A Study On Geomorgraphic Character And Landslide Evolution In Wanzhou City, Three Gorges Reservoir

Landslide is not only a typical model of slope failure, but also an expression form of landform evolution, so the generating process of landslide is related to landform evolution closely. The Three Gorges Reservoir Region has the typical characteristics of valley-type topography, and the layered terrain is very apparent. The geomorphic evolution inthe reservoir is closely related to the regional tectonic setting and the new tectonic movement, the landform also provides a basic condition to the formation of landslides in reservoir area.Wanzhou City is a key area in prevention and control of geological disasters in the Three Gorges Reservoir Region, a number of large gentle-dipped landslides are concentrated in this district, but it is difficult to explain the causes of formation of approximately horizontal strata landslide through the mechanical balance theory. It's generally considered that the weak intercalated layer native in rock is the medium with the formation of landslide zone, when there is a heavy rain, hydrostatic pressure caused by water-filled in pull cracks in the back edge is a key point to make landslide happen by pushing the approximately horizontal strata. In this process, the role of the river valley landform evolution to the approximately horizontal strata of landslides can not be ignored. Gentle-dipped landslides in Wanzhou District have similar combined features of landforms and geology, the elevation distributions of from top to bottom are unstable rock cliffs, ancient landslides, and loose accumulation body, which reflects the evolution of geological features of ancient landslides in different stages.In order to reveal the slip mechanism of gentle-dipped landslides in Wanzhou District and to analyse the relationship between landform evolution and landslide formation causes, Firstly, based on the summy of research combined the regional geological setting and the new features of tectonic activity, the papar analyses the evolution process of a typical river landscape in the Three Gorges Reservoir Region and analyse the evolution of geomorphology and landslide by using the basic model of the system evolution. Secondly, the characteristics of landslide in Wanzhou District is summarized, geological environment and ancient climate environment in the landslide area systematically analysed, as well as the changes in the process of the ancient riverway of Zhuxi River in Wanzhou District are studied in the paper, then explore the relationship between the ancient landslide in Wanzhou District and the local geological environment evolution. Lastly, based on the borehole data of back edge of rupture of Shanwan landslide in Wanzhou District, the author combines geology differentiation and numerical simulation analysis, studies the river valley landform evolution and explore the formation mechanism of the gentle-dipped landslides in this area. This paper has got the following stage, innovative research results.(1) According to tectonic and landscape patterns in the Three Gorges Reservoir Region, with the boundary of Fengjie, physiognomy of the reservoir divided into two units, east part and west part, the east unit is the Three Gorges canyon, the west unit with main area of wide vale is the low hill area of the east part of Sichuan basin. Two topography units are respectively impacted by those two tectonic units, Sichuan col and Bamian Mountain fold belt. Affected by the new tectonic movement of crustal uplift stage, topographic features of the reservoir which with characteristics of high mountains and canyons, as well as multi-level terrace and planation surface, have significant layered profile landforms.(2) Tectonic lines in the reservoir are usually in control of the water system and the extension of the mountain. The landform between the top of the reservoir and Badong is mainly affected by Huangling anticline and Zigui syncline, and large rivers flow from the peripheral of the syncline, such as Xiangxi river. It also has some water systems developing along the syncline core, distributing like branches. Affected by Qiyao fault zone and a series of compact folds, the Yangtze River almost paralleled to or oblique crossing the tectonic line. Fengjie to Chongqing belongs to Chuandong fold belt, which is composed of a series of compact anticlines and broad syncline in NE direction.(3) Since the neotectonics, the elevating speed of the Terrace is higher than the riverbed. The crustal uplift in the reservoir appears spatially unbalanced. There is a trend of rapid uplift between Fengjie and Wushan. The trend exists since the Pleistocene, more distinct in the Holocene, resulting in some inflexions to the terrace in the Fengjie region. However, the development of the terrace is basically similar in the same region. It appears that new terrace uplifts fast, while old terrace uplifts slow. Presently, the direction of the principal compressive stress of the crustal is NE12°～60°, which is parallel to the water level.