Research On Effect Of Increasing The Daily Drop Rate Of The Three Gorges Reservoir Level On Remedied Landslides

Since the impoundment of Three Gorges Reservoir after flood season in 2003, due to controlling the reservoir water level decline rate during drop-fluctuating period, among 2600 submerged slide masses, only 6(0.23%) slumped into the river severely and other 591 occurred small-scale deformation primarily in past 10 years. Both quantity and degree of deformation show a downtrend recently. However, controlling water level decline rate strictly limits the benefit of Three Gorges Reservoir project to some extent. For that reason, whether or not through increasing the reservoir water level decline controlling rate during drop-fluctuating period to increase benefits of flood control and power generation become a concerned important issues to be studied and solved. On the basis of detailed field investigation and view, combining past landslide information and using the double-ring permeation experiment to obtain the correct hydrologic characteristics of soil mass in fluctuation belt of water table, analyzed the failure discipline of submerged slide masses under the action of Three Gorges Reservoir water level fluctuation. Evaluate the stability of Three Gorges Reservoir Region remedied submerged slide masses, under conditions in which the reservoir increase water level decline rate during non-flood season. Comprehensive analyze and demonstrate the sustainability of increasing water level decline rate. Through a series of analysis and study, the author obtained following conclusions:1.There are 39 remedied landslides with good stability in the area from FengJie to WuShan in Three Gorges Reservoir. Affected by the cyclical fluctuation of Yangtze River water level, 14 of them(accounted for about 36%) have deformations which appear near the water leading edge of the landslide during reservoir water level decline period.2.Using the double-ring permeation experiment to obtain the correct hydrologic characteristics of soil mass in fluctuation belt of water table, get the conclusion that the of Three Gorges Reservoir Region fluctuation belt of water table is closely related with grain composition in soil mass. Fluctuation belt of water table is mainly made up with silty clay(1 m/d < K < 2.75 m/d). Due to the erosion by the Yangtze River, the closer the Yangtze River, the greater permeability of soil mass. Further analyze deformation law and get the different effects of reservoir water level fluctuation on landslide deformation and remediation measures in different location.3.At first, select different hydrology and seepage characteristics of the soil mass and study its seepage field transformation characteristics during reservoir water level fluctuating. Second, analyze different effect of different seepage field transformation characteristics on the overall stability of landslide with a normal mechanical model. Last, conclude the effect of reservoir water level fluctuation on deformation law of landslide, by which divided landslide into two types, weight reduction and flowing pressure, and the critical value K = 2.75 m/d.4.By studying the relationship between permeability coefficient of soil mass in fluctuation belt of water table K and daily decreasing amplitude, following can be concluded: ①when H/d is low, K is main control factor. Landslide type changes from flowing pressure into weight reduction with K increasing, because of the impact of H/d is negligible.②when H/d gradually increase, the influence caused by K difference gradually decrease. H/d becomes the main control factor. H that accumulated by both inside and outside of slope in unit time is controlled by H/d directly. Therefore, when H/d is high, the influence caused by flowing pressure effect to slope FOS is obviously higher than that caused by K.5.XiangJiatang landslide is in stable state. When reservoir water level fluctuates, only the secondary landslide may appear deformation. The unsupported parts of it have larger deformation, while the remedied parts have the smaller. XiangJiatang landslide is in stable state when daily decreasing amplitude add to H/d = 1.2 m/day, whereas the secondary landslide is in limit state. When reservoir water level decreases below 150 m, the soil mass of anti-slide pile loading segment have deformations that affect its supporting function. If daily decreasing amplitude continually adds to 1.6 m/day, the secondary landslide would move along the sliding surface and has deformation again.6.For the 39 remedied landslides from FengJie to WuShan in Three Gorges Reservoir, when the H/d=1.2m/d, all of the FOS>1.20, in a stable state; submerged parts of 35 of them FOS ≤1.15,in a limit state, could not slope totally and become disaster. However, in case of both partial as well as complete failure of remediation measures, the FOS of the landslide will decline sharply and have potential to landslide again. By calculation, acceptable daily drop rate of water level threshold value of remedied landslide equal 1.6m/d.