The Formation Mechanism And Analysis On Dam-Breaking Mode Of Tang Jiashan Barrier Dam

Wenchuan earthquake with the magnitude of 8.0 happened in Wenchuan County in Sichuan Province on May 12,2008. It caused a huge loss of life and property, and generated a large number of secondary geological disasters (like rock fall, landslide, debris flow etc) especially in the worst affected mountain regions. The earthquake had triggered Tang Jiashan high-speed landslide which is on the right bank of Tongkou River, and it is about 4.7km upstream of Beichuan County. The landslide blocked the river and formed Tang Jiashan barrier dam, and the dam is 803.4m long,611.8m wide (maximum width),82m to 124m high, it is about 300,000 m2 in area and 20,370,000 m3 in volume. With the gradual uplift of water level, it appeared seepage on downstream of the dam. Obviously, the barrier dam will damage due to seepage or instability of the dam slope which led dam-break has become the issue of common concern.Based on accurately obtained data of the dam on geological structure and the parameters of the soil's physical-mechanical properties, first, this thesis analysised the formation conditions of Tang Jiashan landslide and the formation mechanism of the Tang Jashan barrier dam. Free surface of three sides on terrain, slope within the soft interlayer and the negative combination of the structural plane and the effect of earthquake are the main factors in formation of the Tang Jiashan landslide. The formation mechanism of barrier dam summarized as the following stages:seismic action→through of the sliding plane→high-speed falling of landslide formed the air wave and erosion riverbed→slide of the trailing edge of the slope→landslide blocked and stop sliding→blocking the river by the landslide→the formation of the barrier dam.Second, simulate the seepage field of the dam under four conditions of water level (710m,720m,730m and 740m) of the dammed lake based on the parameters of the soil and analysis the seepage stability of the dam. Simulations show that the dam is stable beside the downstream of the dam, because of the seepage gradient is greater than allowed gradient. On the lake water level of 740m, the dam slope stability simulations show that the dam slope is stable even in a strong earthquake except the surface gravel on the upstream and the slope toe on the downstream, the barrier dam does not occur full or half dam breaking.Finally, the dam stability results show that the dam will not damage except for the surface gravel. When the lake water level uplift to the lowest crest elevation(752m), dam overtopping damage will occur, and the dam-breaking mode is progressive failure.