关键词: 白格滑坡, 潜在危岩体, D-InSAR, PFC2D, 稳定性分析

Abstract:

At 4:00am on October 11, 2018, under the influence of heavy and continuous rainfall, a large-scale rocky landslide occurred in the Baige village of Bolo Town, Jiangda County, Tibet Autonomous Region, which is located at the upper reach of the Jinsha River. During its sliding, the landslide body is cut out from the upper part of the high and steep slope and falls rapidly, and the lower rock mass is continuously scraped, which increases the volume remarkably. With the disintegration of the landslide mass, the landslide mass is transformed into a fast and remote debris flow sliding. The massive debris flow materials rapidly flowed down to block the Jinsha River, forming a barrier dam. Then the lake rose and flooded many roads. At 5:00pm on the October 12^th, the barrier dam was overtopped and gradually washed by the river to form a drainage channel. At 9:00am on the 13^th, the dam was completely flushed open, accomplishing the flood discharge and relieving the danger caused by the landslide. At 5:00pm on November 3, 2018, the trailing edge of the Baige landslide experienced a sliding rupture, which led to the debris flow, at a high speed, piled up the dam from the first landslide, and blocked the Jinsha River again. The height of the second barrier dam was 50m higher than the first one, forming a larger barrier lake. After the landslide occurred, the water level of the upper reaches of the barrier lake continued to rise, and Jiangda County, Boro Town, Baiyu County Jinsha Town and other towns on the upper reaches of the Jinsha River were flooded. After the second floodwater released, a large scale flood occurred in Jinsha River, which caused the flooding of cities and towns in the middle and lower reaches in Sichuan, Yunnan and other riverside areas, and destructed roads and bridges, posing a great threat to the lives and property of people and the safety of infrastructure such as hydropower stations. The water level of the dammed lake was lowered by artificially constructing a diversion channel to eliminate the danger of dam break and avoid the occurrence of greater flood hazards. On the basis of field investigation on the landslide site, it is found that after the first landslide, three potential unstable rock masses were found at the trailing edge and both sides of the landslide. According to radar monitoring, three potential unstable rock masses at the trailing edge of the landslide are still continuously deformed, with obvious activity, and there is a risk of blocking the Jinsha River again. The author was monitoring constantly the unstable rock of the trailing edge of the Baige landslide for 7.5 days adopting D-InSAR. The surveillance results indicate that there is a slight sliding on the upper side of the landslide and there are four major deformation regions on the upper edge of the landslide. Besides, four measuring data points, selected within the four major deformation areas, show that the deformation value is 200mm and the deformation rate on the landslide top reaches 300mm/day, which suggests that the current landslide is still not stable and there is the risk of blocking the Jinsha River by the landslide. This paper, using PFC2D, simulates the stability of unstable rock on the trailing edge of landslide under the influence of gravity, torrential rain, and earthquake and analyzes the landslide’s stability scientifically in terms of simulation results. The simulation results show that the slope only deforms slowly under static action, without obvious destabilizing sliding. The initial deformation of the slope is basically consistent with the results of radar monitoring displacement, indicating that the sliding body of the slope still has a sliding trend under static action, and is not stable. Under the action of heavy rainfall, with the increase of time step, the deformation and displacement of slope is also increasing. In the process of operation, tensile cracks gradually appear in the slope, and continue to develop until it is cut through, and instability failure occurs. The ground motion is input from the bottom of the slope model in the form of velocity. When the model is running, tensile cracks first occur at the back edge of the slope on the right side. As the shear failure occurs in the middle of the slope and the tensile crack at the back edge goes through, the whole slope becomes unstable and fails. But on the whole, it’s basically stable. The simulation results show that the unstable rock in the trailing edge of the landslide will still lose stability under the inducing factors such as heavy rainfall and earthquake. It’s necessary to take appropriate engineering measures such as slope cutting to control the unstable rock, and the real-time monitoring and early warning system should be set up to eliminate the hidden danger caused by the slide of unstable rock blocking the Jinsha River again in time. At the same time, this paper also provides reference significance for further understanding the development and evolution process, as well as the deformation failure mechanism of landslide and debris flow in alpine regions. It also provides theoretical guidance for emergency measures and disaster prevention and mitigation after a disaster happens.

Key words: Baige landslide, unstable rock, D-InSAR, PFC2D, stability analysis