Research On The Time-varying Characteristics Of Slope Deformation In The Head Area Of Three Gorges Reservoir

According to statistical analysis at home and abroad, The stability of the reservoirs slope whether it is rock bank or soil bank, would be deteriorated or destructed during the reservoirs impoundment. Such as Malpassetin arch dam failures in France, Vajont catastrophe massive slide displaced the water in Italy, Grand Coulee reservoir slope in America,206 earth dams in North America, and Fengtan, Zhexi, Dongjiang and Baiyutan Reservoirs in China. The above mentioned reservoirs slopes occurred slope deformation or slope failures events after the impoundment. The water conservancy workers paid great attention to the reservoirs instability problems. Furthermore, these problems even become important factors in determining the engineering scale or deciding whether or not a project of the construction.Since Three Gorges Reservoir (TGR) begin to impound in June 2003, the water level raised up to EL.175m in December 2014 and successfully operating for 2 years. The height of the rising of the water level in the head area (from the damsite to the Niukou river) raised about 110m and the maximum annual amplitude up to 30m and the initial seepage field have changed. The physical and mechanical properties of the rock(soil) along the reservoirs slope deteriorated due to the water-rock interaction. So the slope stability have been affected by the above mentioned factors. Some large and medium-sized landslides such as Yemaomian, Yanbao, Laoshewo, Shuping, Baishuihe, Fanjiaping, Daping, Huanglashi, etc. in the Yangtze river, occurred obvious displacement. The deep seated rock Qianjiangping landslide occurred soon after the Three Gorges reservoir water level up to 135m in 2003. According to the statistical documents, there are 152 events of geologic hazards appeared slope deesince TGR impounded in the head area of the reservoirs."Slope deformation time-varying characteristics in the head area of Three Gorges Reservoir" was selected as the research subject. Based on the in-depth engineering geological investigation of the reach from the TGR dam site to Niukou, the deformation of bank slopes in the area, which occurred during June 2003 (began to impound) to December 2014, was investigated in detail (Chapter 2). The relationship and evolutionary process of the slope deformation with the impoundment and the rainfall spatial-temporal distribution were analyzed in the paper, based on the mathematical statistics and the analysis principles of engineering geology (Chapter 3). According to the space-time relationship of deformation characteristics and the coupling of reservoir impoundment and rainfall, the influencing factors were classified into outer dynamic factors, internal factors and other factors (artificial disturbance, earthquake, vegetation covering, etc.). The time-varying process of the slope deformation to outside factors changing was simulated (Chapter 4). Based on the research mentioned above, the evaluation index of regional geological hazard activity in reservoir area, such as point density, terrain change ratio and area ratio were contrastively studied. The active intensity index was put forward, and the evaluation system of geological hazard activity degree of reservoir was established with consideration of active strength index. The geological hazard activities of bank slope in TGR were divided into 5 stages, since the first impoundment (Chapter 5). Finally, Shaxi landslide and Kaziwan landslide were taken as examples to analyze the time-varying process of the slope deformation in detail, and to classify the bank slopes according to the time-varying curve. As the above, the main conclusions and innovative results are shown in the following:1. The field investigation and analysis shows that geological hazard masses in the head area of TGR mainly develop in Jurassic clastic rocks and Quaternary loose overburden in the middle and lower of the first grade slope, especially in the bedding slope made up of the middle-upper Jurassic mudstone and siltstone. They mainly distribute at the right bank of Yangtze River between Guojiaba and Shazhenxi, the right bank of Xiangxi River, both banks of Guizhou River and Qinggan River, and the right bank of Tongzhuang River. The spatial and temporal distribution is territoriality and timeliness obviously. Moreover, the spatial distribution has the characteristics of stripped, vertical zoning and relative concentration, while timeliness has the characteristics of periodicity and hysteresis.2. On the basis of the engineering geological analysis and the statistics, the range, pattern, degree, and spatial-temporal distribution of the effects to reservoir bank slope of reservoir filling and were studied, and then the deformation evolution process of bank slope was established.With the rising of water level (135m～156m～175m), the first water storage of each stage had the largest influence on the deformation of the slope, and the effect was decreasing with the increasing of the water level in each storage stage, the peak of scale-time curve decreases cyclically.In the initial stage, the influence of reservoir impoundment on reservoir bank slope firstly was the soaking deformation of rock mass, which destructed the structure and strength of the rock or soil. Meanwhile, a series of chemical reactions such as hydrolysis, dissolution and carbonation occurred due to the water immersion, which led to the reduction of cohesive force and shear strength of rock and soil materials. Thirdly, it was also a significant reason that the lag of the periodic fluctuation of reservoir water level and the dynamic effect of the groundwater level which caused the change of the groundwater seepage field and the pressure in the slope. The humidification