Research On Reactivation Mechanism And Hazard Assessment Of Ancient Landslide In Upper Reaches Of Minjiang River

Due to the steep topography in China even in the global,active internal and external dynamic geological effects,and extremely complex geological evolution process in the eastern margin of Tibetan plateau,a great number of large landslides have intensively developed on both sides of the big rivers.Affected by frequently occurrence of extreme weather and the intensifying human activity in recent years,the problem of ancient landslide reactivation is increasingly prominent,threating the local engineering facilities and town planning,as well as the safety of public life and property.In this study,the upper reaches of Minjiang River in the eastern margin of the Tibetan plateau are selected as a research area to study ancient landslide.By means of remote sensing interpretation,ground survey,physical and mechanical properties of rock and soil mass tests,physical model experiments,and numerical simulation,the distribution characteristics,the reactivation deformation mechanisms and the potential risk of forming a disaster chain of the ancient landslides are studied.The following results can be obtained:(1)Based on the identification marks,i.e.,morphological feature,boundary feature,material structure feature,surface feature and accumulation feature of landslide,357 ancient landslides were identied in the upper reaches of Minjiang river,which were mainly distributed on both sides of the Minjiang River and its tributaries.Most of the ancient landslides occurred in three periods,namely about late Pleistocene 105 ka B.P.,25~20ka B.P.and in modern times from 1840 to about 100 years ago.Comprehensive analysis has shown that seismic action was the main inducing factor of ancient landslide in the area,and glaciation and extreme rainfall induced a small amount of ancient landslides.The spatial distribution of ancient landslides was controlled by topography,stratigraphic lithology and geological structure.(2)By combing and summarizing typical cases of the reactivated ancient landslides,the reactivation triggering factors of the ancient landslides in study area mainly include rainfall,reservoir water level changing,earthquake,excavation or erosion of the slope toe,etc.The different triggering factors causes ancient landslides to present four different reactivation modes,i.e.,Toe excavation/erosion + multistage sliding belt + retrogressive local reactivation type,reservoir water level change + multistage sliding belt + retrogressive local reactivation,rainfall + multistage sliding belt + progressive local reactivation type,multiple action + multistage sliding belt + overall creep type.The reactivation deformation degree of ancient landslide is mainly controlled by slope shape,undulating shape of sliding surface and driving power of external inducement.(3)Taking the Xuankou landslide in Zipingpu reservoir as an example,which developed in Xujiahe group formation of the middle Triassic,by the means of comprehensive monitoring analysis and numerical simulation,the reactivation mechanism of rocky ancient landslide in the reservoir under the action of rainfall and the change of reservoir water level is revealed.Combined with the rapid decline of reservoir water level and short-term rainstorm is likely to induce the reactivation and instability of the leading edge of landslide.Based on surge calculation using the recommended formulas,the risk of forming landslide-surge disaster chain after the leading edge block sliding is predicted and evaluated,which has an impact on the upper and lower reaches within about 20 km.(4)Taking the Shangyaogou landslide in Songpan as an example,which is an accumulation ancient landslide developing near the Minjiang fault,by the means of rock and soil mechanics tests and physical model experiments,the reactivation mechanism of the landslide under the action of toe erosion and rainfall is revealed.The reactivation process of Shangyaogou landslide is tensile crack occurring in the front part caused by toe erosion–crack propagation–acceleration creeping–successive failure and sliding.Numerical simulation shows that the shangyaogou ancient landslide may be transformed into a debris flow disaster chain when it has been reactivated.When the whole landslide slips,the farthest movement distance is about 550 m,having a high hazard risk for residential areas at the exit of gully.(5)By numerical simulation method,the effects on reactivation of ancient landslide under coupling effects of fracture and rainfall were studied.When there is a crack at the rear of the ancient landslide,as the crack depth increases,it is more likely to induce the overall instability of the sliding body along the original sliding belt,which is characterized with push forward model.When there is a crack in the front of the ancient landslide,the rainfall time required for the reactivation deformation response of landslide is obviously less than that of the landslide with a crack in the rear part.Compared with the development of cracks in a single part of landslide,when cracks are developed in both the front and the rear of landslide,the rainfall required for reactivation of landslide is reduced,and the range of reactivation deformation is larger.With the increase of the number of cracks,the influence range of seepage may get larger,and the reactivation of ancient landslide will be accordingly intensified.