| The Three Gorges reservoir has experienced several storage stages. At the first time,the storage water level reached 175m in October 2010. Since 2013 when the reservoir ran normally, the water level periodically fluctuated between 145m and 175m, and the range of variety of water level (hydro-fluctuation belt) was up to 30m.The local part of Yi-Ba highway mainline and the Xingshan connection line, which are designed and constructed along the Xiangxi River shore, are the tourist passage that lead to Xingshan County and the Shenlongjia forest. In our study area, the altitude of the highway along Xiangxi river bank is between 176m and 200m, which very close to normal water level of reservoir (175m). Thus, the slope of waterfront highway and the slope of abutment are often in the condition of periodically fluctuated water level (water saturated air drying). High fluctuations of water level (up to 30m), water storage, releasing waters and the frequently changing water level, and as well as the heavy rainfalls, will become an important force of bank rebuilding, and also will play a major role in changing hydrological and geological conditions in the bank slope of the Three Gorges reservoir. As a result, it will lead to a new bank rebuilding and reservoir landslide disasters etc., and seriously threaten the traffics and safety of lives and properties of the people in the reservoir area.In this paper, based on the field investigation, by the means of field sampling, rock acoustic wave test, rock physical and mechanical properties lab test, centrifuge model test, numerical simulation analysis and field monitoring and analysis etc.All analysis and research,which concluded the deterioration mechanism of bank slope rock mass quality,bank collapse mechanism of soil landslide,rock mass quality classification and the types and stability analysis of bank slope of rebuilding in the research area,had been carried out.Then,we conclude as follows:(1) The results of the type classification for slope rebuilding and stability partition.According to thecomprehensive survey and analysed of the research area, we divided the slope rebuilding into three categories, they are, erosion and abrasion type, collapse slump type and whole sliding type.Through the rock and soil tests of physical and mechanical properties, the physical and mechanical parameters of rock and soil are obtained, those parameters are basis parameters for centrifugal model tests and numerical simulation calculating.(2) The result of testing the deterioration mechanism of the bank slope rock According to the tests on the typical rocks, the results showed the rock’s fracture of the siltstone slope and the limestone slope, which is impacted by fluctuation of reservoir water level, was developed, as more broken and broken rock.Through the uniaxial compressive strength deterioration tests on wetting and drying cycle rock sample, the results suggested that the degree of strength deterioration of siltstone and limestone follow the growth trend as the number of cycles increases.Under the same conditions, the siltstone rate of degradation was significantly higher than limestone’s, which also show the effects of reservoir’s water on soft rock is more apparent.The test of slake durability suggested that, slaking resistant index (Id) decreases when rock disintegration cycles (N) increases, There is a logarithm relationship, that is, Id=a×Ln (N)+b, a and b are the coefficient related to lithology. In view of siltstone, also found that the coefficients a and b also relate to the sample’s altitude, rock have a higher altitude is more easy to weathering and disintegration, while a and b decrease.(3)The results of slope rock mass classification under the fluctuation effect of reservoir water level.Since there are some drawbacks on the existing methods, and taking into account the cyclical movements of the Three Gorges Reservoir water levels,we introduced the two indexes, they are "the total deterioration degree of slope rock’s mass" (Dh(n))and "the annual deterioration degree of rock’s mass " (d(h)). For wading rock slope rock mass quality classification assessment, we established a slope rock mass classification evaluation prediction formula,that is [BQ]h(n)＝(1-nd(h)[BQ]h(0).Based on the method of BQ, two typical bank slope rock mass quality classification evaluation were carried out.and we forecasted the rock mass quality of the L-slope with siltstone in different altitude, the proposed prediction formula are:The above formula shows that the [BQ] of slope rock mass are correlate to altitude and the running years of reservoir.(4) The results of the centrifugal model test on collapse’s mechanism in the highway reservoir bank.Using the centrifugal model test, we studied the collapse mechanism on the process of "start-development-collapse" on landslides under the two conditions which included rapid drawdown of water level and rainfall coupled rapid drawdown. The results suggested that:The rapid drawdown of water level is one of the main predisposing factors to the highway reservoir bank landslides.When the moment of water level dropped, the slope is sliding overall along the deep soft surface,then the displacement of sliding gradually increase.At the same time, the slope which is located in the original water line immediately produced a small cracks, and the cracks which were below the original water line gradually appeared and spread to both sides. The mutation analysis of pore pressure monitoring value showed that, the cracks gradually extended to the slope interior step by step. The rainfall reduced the shear strength of the soil slope, and a small amount of displacement was shown in the slope, but it did not affect the overall stability of the slope.The two groups of condition test suggested:both slope had the whole sliding under the conditions of rainfall and drawdown of water levels, and the sliding surface occur and develop along deep weak interlayer.(5) The results of the numerical simulation and monitoring feedback for highway slope’s stability.The analysis results of simulation are as follows:The simulation results of Feitai-Mountain landslide showed that while the reservoir water level decreases with the rate of 0.6m/d and 1.2m/d, the stability coefficients are both greater than 1, indicating that the landslide is stable at present. However, the drawdown of reservoir water level have strong effects on the stability of landslide. The faster speed the drawdown, the less stable the landslide.The simulation results of Zhoujia-Slope landslide showed that while the reservoir’s water level decreases with the rate of 0.6m/d,1.2m/d and 2.0m/d, the stability coefficient decreased apparently.The reduced rate and amplitude of stability coefficient is closely related to the drawdown rate of reservoir’s water level. It can be concluded that water level drop is negative to the stability of reservoir’s landslide. The rapid decrease of reservoir water level is a main factor to the relive of the reservoir bank potential landslide and ancient (old) landslide. At the same time, during the time that the reservoir water level dropped to 145m and kept stable, after the stability coefficient of three kinds mode leads to the minimum, all of them increased a little (rebound).Based on the monitoring results of typical deep slope (landslide) deformation in highway construction, it concluded that the deformation’s occurrence and development of accumulative landslide were mainly affected by the construction as well as rainfall,especially strong rainfall.During the road construction,these processes which including the cutting,heaping loading, blasting, and frequent rolling of mechanical vehicle,changed slope shape, and break the initial equilibrium of the slope stress state. Moreover, the infiltration of rainfall and surface water weakened the shear strength of slope material, increased the weight of slope, accelerated slope deformation, and finally leaded to the generation of landslide. This is also the reason that engineering landslide mainly occurred in the rainy reason.Based on all the above consolidated results,considering the combined effects of fundamental factors and influencing factors to slope stability,the qualitative evaluation of bank slope stability had been made by means of multi-level variable fuzzy sets based on dynamic weight, and the stability type of bank slope in study area had been divided into stable, basically stable,relatively unstable and unstable. |
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