2.1 Description of the Zhongbao landslide
The Zhongbao landslide is located at the north side of the Yancang river, about 59 kilometers to the Wulong District, Chongqing, China (Fig.1 a). In plane view, the shape of the Zhongbao landslide approximately likes a tongue, with 850 m length and 200 m width (Fig.1 b). The thickness of the landslide is estimated 25m -47 m, and its’ volume is approximately 4.6 × 106 m3 (Fig.1 c). The slope of the landslide surface is 25°- 45°. The main sliding direction is 210°, approximately perpendicular to Yancang River. The Zhongbao landslide began to slide at 16 p.m. July 23, 2020, and destroyed three buildings and main roads. Over 1.0× 104 m3 soil and rock masses slipped into the river and formed a barrier lake.
The field investigation indicates that main geologic units of the area are deposits and shales. The uppermost part of the surficial deposits is eluvial deposits (up to 0.5~3.5 m thick), whereas stratigraphically lower layers comprise mostly alluvial deposits and colluvial deposits. The eluvial deposits consist of clay and 10% ~ 45% of gravel clasts (Fig. 2 a, b). The diameter of gravel clasts ranges from 50 mm to 300 mm. In-situ exposed outcrops over 10 m high (undisturbed by the landslide) crops out near the western limit of the depicts the lithological structure (Fig.2 c). Dominant pale-yellow shales, 0.2 m ~ 0.5 m thick, are in the pelitic structure with developed joints. The mainly component of shales are smectite and other clay minerals which are easy to be weathered and softened by water. The rocks dips 25° toward 130°.Three dominate joints are found in the study area (Fig. 2 d). The first joint dips 75° toward 351°with a width of 3~ 15 mm and a length of 1.5 – 4.9 m, and its space is 1.30 – 4.20 m. The second one is dips 84° toward 128°with a width of 2 -12 mm and a length of over 10 m, and its space is 1.10 -2.70 m. The third dips 76° toward 276° with a length of 2.1 – 4.5 m and its space is 0.80 -3.60 m. Those joints are partially filled with clay.
2.2 Landslide monitoring after failure
2.2.1 Emergency monitoring system
After landslide failure, emergency work and stability evaluation of the reactivated deposits are necessary, but conventional monitoring technologies are not equipped in monitoring the damaged landslide, because of the instability of the landslide. Therefore, some new methods are taken to monitor the deformation of the landslide(Figure 3):
(1) Ground-based radar
HC-GBSAR1000 ground-based radar monitoring system uses the Ground-based repeated orbit interferometric SAR technology, which accurately measures the displacement of the landslide’s surface by comparing different pictures at different times based on the principle of phase interference. This technology has technical advantages of all-weather, high-precision, and continuous measurement in time and space. Therefore, it is used in emergency monitoring of the landslide.
(2) Unmanned Aerial Vehicle
Five sensors are equipped in the unmanned aerial vehicle to take pictures from five views. Meanwhile, the location of the vehicle during flying are recorded, including its height, speed and flying direction. Based on the digital aerial stereo images, the image points and coordinates are automatically recognized, and the three-dimensional coordinate of the study objection is formed by using analytical photogrammetry method. This technology could rapidly obtain a digital orthophoto, a digital elevation model, and high-resolution three-dimensional models, which provide fundamental data for the evaluation and monitoring of the landslide.
(3) Airborne Lidar
The airborne Lidar integrates multi-technologies, such as laser, computers, high dynamic carrier attitude determination technology and High-precision dynamic GNSS (Glogbal navigation satellite system) differential technology. This technology recognizes objects by laser technology, and analyzes those datum by computer technology. Airborne lidar is composed of a laser scanner system, a POS system (GNSS) and a high-resolution camera system. The technology can avoid the effect of trees and obtain a high-resolution photograph and 3D point cloud data rapidly and accurately. Those datum are used to analyze the deformation and the volume of the landslide.
(4) Thermal infrared temperature monitoring
The thermal Infrared Camera FLIR X8400sc is a device which converts energy into electrical signal. It has a precision of ± 0.1 °C, a temperature resolution of 0.04 °C, a resolution of 1280 × 1024 pixels and a spectral range of [1.5 5.1] m. This equipment measures the temperature of the landslide surface so as to obtain the temperature of the field.
2.2.2 Conventional monitoring system
Conventional monitoring equipment has been widely used in natural hazards monitoring because of their low cost, simple structure and easy construction. In addition, they are applied for its wireless transmission in measurement data. In this paper, those monitoring technologies are used (Fig. 3):
(1) GNSS displacement monitoring
The principle of GNSS monitoring is to determine the position of the GNSS receiver using distance intersection method by measuring the distance between four or more satellites and receivers. The base station is constructed in a stable area, and monitoring stations are built in a deformation area. The displacements of the monitoring stations are calculated by comparing the location between base and monitoring stations.
(2) Displacement monitoring in depth
The flexible inclinometer probe, invented by Chongqing Engineering Research Center of Automatic Monitoring for Geological Hazards, measured its angle by three-dimensional micro-mechatronics measurement units (MEMS) to calculate the horizontal displacement. The flexible probe monitoring system includes a flexible probe, controller and PC software. One probe contains 80 measurement units and a vertical spacing of 1.0 m.
(3) Rainfall monitoring
A tipping-bucket rain-gauge is used to measure precipitation, because it is characterized by real-time recording rainfall with high accuracy and almost without human factors. The measurable range of the equipment is 0.01 mm~4 mm/min, and the precision is ±2%.
(4) Sound and light alarm
The received data being collected and analyzed by the monitoring terminal, is uploaded to the remote control terminal and then the alarm information is pushed to the sound and light warning terminal for efficient and timely warning. Sound and light alarms are carried out in time to remind nearby vehicles and personnel to react to disasters.