Distribution of landslides adjacent to the northern side of the Yarlu Tsangpo Grand Canyon in Tibet, China

This paper presents results of recent studies on distribution and category of landslides in one section of the Sichuan–Tibet Highway, adjacent to the northern side of the Yarlu Tsangpo Grand Canyon, Tibet, Southwestern China. In the tectonic setting predominated by compression and strike-slipping, active faults are dominant and result in the genesis of the great alpine relief together with fluvial incision and unloading. In this section, with a distance of about 290 km between Ranwu and Lulang, 34 landslides occurred. Among them, the 12 large and super-large landslides comprise the most dangerous part of the highway system to road users over the past 50 years. The landslides usually occurred in slopes comprised of moraine with a large thickness, fluvio-pluvial and lacustrine deposits and fractured rocks. Based on the examination of the physical geography, structural geology, Quaternary geology, stratigraphy and petrography, this paper presents the temporal-spatial distribution of landslides along the section and classification of them into three types with respect to mechanism and composites of landslides.

Type 1.	Landslide initiated at high elevation and transformed into a distal debris flow damming the river with a long reoccurrence interval
Type 2.	Landslide at stream banks, in different ranks of mass movements in dissecting layers of moraines
Type 3.	Landslide of fractured rock mass with quick translation movements.

Each of the three types is presented by distinctive case examples. Type 1 is Yigong Landslide; type 2 includes Dongjiu Landslide group, 102 Landslide group, Suotong Landslide group, Songzong Landslide; and the type 3 is Layue Landslide. A binary landslide, which is seldom seen in nature and which occurred in the outlet of one creek under the integrative function of fluvial incision and road cut, is also presented. These are distributed in different parts of the area with characterized structural geology, topography and composites (gneiss and Quaternary deposits) with accumulative quantity between Suotong and Dongjiu. The most occurred with frequent and rapid translation, shallow shearing planes, various influential factors and unique triggering factors, and a variable period of reoccurrence with a tendency of increasing magnitude. In this region, six influential factors greatly contribute to their occurrence: (a) intensive neotectonic movements, (b) ongoing alpine canyon relief, (c) earthquakes, (d) heavy precipitation, (e) stream erosion, and (f) human activities, to a certain extent. Mostly, the latter four factors play major roles in triggering landslide occurrence. The earthquake causes the rock and soil to fracture and deform, and tends to be unstable, or cause earth-induced landslides. The heavy rainfall in monsoon seasons which saturates the soil and lower the shearing strength is mostly responsible for the occurrence of landslides. The stream erosion disturbs slope toes causing progressive failure. Road construction and deforestation decrease the natural slope stability. Comments and suggestions on the physical measures for remediation of landslides and safety of highways are also made.