Study On The Reactivation Characteristic And Deformation Self-adaptive Of Landslides In The Three Gorges Reservoir Area

The ancient landslides and old landslides generally occur along the bank slopes.The Three Gorges Reservoir Area(TGRA)is particularly vulnerable to landslide and more than 2600 submerged landslides were identified.A large number of landslides had reactivated after the trial impoundment to 135 m above sea level(ASL),causing damage to the roads,houses and buildings,threatening the navigation of the river.However,the response law of landslide reactivation in the entire area to the reservoir fluctuation is lack of comprehensive and systematic summary.In the more than 10 years since the impoundment to 175 m ASL,it didn't lead to the abundant reactivations of landslides,but the deformation weakened gradually.The deformation adaptability of landslides has not been well studied,which is related to the bearing capacity evaluation of landslides,and also facilitate the study of resolving the contradiction between the benefits improvement and the geohazard prevention in the TGRA.The spatial distribution and the fundamental factors of landslide occurrence in the entire TGRA was analyzed using GIS.Based on the historical evolution process of landslide,the formation process and engineering geological structure characteristics of landslide are analyzed firstly,and the relevant conceptual model is put forward,and the spatial-temporal evolution of different types of landslides response to reservoir fluctuation is studied.A centrifugal landslide model is constructed and tested to investigate the deformation characteristics and long-term evolution of the landslide including a plate-shaped sliding surface.Finally,the deformation characteristics and mechanism of deformation self-adaptive of different types of landslides in the TGRA are revealed using field monitoring data.The main results of this paper are as follows:(1)The bedrocks are the primary control in determining the regional distribution of landslides,and can be divided into four main lithological combinations in the area.The majority of landslides occurred within the sandstone,shale,and mudstone with coal seams combination.The marl and limestone with mudstone lithological combination are found to be the most prone to landslides.The highest number and densest concentration of landslides occur in the consequent slope,and the difference of landslide concentrations between consequent slope and reverse slope results in the subregional differences of distribution of landslides.(2)The sliding-bending landslides typically originate in consequent strata that dip moderately outwards,or steeply without a free-face,and sliding-tearing and sliding-shearing failure modes primarily occur in which dip gently outwards.In the strata that dip into the slopes moderately,the rock strata exhibit bending-tensile motion,and develop a creep-slip-shearing fracturing type of landslide when dip into the slopes gently.Four types of landslide slip surface,arc,polygon,chair and plate-shaped,are proposed.The corresponding landslide originated conditions,genetic models,representative cases and the main distribution in the TGRA are analyzed and summarized.(3)The deformation characteristics of muyubao landslide and Bazimen landslide are analyzed,which are the typical cases of hydrodynamic and buoyancy landslide.The sharp increases in displacements of the Muyubao landslide are governed by the high water level of the reservoir rather than the rainfall.During the reservoir drawdown from 160 m to 145 m,the deformation is dominated by the hydrodynamic pressure,which is produced by the variation in the groundwater level lagging behind that of the reservoir water level.When the water level above 160 m,the buoyancy becomes the fundamental factor governing the deformation behavior because of the better slope permeability.The precipitation has a significant impact on Bazimen landslide and the movements will occur within 3 days after the rainfall.The effect of reservoir drawdown on landslide is not only related to the drawdown rate,but also to the duration of reservoir drainage.It takes 30 days to induce the landslide deformation when the drawdown rate is 0.2m/d,and only 14 days to when the rate of decline is 0.65m/d.The relationship between the drawdown rate and the minimum days of induced deformation is linear inverse.(4)During the 14 years from 2003 to 2017,there are 526 reactivated landslides with significant movements in the TGRA.The frequency of landslide reactivation is very high within three years after 135 m impoundment,one year after 156 m impoundment and the year of 175 m impoundment.The landslides with the arc-shaped and plate-shaped sliding surface are the most numerous and are mainly hydrodynamic landslides.The number of other two types of landslides is relatively small,most of them are buoyancy landslides or composite landslides.The permeability of landslides in the TGRA is mostly distributed between 0.1m/d and 2.6m/d.The permeability of hydrodynamic landslide is mainly concentrated in 0.01?2m/d,that of buoyancy landslide is mainly concentrated in 0.5?5m/d,and that of composite landslide is mainly distributed in 0.01?4m/d.(5)The centrifugal model of the landslide with plate-shaped slip surface is subjected to simulated two-cycle periods of reservoir water fluctuation.The infiltration of water and the hydrodynamic pressure directing into the slope during impoundment are conducive to increase the stability of the slope.The failure processes were characterized by progressive retrogression and the cracks caused by the reservoir drawdown.On the other hand,the seepage erosion has a certain damage effect on the internal structure of the slope.The front of the slope was dominated by the horizontal displacement,while the consolidation and compaction deformation in the vertical direction is primary during the first reservoir drawdown.When the water level dropped again,the front slid along the fractures and fell into the river,but the mid-rear parts had no obvious deformation and presented a phenomenon of self-stabilization.A good coincidence of deformation characteristics is obtained between the experimental results and prototype landslides in Yunyang and Fengjie county.(6)Most of the landslides have experienced deformation for a certain period of time and gradually reached self-adaption.The time required for the self-adaption of the landslides is 2?4 years,that of the buoyancy landslides is the shortest.The mechanism of the deformation adaptive of the landslide with plate-shaped slip surface is that the compactness of sliding mass in the non-submerged area is increased due to the consolidation and compaction deformation in the vertical direction.The self-adaptive mechanism of the landslide with arc-shaped sliding surface is that the rear sliding mass pushes the toe and results in the compaction of the sliding mass and the release of the gravitational potential energy.In addition,the increase of the permeability of the submerged sliding mass caused by the periodic reservoir fluctuation is the main mechanism to gradually self-adaptive for this kind of landslide.