Background of the Youxi terraces landslide area
Youxi County is located south of the 26th parallel north and about 100 km east of the East China Sea, with a humid mid-subtropical monsoon climate. Its summer is warm/hot, winter is cool/chilly, and spring and summer are rainy, with abundant precipitation. The average annual precipitation is between 1500 and 1750 mm, with extremely uneven rainfall throughout the year, including many typhoons and rainstorms from May to August, which is the main flood season.
Lianhe Township of Youxi County is surrounded by mountains, with an elevation of 260–1137.6 m, and a medium–low hilly landform. The relative elevation difference is 877.6 m, and the slope has a gradient of 25°–45°. The mountainous terrain is high with steep topography and sharp gully cutting. According to the regional 1/50,000-scale geological survey report (G50E010017), the fold structures developed in the area of Lianhe Township, Youxi County, are mainly located in the Nanping-Yuxi compound syncline, with the axis located in the area of Xiqin, pointing in the 20°–30° northeast direction. The area is composed mainly of Presinian strata, with a gentle dip of about 30° in the axis of the rock layer, while the flank can reach 50°–60°. The secondary fold structures are developed in the axial part of the compound syncline and are parallelly arranged in the northeast direction. The fault structures in the area include mainly two groups: the north–northeast fault structure of Baizhangji in Wencheng County and the northwest fault structure of Dingdi in the highlands, with the former being the main fault structure. These two groups of fault structures are large in scale, generally extending up to tens of kilometers, cutting deep into different tectonic layers. In the fault structures, extrusion, rushing zones, and conglomerate are normally distributed, while silicification, chloritization, and other alterations are generally observed with the development of small fractures. The Youxi Lianhe terraces are located at the intersection of these two fault structures, with fragmented and unstable rock layers (Fig. 2).
The Youxi Lianhe terraces have a history of 1300 years and are one of the earliest large-scale ancient terraces dug by the Han people in the history of China. The relative elevation difference of the terraced landslide area is 877.6 m, and the slope gradient is 25°–45°. The area has a medium–low hilly landscape, with abundant rainfall all year round or seasonally, and the geological environment is relatively fragile. According to the statistics of “Fujian Province Geological Disaster Prevention and Control Information Network,” in the Lianhe Town of Youxi County, there are 105 locations of potential geological disaster and 68 locations of high and steep slopes. The core area of the Youxi Lianhe terraces connects the five administrative villages of Lianxi, Dongbian, Lianyun, Yunshan, and Xiaoyun, with 27 locations of potential landslide, among which, Yunshan and Lianyun are along the 016 rural road, which has three existing locations of pulling deformation with serious faults on the pavement, which may result in potentially large-scale landslides. Every year during the flood season there are geological disasters of different degrees, including the most typical landslide or mudslide disaster that occurred in 2010 in Xiaoyun, destroying approximately 110,000 square meters of terraced rice fields to varying degrees. The influence of the geological environment on the terraced rice field ecosystem is progressively noticeable, as the situation becomes increasingly serious (Fig. 3).
According to the field survey, the overall slope of the terraces in Lianyun is 6.53°–35.87°, the minimum annual deformation rate along the slope is 13.04 mm, the maximum annual deformation rate is 165.28 mm, and the average throughout the years is 57.55 mm. The overall slope of the terraces in Yunshan is 5.22°–38.04°, the minimum annual deformation rate along the slope is 10.02 mm, the maximum annual deformation rate is 148.79 mm, and the average throughout the years is 30.42 mm. The overall slope of the terraces in Dongbian is 5.39°–28.26°, the minimum annual deformation rate is 10.0 mm, the maximum annual deformation rate is 148.75 mm, and the average throughout the years is 32.97 mm.
The creeping of the terraced slope on the east side of Lianyun has been most severe in recent years. According to the preliminary site survey, although the ground cracks on the back edge of the landslide developed only slightly, cracks are evident in the buildings and roads of greater stiffness. While the main structure of the Lin’s ancestral hall at the back edge of the landside is intact and no cracks are observed, the crack between the main structure and the auxiliary structure is obvious (Fig. 4). The tensioned ground cracks near the blue-topped shed in the northeast direction continue to extend on the surface, and the direction of the fissure is basically parallel to the slope direction (Fig. 4). Obvious cracks can be observed on the road surface nearby, which are mainly induced by the sunken slabs at the back edge (Fig. 5). In addition, shear cracks of an echelon arrangement are observed on the concrete floor of a pig farm in the east part of the village. The landslide has caused a slight subsidence of the slabs at the back edge, and the front edge is located in the terraced area, with bulging at some locations. The main surface cracks are distributed in a scattered manner. The perimeter of the landslide is not clear; currently, the landslide is in a slow creeping state without having entered the accelerated deformation stage.
