Analysis Of Blocking Mechanism Of Tangjiashan High-speed Short-run Landslide And Dam-breaking Mode Of Tangjiashan Barrier Dam

After the1950s, many large high-speed rocky landslides occur all over the world. The catastrophic events promote high-speed landslide studies unprecedented prosperity in the80s and90s. Many creative theory have been proposed successively and successfully applied in practice. However, the systematic studies of collision braking mechanism of the earthquake-induced landslide and dam-breaking mode of barrier dam, etc are less, a few achieved scientific research also has significant limitations. In addition, the successful cases on emergency rescue of barrier dams are rare, let alone that the comprehensive system of disaster prevention and mitigation plans and experience summary have not been proposed yet. By the impact of strong earthquakes and heavy rainfall, large-scale landslides often occur in mountain and canyon area of southwest China. If the blocking magnitude is huge and no effective measure or condition to lower the level of lake to exclude the danger, some unimaginable catastrophic would be caused.On May12,2008, Wenchuan earthquake with the magnitude of8.0happened in Wenchuan County in Sichuan Province. It caused a large number of further earthquake-induced geological disasters, such as rock fall, landslide and debris flow, especially in the worst affected mountain region. The earthquake had triggered Tangjiashan high-speed landslide which is on the right bank of Tongkou River and upstream of Beichuan County. The landslide blocked the river and formed Tangjiashan barrier dam, with length of803.4m, width of611.8m (maximum width), height of82m to124m, area of300,000m2and speculated volume of20,370,000m3. As the water level gradually rose and the hydraulic head pressure difference between upstream and downstream was continuously increasing, it had been appeared infiltrating on downstream slope of the dam. Whether the barrier dam will occur seepage, piping or slope failure, and then leading to dam-breaking has become the focus of common concern.The disaster has brought us the sacrifice and sorrow, at the same time, also offered us the opportunity to study and the way forward. Because Tangjiashan landslide have the characteristic such as earthquake-induced, high-speed starting, collision and braking to stop, infiltrating, successful flood relief, rescue, etc factors, it becomes an extremely valuable research material in many landslide-blocked river event. If in-deep and systematic study could be carried out, the achievement should play a connecting role and would be better able to guide the rescue work for similar incidents in the future.In the specific implementation process of study, the dynamic process analysis of Tangjiashan high-speed landslide and the stability study of barrier dam are adopted as the main line; the dam-breaking mode of the Tangjiashan barrier dam is determined for the purpose; the mechanism study of the process of Tangjiashan landslide blocking the river are carried out systematically and comprehensively. The innovation points and the main achievement are summarized as follows:(1) Based on the accurate access to the topography of Tangjiashan slope before and after earthquake, geological structure of the barrier dam and physical and mechanical parameters of soil, the formation conditions and failure mechanism of Tangjiashan landslide are analyzed firstly. It can be derived that the three free facing terrain features of Tangjiashan slope, consequent bedding structure and intense seismic forces is the most important factor triggering the Tangjiashan landslide. The new failure mechanism of rocky landslide with consequent bedding structure is put forward:"earthquake prompted tension rupture of landslide trailing edge-rock wedge splitting and interface shear sliding on central landslide plane-brittle shear breaking of the bottom rock masses-sudden high-speed set out". And the elaborated formation mechanism of tension rupture, the leverage of wedge splitting rock masses and the fast rolling friction effects of elastic rock debris are described in detail. Furthermore, a stability-loss criterion about cataclinal rock slope sliding along the structural plane under earthquake is deduced.(2) In accordance with the approaches and the theories of elasticity and fracture mechanics, failure criterion of locked-up segment of consequent bedding rock slope is deduced. And the calculating formula of deformation energy of locked-up rock masses, while the locked-up segment is sharply cut off under earthquake, is also deduced. Considering the advantage releasing direction of deformation energy of locked-up rock masses, the energy conversion principle is applied to propose a more reasonable and accurate burst starting velocity formula of the consequent bedding rock landslide. Moreover, applicating the elasticity-plastic theories and the discrete element numerical simulation method, the whole process dynamics analysis of high-speed short-run landslide and the study of blocking-river mechanism are conducted.(3) Trough the analysis of the impact effect of water and air wave, bottom friction effects, collision effect as well as temperature effect, the braking mechanism of consequent bedding rocky landslide and how the landslide brake type control the geologic structure of barrier dam are mainly discussed. A reasonable explanation for the characteristics of the internal geological structure of Tangjiashan barrier dam and the description of its formation mechanism are provided and delivered in this essay.(4) Through simulating the internal seepage field of barrier dam at different water level conditions by the employment of three dimensional software, Vmodelflow, the thesis has analyzed the following aspects:influence upon the seepage stability of barrier dam inflicted by the water level rise and the seepage stability of barrier dam at different water levels. Thus the dam-breaking model of barrier dam,after the water exceeds its capacity, is speculated.(5) By the employment of the Geostudio, the stability of upstream and downstream slope at740m water level is simulated. The outcome shows that the barrier dam, as a whole, still remains solid and stable, except the gravel soil could destroyed, at the shallow upstream slope and the downstream slope’s toe, in a limited scope. Thus the dam can still remain stable and no overall or partial outburst collapse will take place, even if a Ⅷ seismic intensity afterquake occurs.It is also indicated that when the water level is over the lowest crest elevation.(752m), the dam will occur flood overtopping failure, and its dam-breaking mode could be progressive outburst to gully.