Catastrophic Mechanism and Stability Analysis of Jiangdingya Landslide in Zhouqu County, Gansu Province, China

On July 12, 2018, a large-scale landslide occurred in the Jiangdingya area, Nanyu Town, Gansu Province, China. According to remote sensing interpretation and field investigation, the landslide volume is about 480×104 - 550×104m 3 , which caused the Bailongjiang River to be blocked and the water level to rise, forming a dammed lake, flooding Nanyu Town. The landslide is characterized by chain disasters, which poses a serious threat to the ecological security of the upper reaches of the Yangtze River. In this paper, the catastrophic mechanism of Jiangdingya landslide is analyzed, and the stability of the slope is studied. According to the disaster characteristics, comprehensive safety control measures are put forward. The research results of this paper are of great significance to the prevention and control of chain disasters and the comprehensive treatment of slopes.


Introduction
China is one of the countries with serious landslide disasters in the world. Due to the special landform and geological environment conditions, landslide disasters occurred every year in history. There are nearly one million geological disaster points in China, among which there are more than 34,000 serious geological disaster points. Nearly 1,000 people die every year due to geological disasters, and the direct economic loss is 20 billion yuan. The annual cost of landslide remediation of the state is as high as 50 million yuan [1]. The Zhouqu County of Gansu Province is located in the south of the West Qinling tectonic belt, with high mountains and deep valleys, well-developed geological structure and strong neotectonic movement, which is a seismic prone area. The rock mass in this area is broken, the structure is loose, the slope is large, the geological environment is fragile, and the soil erosion is serious It is one of the areas with high incidence of landslides, collapses, and mudslides in China. There is a high frequency of resurrection of landslides in this area, such as Xieliupo landslide, Nanyu landslide, Suoertou landslide and so on. At present, some landslides continue to deform and cause new disasters. At about 17:00 on June 13, 1991, the Jiangdingya landslide of Nanyu Landslide Group in Nanyu Town, Zhouqu County, Gansu Province, China, experienced a largescale sliding [2][3][4][5] The lower part of the landslide body was firstly sheared and destroyed, then slowly declined, and instantly blocked the Bailongjiang River. Then, the upper part of the landslide mass was pulled to slide rapidly. The huge thrust of landslide soil and the jacking action of the steep wall on the other side of the Bailongjiang River made the sliding tongue of the sliding body climb along the steep wall, and finally form an earth dam with a height of 20m and a width of 50m. As a result, the Bailongjiang River is cut off for up to 8 hours, the backwater in the upper reaches of Bailongjiang River is about 2.8km, and the accumulated water is 754.9×104 m3. Half of the Nanyu Town is submerged, and the loss is serious [6]. In the following years, the Jiangdingya landslide was basically stable. At about 8: 00 on July 12, 2018, the Jiangdingya ancient landslide was reactivated. The reactivated Jiangdingya landslide (H1 landslide) belongs to the local part of Jiangdingya ancient landslide (H landslide) The earthwork volume of landslide reached 480×104 -550×104 m3, which caused the Bailongjiang River to be blocked and the water level to rise. The Jiangdingya landslide has the characteristics of chain disaster, which not only threatens the safety of Nanyu Town, but also interrupts the safe operation of roads and hydropower stations. The blocking of large-scale rivers will endanger the safety of the Zhouqu County in the upper reaches and Longnan City in the lower reaches. It will also pose a serious threat to the ecological security of the upper reaches of the Yangtze River with immeasurable losses. Therefore, the study of landslide development characteristics and formation mechanism has gradually become one of the core problems and hot issues in landslide disaster research.

Natural Geological Structure
The Jiangdingya ancient landslide is located in Nanyu Town,

Zhouqu County, Gannan Tibetan Autonomous Prefecture, Gansu
Province, China. The geographic coordinate of the landslide is 104°25'7"~104°25'47" E and 33°43'03"~ 33°43'43" N. Zhouqu   The fortification intensity of Zhouqu County is degree, and the peak ground motion acceleration is 0.2g. The stability calculation results are shown in Table 1. It can be seen that the landslide is in a stable state under natural conditions, and the stability of the slope will be reduced to a certain extent under the influence of earthquake or heavy rainfall. Under the action of continuous precipitation, the

Conclusion
In this paper, the catastrophic mechanism and stability analysis of Jiangdingya landslide are studied, according to the characteristics of disasters, the comprehensive safety control scheme is put forward, the following conclusions are obtained:

2.
The Jiangdingya landslide is affected by the fault zone, with broken rock mass and special topography accumulating landslide potential energy, which provides conditions for landslide sliding. The geological environment in Jiangdingya area determines the diversity, susceptibility and regional variability of geological disasters.

3.
The occurrence of the Jiangdingya landslide is closely related to the superposition of geological structure, environmental evolution, precipitation infiltration, earthquake damage and human activities. Among them, the geological structure is the decisive factor, and earthquake and rainstorm are the inducing factors.

4.
The Jiangdingya slope is in a creep state under the coupling effect of endogenic and exogenic dynamic forces.
Precipitation infiltration and agricultural irrigation increase the water content of slope and weaken the shear strength of slope continuously. The water erosion has continuously weakened the stability of the slope, which easily leads to the revival of new large-scale landslide.

5.
In view of Jiangdingya landslide, risk management measures should be taken to identify the landslide disaster risk, strengthen mass prevention and treatment, strengthen landslide early warning, and avoid the recurrence of major geological disasters.

6.
Under the influence of earthquake or heavy rainfall, the stability of slope will be reduced to a certain extent. The comprehensive treatment of anchor cable lattice structure engineering, anti-slide pile engineering, interception and drainage engineering and slope crack treatment engineering has a positive effect on landslide disaster prevention and ecological restoration.