Research On Reactivation Mechanism Of A Large Bedding Landslide In Xiangjiaba Reservoir

Landslides are a common geological hazard worldwide and have become more frequent in recent years owing to the decreased rainstorm recurrence period,increased earthquake frequency,and other factors.Water conservancy and hydropower projects bring large economic benefits for human society but also influence the geological environment around the reservoir bank.If triggered by slope structure,rainfall,and reservoir water,revival instability of large or giant ancient bedding landslides can easily occur around the reservoir bank.At the same time,the landslide reactivation mechanism is extremely complicated owing to variable geological environment characteristics,formation conditions,and other influencing factors,and it has become difficult problem in disaster prevention and reduction work.Therefore,it is of great significance in theoretical and practical to study the reactivation mechanism of bedding landslides in reservoir areas.In this paper,a large reactivation bedding landslide in Xiangjiaba Reservoir was chosen as the research object.The temporal-spatial evolution characteristics of reactivation landslide were studied on the basis of data collected from detailed geological surveys and many years monitoring,and the reactivation evolution process and model were revealed based on the characteristics of ground surface cracks distribution.Microscopic and residual strength characteristics of slide zone soil were studied with advanced laboratory facilities and instruments,and the internal reactivation mechanism of landslide was revealed.Three-dimensional numerical simulation was employed to analyse the response laws of deformation under the influence of groundwater,and the reactivation mechanism of landslide deformation induced by rising groundwater was revealed.The effectiveness of combined prevention,drainage tunnel and anti-slip key,was proposed and demonstrated according to the results of 3D seepage field simulation of groundwater with drainage tunnel.The main research findings and conclusions are summarized as follows.?1?Study on the fundamental conditions of landslide reactivation.The large bedding landslide in Xiangjiaba Reservoir represents partial reactivation of an old landslide and is in the shape of“?”with wide front and narrow back.The front faces empty space,directly to the Jinsha River.The strata?steep upper layers and gentle lower layers?is mainly developed in the Upper Shaximiao Formation of the Middle Jurassic(J_2S).The rear part rock is mainly soft mudstone.There are several soft muddy interlayers in the leading edge.The landform and physiognomy,geological structure,and formation lithology provide qualifications for the reactivation of landslide.?2?Study on the temporal-spatial evolution characteristics of landslide reactivation.Affected by internal?landform and physiognomy,geological structure,formation lithology?and external?rainfall,reservoir water?factors,the reactivation landslide goes through a process of creep-accelerated deformation-stabilized deformation and shows rear part advancing-middle part bedding slip-leading edge shearing.The temporal-spatial evolution characteristics of reactivation landslides are as follows.?1?The deformation direction in different zones of the landslide is almost same.The P-1 fill zone on the east side of landslide produces a deformation in the direction of north by east due to the different consolidation characteristics caused by partial filling,and presents a characteristic of left rotation during deformation.The rear part G zone angd middle part P zone produce a deformation in the direction of nearly due north.The leading edge part Q zone deforms to the north by west and occurs uplift under the pushing.?2?The deformation in different zones varies spatially.From 2013 to 2015,the average horizontal and vertical deformation values in each zone show different characteristics during dry and rainy seasons.?3?The displacement velocity fluctuates with time.The accumulative displacement changes with time is in the form of“step”and increases significantly during rainy season.The deformation rate fluctuates up and down over time,the landslide presents steep periodic rising in the accelerated deformation stage,and the peak presents periodic change.?4?The deep deformation differs in different zones.The surface deformation of the rear part is synchronized with deep deformation,while the surface deformation of leading edge is slightly larger than deep deformation.There is secondary shear in the shallow part of landslide.The variation of groundwater is mainly controlled by rainfall and shows a positive correlation with daily rainfall,but the maximum peak of groundwater lags behind the maximum rainfall.Landslide deformation is mainly controlled by rainfall and shows a positive correlation with monthly rainfall.The influence of reservoir water on groundwater is mainly reflected at the leading edge of the landslide.Only deformation of the leading edge is influenced by reservoir water,and the deformation of sudden drop of reservoir water on the slope is greater than sudden rising.?3?Study on the microscopic and residual strength characteristics of slide zone soil.The main internal mechanism of landslide reactivation is owing to destruction of the slide zone soil original structure,reduction of the shear strength,increase of the fine particle contents and strong orientation during the formation of landslide,all of which provide basic qualifications for the reactivation of landslide.At the same time,slide zone soil is rich in strongly hydrophilic clay minerals with content up to 26.5%-40.7%.If the soil is saturated,it expands and softens,and the shear strength reduces again,particularly the residual strength.The results of ring shear experiments on slide zone soil show that single-stage shear tests take much more time but produce highly accurate residual strength,while the residual strength of multistage shear tests is larger.The relationship between shear rate and residual strength stress ratio exhibits a concave pattern with the lowest mobilized stress ratio at 1.09 mm/min.The relationship between residual strength and fine particle contents exhibits a quadratic polynomial correlation.When the fine particle contents exceed the critical value,the residual strength will decrease gradually.To some extent,higher fine particle contents can reduce the residual strength.?4?Study on the landslide reactivation mechanism and prevention measures.Affected by heavy rainfall and reservoir water,the landslide safety factor decreases with significantly increasing groundwater from 2013 to 2015.The deep deformation of rear part is approximately consistent with that of surface,which pushes the middle part to produce deformation and the deep deformation is slightly larger than that of surface.During the deformation process,a part of the landslide deforms in the direction of north by east and the other part deforms in the direction of north by west.Deformation value is larger in the east than in the west.Affected by the anti-sliding part,the surface deformation of leading edge is slightly larger than that in the deep deformation.The landslide deformation model is a type of advancing bedding shearing.The secondary shear is located at the rock-soil interface.Under the influence of the anti-sliding part and strong pushing of the rear part,displacement and stress are concentrated in the groove of the rock-soil interface.Affected by the terrain,the east and west rock soil mass at the lowest point of groove cause a small part of south and north rock soil mass near lowest point to deform in different direction.According to the 3D seepage field results of groundwater with drainage tunnel,numerical simulation results of combined drainage tunnel with anti-slip key of landslide show that the range of landslide deformation decreases gradually with decreasing groundwater,the anti-slip key area is the most obvious.The maximum value distribution expands somewhat but the value is smaller than that of the landslide only controlled by drainage tunnel placed in a deep position.The safety factor is larger than that of the landslide only controlled by drainage tunnel.Landslide stability can meet the specification requirements when the drainage tunnel is placed in a deep position and combined with an anti-slip key is adopted.