(4) The geomorphological map of valley in Wanzhou and Badong shows that, in the prophase, the slope is mainly doing the undercutting movement. When the base level of erosion go down, it appears that the cliff on the bank slope may collapse backwards constantly or slide downhill. In the later stage it appears to be horizontal transformation. Generally speaking, the evolution of the valley slope is mainly the horizontal decline.The evolution process of the Three Gorges valley geomorphology is affected by the comprehensive effect of the inner agent subsystem, external force subsystem and geologic body subsystem, the former two subsystem are relatively more active, they are easily to change in material and energy. The geomorphology evolution is often a slow process, while the landslide is a mutation process.Considering geomorphology and landslide as an open system, using the information entropy theory in the risk prediction for the regional slope, choosing landform and geomorphology, surface rock group, the thickness of loose accumulation layer, bad geological phenomenon, reservoir bank rebuilding, land-use type and management reinforcement projects to predict the risk of slopes in Wanzhou.(5) Wanzhou City has a significant slope "on the collapse down" feature. After the edge of cliffs slope common to collapse mainly cliffs recede; in the middle of the slope is nearly a level ancient landslide body, the ancient landslide margin after the general development of fracturing tank, split tank connected at the bottom of the deep sliding surface, sliding surface by the rock the formation of a native soft interlayer. Leading-edge developments slump-slope accumulation body, accumulation of body surface and bottom are presented as a multi-stage step-like deformation of the main body of the region. Urban landslide hazards mainly come from the leading edge of the slump-deformation accumulation.(6) Zhuxi River Slope overhead, during rainstorms by the river Side erosion and the downward cutting action, a large area of Slump, forcing river constantly changing, the overall change was from south to north trend.River terraces are formed by the landslide in Wanzhou District, besides the average height of Grass Street slip is low (slip surface elevation of 191m), the other seven ancient landslide slip surface elevation are distributed in 215-240m elevation range corresponds to the Wanzhou district gradeⅣterrace elevation is Q22～Q32 (Middle Pleistocene in the Quaternary and late) product. Soft interlayer between the native rock work as montmorillonite and other minerals mainly hydrophilic, with water forming easy to soften the sliding zone. The ancient landslide was formed from a global warming period, rainfall in the bank slope form the back edge hydrostatic pressure within the fractures with flat push sliding.The ancient landslide was formed earlier than the edge accumulation body, and in the river continued to the next cutting process, the leading edge of the ancient landslide accumulation of body transformation by rivers. Wanzhou red layer structure of alternating layers of rock, in the water level change with easy to collapse, in the collapse of river sediment accumulation at the same time there is formed with the terrace height corresponds to the base platform for hybrid loose accumulation body.Zhuxi River south edge accumulation landslides are related to the formation of the time gap two grade terraces occered or occurred after the formation of terraces. Local fringe slide occurred in the T1 period, such as sub-landslide east of Grass Street, Peace Square, loose body landslide front. Loose bodies piled up with multi-level sliding mode.(7) Mountain Bay Landslide fracturing tank borehole data show that from button to up distribution of sediments are:a complete rock,broken rock, gravel, earth (rock), and silty clay (mud). And silty clay deposits from the collapse and mountains bay weir deposition, landslides deposits have obvious features of collapse cycle, and loose deposits dating show that the formation of a new era of bottom-up changes followed, corresponds to the city of gradeⅢandⅣterraces formative years.In Mountain Bay landslide typical fracturing tank, the average failure rate of accumulation is roughly 44.43m2/ka when the Collapse cycle is below six cycles,and the average failure rate of accumulation 36.92m2/ka when the average collapsed cycles is 7-10,the average thickness of 3.2m in a single collapse cycle deposits, landslides trailing edge escarpment retreat rate is between 0.31 and 0.37m/ka.