IoT-based monitoring and early warning system and its application
Based on the above-mentioned potential hazards caused by landslide in the later stages and the need for effective monitoring and early warning, the latest 5G and IoT technologies are combined to establish a landslide remote monitoring and early warning system for long-term monitoring and early warning. The deformation and destruction trend of the landslide body are tracked in real time, so as to discover and forecast dangerous situations in time for mitigating measures to prevent the loss of life and property caused by sudden disasters.
A complete landslide monitoring system includes data sensing, acquisition, transmission, reception, processing, analysis, and information release mechanism, while a complete landslide early warning system includes data sensing, acquisition, transmission, reception, processing, analysis (judgment), early warning, and response. Therefore, the main differences between monitoring and early warning are the evaluation of the monitoring data according to the early warning guideline and the timely issuance of early warning and risk avoidance responses, as shown in Fig. 6. It is necessary for the landslide monitoring and early warning systems to be able to have automatic identification and judgment functions, according to the early warning guideline, and automatic issuance of early warning information. In this paper, the 5G, IoT and landslide early warning technologies are integrated in one monitoring system. The monitoring data are obtained and read by monitoring stations (rainfall, ground fissure, ground deformation…) in the field. They are transmitted then by 5G and analyzed in servers. Finally, the early warning can be sent according to the effective warning criterion for gradual landslides.
Data acquisition
The data of the system come from the regular data acquisition with the equipment. At present, the system supports automatic monitoring equipment for GNSS-based 3D surface displacement, rainfall, cracks, groundwater level, second-order power, etc. The equipment communicates directly with the equipment-side server, and the collected data are regularly transmitted to the equipment-side server through the network.
To facilitate data exchange with the application-side server, the equipment-side server uses a cache database and periodically transmits data to the application-side server according to the data request specification of the application-side server. After the application-side server receives the data transmitted from the equipment-side server, the data are pre-processed before entering into the storage center database (see Fig. 7 for the data flow architecture).
The user side can request to query the data of the application-side server through the HTTP protocol. Both query by time range and query by specified equipment type are supported. The application-side server will automatically aggregate the data according to the time range of the query. For equipment-specific data that calculate relative values (such as data of GNSS equipment and crack detection equipment), the system automatically calculates the relative data values based on the reference time of the location of the monitoring equipment.
Data transmission
As the latest breakthrough in communication field, 5G communication has present many advantages such as high speed, large capacity, and low latency. In the IoT, high-quality and real-time monitored data transmission is often necessary. In particular, many IoT terminal devices will play an important role in the 5G communication system, such as autonomous driving or a surveillance system with a high-definition camera. In 5G networks, a multilayer structure will be considered, owing to the heterogeneity of equipment and services. Regarding the IoT, it is a potential technology to realize the interconnection of all things. The popularity of small and inexpensive computing devices with sensing and communication functions is paving the way for the widespread application of IoT [28]. In the monitoring and prediction project of Youxi landslide, the corresponding IoT terminals are used to monitor rainfall, ground fissure and surface deformations in real time, and then 5G communication channels are chosen to transmit data.
An automatic data acquisition equipment was developed based on the data output interface and communication protocol of a high-precision mechanics sensor. The equipment includes a data acquisition module, a data storage module, a data transmission module, and a solar power supply equipment. The data acquisition equipment is connected to the sensors and the solar power supply system through shielded cables to form a complete data acquisition system.
In order to improve the stability and reliability of data transmission, each monitoring location in the monitoring area is connected with wireless sensor network technology, and the data are transmitted centrally from the field to the indoor monitoring center using the dual communication mode of BeiDou satellite and GPRS (referred to as BD/GPRS mode) after data acquisition.
There are two modes of aggregating data from monitoring locations within the monitoring area, with the mesh networking mode and the linear networking mode.
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Mesh networking mode: monitoring locations (nodes) can be networked for data transmission at will within the effective range. When one of the monitoring locations fails, other adjacent monitoring locations can be selected for data transmission until the data are finally transmitted to the sink node. The advantage of this transmission mode is that the data transmission is secure, and the data transmission within the whole monitoring area will not be paralyzed due to the failure at one monitoring location (see Fig. 7).
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Linear networking mode: monitoring location Ni and its effective proximity location Ni−1 form a network, Ni−1 and Ni−2 form a network, and so on until communication between the monitoring location N1 and the convergence point is obtained, such that data of the N locations on the same line are all converged to the convergence point. The advantage of this transmission mode is the long effective transmission distance, suitable for strip-shaped monitoring area. The disadvantage is that the data transmission stability is not guaranteed, when one of the nodes is damaged, the data transmission of the whole system is paralyzed.
Therefore, this system uses two transmission modes in a complementary form, which improves both the effective distance of data transmission and the security and stability of data transmission.