wedge structure running follow surface water stream. The small gullies develop larger and larger, and created deep gullies on the surface of slopes, and in some cases, they link together and as the result the slopes failure occurred. Pictures of debris flow and gully on HCMR-Hue section-Vietnam is presented in Fig 6.6.