Characteristics And Reslip Mechanism Of Ancient Landslides In The Lower Reaches Of The Yangtze River

In recent years,with the increase of extreme heavy rainfall in the lower reaches of the Yangtze River,there are more and more intermittent creeping landslides in the gentle slopes of the Ningzhen area.Traditional understanding of the gentle slopes in the lower part of the mountain is conducive to the stability of the whole mountain.Results of geological geological survey reveal that the terrain is steep and gentle,the landforms are mostly broad-shaped chair-shaped,and Zuihan forest and Madao forest are common here,which are typical features of pale landslide features.Ancient landslide body is usually in a temporary stability state,and under certain natural or human factors,it often slips again to form a new landslide.At present,there are few studies on this type of landslide in the lower reaches of the Yangtze River,and there is a lack of systematic understanding of its characteristics and mechanisms.Due to the developed economy and large population density,the landslide is very harmful,as a result,it is very necessary to carry out research on the mechanism of such landslides in order to provide a basis for forecasting and scientific prevention.In this work,we took Ningzhen area,which is in the lower reaches of the Yangtze River as a main research area,and carried out analysis of the tectonic movements,climate,sea level rise as well as the evolution of geological environment of the Yangtze River since the Cenozoic and the Quaternary.We also introduced the Quaternary stratigraphic structure of the area and the sedimentary characteristics of fine-grained soils are used to analyze the characteristics and genetic mechanisms of ancient landslides in Ningzhen area.Then,the re-sliding of the ancient landslide is derived from the extreme heavy rainfall in recent years.Taking the Paomashan landslide in Zhenjiang as an example,combined with the automatic deformation monitoring,the deformation characteristics,genetic mechanism and regional distribution of the ancient landslide re-sliding are explained.Finally,the corresponding monitoring and early warning measures,stability evaluation methods and enlightenment for engineering activities and disaster prevention and reduction are proposed.Main findings and conclusions obtained through the research are as follows:(1)The EW-oriented basement structure of the pre-Sinian,the NE-trending structure formed at the end of the Early Paleozoic,series of quasi-story caprock folds formed by the Indosinian movement have laid a foundation for this area.The basic pattern of geomorphology and geological structure;Jinning Movement and Indosinian Movement determined the development and evolution characteristics of geotectonic structure in this area;the NNE and EW faults formed by the Yanshan Movement profoundly affected the structural pattern of the area,and determined modern landscape of this area.(2)Regional geological tectonic background and regional faults control the development of ancient landslides in this area,while the weak structure layers are the dominant structural surface of the ancient landslide bodies,controling the ancient landslide boundaries and their re-sliding.Ancient landslides and their re-sliding in Ningzhen area mainly occurred in low hills and hills covered by the Quaternary sediments.There is a trend of landslide group development,and it is located in a zone with dense or active folds or folds.(3)Differential elevating and descending of the crust,climate change,transgression of the sea,and evolution of the geological environment such as the Yangtze River system jointly determined the special topography,stratigraphic lithology and sedimentary characteristics of the fine-grained soil,as well as the soil composition of the slope of this area.Under the influence of some natural factors,the slope slides along the weak structural layers to form an ancient landslide.(4)Although the Xiashu loess in the south bank of the lower reaches of the Yangtze River has similar characteristics to that of the northern loess,its composition is not entirely composed of Malan loess in the northwestern region,and the other components were formed by weathering of the mountain in this area.(5)The special topography,stratum lithology and structure of Paomashan Mountain provide the basic conditions for the re-sliding of ancient landslides;and the temporary confined water caused by extreme heavy rainfall and unreasonable engineering activities are induced by the re-sliding of ancient landslides factor.(6)According to the monitoring results towards the deformation of the ancient landslide body in Paomashan Mountain,the deformation process cost a long time,which can be divided into three stages:initial deformation,constant velocity deformation and accelerated deformation,the formation of temporary confined water and dissipation basic cause of the deformation characteristics of its intermittent creep.(7)Due to the influence of temporary confined water,the types of failure of the gentle slope in the front of the Ningzhen area are mainly top-slip-slip type and high-angle shear-slip type.The type of failure of the re-sliding of the ancient landslide of the Paomashan Mountain is high angle.Shear slip type.(8)"Calculation of water first",the key to controlling landslides is controlling precipitation.We should not only pay attention to the monitoring of the groundwater level in the slope,but also try to lower the groundwater level and prevent the formation of temporary confined water effectively through the construction of intercepting drainage ditch,underground drainage corridors,and setting up precipitation wells.