of rock and soil was completed after they had soaked for a certain period of time, and the slope of reservoir bank adjusted and released stress to adapt the new environment. The mechanical properties of rock and soil were generally close to saturation, and the stability of bank slope was mainly controlled by dynamic water pressure caused by the water-level fluctuation in the period of water lowering.The research on the relationship of the height of reservoir water level and the frequency of the bank slope deformation shows that the most frequent disasters happened at the water level range of 145m to 150m, and the maximum volume disaster event happened at the water level range of 150m-155m. Therefore, the water level range of 145m to 155m can be thought as the most unfavorable water level of the reservoir bank slope stability. The deformations of reservoir bank slope happened mostly in June to September, and the time of these hazards lagged about 10～15 days from reservoir impoundment.3. Based on the research of the evaluation of geological hazard, such as point density, area ratio and terrain change ratio, the concept of active intensity index and its calculation method was proposed. It shows that it was not suitable to evaluate the active strength of a regional geological hazard with only consideration of the area density ratio of geological hazard, which was pure a percentage of space geometry without the energy size. The active intensity index of geohazards takes into account the major affecting factors as far as possible, and describes the intensity of geohazards with the expression form of energy. The index is superior to others in evaluating the active intensity of geohazards.Grey-Markov chain method, grey-period extension method and spectrum analysis method in regional geohazard prediction were studied. The results show that the spectral analysis method is good to predict the regional cluster geohazard. Using the method of spectrum analysis, on the basis of fitting the scale of geohazards in 2003-2014, the prediction model of the geohazards in 2015-2030 was established as follows:S(t)=2.2005+0.7258cos(0.18027πt+0.4113)+1.7896cos(0.3604πt+0.6886)+1.41 lcos(0.5406πt+0.6708)+1.4682cos(0.7208πt+2.8036)In turn, a forecast analysis was undertaken. It shows that the scale of geohazards in the research area will reach the next peak and then gradually tend to lower level in the end of 2015 to early 2016.4. The cluster analysis of geohazard activities in the research area in 2002-2014 was carried out. Combining with slope surface deformation, TGR storage schedule, and point density, terrain change rate, etc, the activity degree of reservoir slope deformation were divided into 4 levels,1 level (weak activity),2 level (apparent activity),3 level (strong activity) and 4 level (very strong activity). Lastly, an assessment system of geohazards in reservoir impoundment was established.According to the evaluation system of geohazards in reservoir impoundment and geohazard activity intensity index, combing with the characteristics of time-varying evolution, the feature point (curve inflection point), the impoundment stage and surface topography, the degree of geohazard activity caused by the influence of reservoir impoundment could be divided into 5 stages:quasi-stationary-phase, active period, strong activity period, shock decaline period, and dynamic equilibrium stage, and the evolution of reservoir slope was studied.In the whole process of reservoir impoundment, the active relationship of the geohazard was as follows:strong activity period (13)> active period (12)> shock decline period (I)> dynamic equilibrium phase (15)> quasi-stationary-phase (II).To reduce the effect of impoundment on reservoir bank slope, if appropriately extend (T1 to T2) time and increase the initial water level and supply the full impact space-time that the reservoir impoundment had on the reservoir slope in the corresponding elevation, stress adjustment can be completed or almost completed in low level and the peak level of geohazard activities effectively weaken at the next stage of storage, which can decrease the effect of impoundment on reservoir slope.According to the behaving rule of TGR impoundment, it start about 2afrom initial water storage, lasting about 10a, which was about 5 times the previous period. At present, TGR is in a transition period (curve inflection point) from shock decline period to dynamic equilibrium phase. To shorten the duration of shock period (T4-T5) as soon as possible, it can also be considered to increase water level fluctuations in the first 3 cycles, to speed up the distribution and adjustment of its impact in the low level (controllable). According to preliminary estimates, if the water level fluctuations is increased by 10% in a period, shock decline period can shorten of about 1.5a.5. In the light of the three stages, contacting time-varying curve characteristic of slope deformation in study area, the deformation of reservoir slope was classified as 3 types:step type, smooth type and composite type. It was considered that the essence was the conversion of the sensitivity of the slope rock mass to the fluctuation of reservoir water and the favorable and unfavorable result of the water-rock interaction.The composite type was relatively rare in the reservoir area. In the process of deformation developing, since the reservoir impoundment was at initial stage, and the permeability of the rock and soil was low, the slope time-varying curves present a smooth features, but with repeatedly and periodically storage of water, the rock and soil particle material was partly taken away, resulting in slope permeability changing greatly, and late deformation curve presenting in a step type, the overall slope deformation-time curves performing smooth-step type.