Application of GPS in Wushan high slope monitoring

Huang Zhou 1 Chen Cheng 2 Tan Jieheng 1

(1 Geological Environment Monitoring Station of Wushan County, Chongqing, 434700; Institute of Geostress, Seismological Bureau of China, Beijing, 100085)

Wushan county urban area is one of the key areas for resettlement of the Three Gorges Project, and it is also the area with frequent landslides in the reservoir area. Under the influence of long-term rain and human engineering activities, some high slopes have been partially deformed and cracked. In order to ensure the relocation of immigrants in the reservoir area and the safety of people's lives and property, it is particularly important to monitor the high slope of Wushan county in the Three Gorges reservoir area. This paper discusses the related problems of deformation monitoring of Wushan high slope by using GPS technology, discusses the selection principle of GPS monitoring points, and summarizes the results of GPS observation in recent two years.

Keywords GPS baseline; High slope edge

1 overview

Wushan new county is located on the hillside above the old county on the north bank of the Yangtze River. The geological conditions of the site are complex and the terrain is broken, so the site area suitable for construction is quite limited. In the construction of new towns and road excavation, a large number of artificial slopes are inevitably produced. Most of these slopes are above 40 degrees, and most of them control urban land. Once unstable, it will block traffic, seriously threaten people's lives and property safety, endanger the construction of resettlement projects, affect the progress of resettlement work in the Three Gorges Project, and be a stumbling block to resettlement projects. Therefore, it is particularly important to carry out slope monitoring to ensure the relocation of immigrants in the reservoir area.

The purpose of this work is to comprehensively monitor the high slopes and high retaining walls in Wushan New Town by using GPS technology, and to accumulate data for studying the stability of these slopes. There are generally two schemes for GPS displacement monitoring: the first scheme is to establish an unattended GPS observation system at the monitoring point, and realize real-time monitoring, deformation analysis and prediction through software control. The second scheme is to use multiple GPS receivers to observe at monitoring points regularly, and then analyze and predict the deformation after post-processing the data. The first scheme can be used for real-time monitoring with a high degree of automation, but the cost of the monitoring system is very expensive because GPS receivers need to be installed at each monitoring point. Although the second scheme has high labor intensity and low degree of automation, it has low cost and obvious advantages when there are many monitoring points.

The second method mainly uses GPS to monitor Wushan high slope, which greatly reduces the cost of monitoring system.

2 Wushan GPS monitoring network system

2. Design of1GPS reference network

The baseline vector obtained by GPS measurement belongs to the three-dimensional coordinate difference of WGS-84 coordinate system, but we use the coordinates of national geodetic coordinate system or local coordinate system in engineering practice. Therefore, the coordinate system and initial data adopted by GPS network should be defined first, which is the so-called benchmark problem.

The datum design of GPS network includes position datum, scale datum and azimuth datum. The position reference can generally be given by the coordinates of the more advanced GPS network reference station, and can also be determined by selecting the coordinates of the existing city control points. Because the first-class GPS network undertaken by the Ministry of Land and Resources has not been built in the Three Gorges reservoir area, it can only be realized based on the existing urban control points. According to the information provided by relevant departments in Wushan, we selected some control points for joint measurement. After the joint survey, the position benchmark, scale benchmark and orientation benchmark of Wushan GPS monitoring network are determined accordingly.

2.2 Wushan GPS monitoring network selection principles

We know that GPS monitoring points should pay attention to the following points:

(1) The monitoring point should be located at a higher position where the foundation is relatively stable and the receiving equipment is easy to place;

(2) In order to reduce the blocking or absorption of GPS signals by obstacles, there should be no tall obstacles above 15 around the field of view;

(3) the point should be far away from the high-power radio transmitter, and the distance should not be less than 200 m; Stay away from high-voltage transmission lines, with a distance of not less than 50m, to avoid the interference of electromagnetic fields on GPS signals;

(4) There should be no large area of water or objects that strongly interfere with satellite signal reception near the point, such as tall trees, to weaken the influence of multipath effect;

(5) The location should be determined according to the morphological characteristics, deformation characteristics, dynamic factors and monitoring and forecasting (deformation orientation, deformation amount, deformation rate, spatio-temporal dynamics, construction dynamics, development trend, etc.) of the slope. ), and these points can truly reflect the deformation sensitive parts of the disaster geological body.

However, in the actual site selection and investigation, there are many high slopes and high retaining walls in Wushan New County, especially the high-rise buildings are dense, and some sites can not fully meet the above requirements.

2.3 GPS monitoring network construction

The whole network of Wushan GPS monitoring network consists of more than 40 deformation monitoring piles, which are distributed throughout Xinxian County of Wushan County. The layout of monitoring network adopts synchronous graphic expansion, which is the most commonly used way in GPS network layout. That is to say, multiple GPS receivers are placed on different reference stations for synchronous observation. After a period of observation, several receivers were moved to the next group of stations. Between the two groups of observations, there are some similarities between the two synchronous graphs until the whole network is covered. This method is simple in operation, high in graphic strength, fast in expansion and easy to organize, and is widely used in practical work.

3 GPS monitoring data processing and accuracy analysis

Because the deformation to be monitored is relatively small, in order to accurately measure its change, the deformation monitoring network is required to have high accuracy. In view of this, in the design of Wushan high slope monitoring network, it is required that the GPS high slope monitoring network should cover the high and steep slopes where immigrants move into the county and important resettlement projects. The plane calculation accuracy of the datum coordinates of the monitoring network is better than 2mm, and the elevation accuracy is not less than 4mm；; The calculation accuracy of the coordinate plane of the deformation monitoring point is better than 3 mm, and the elevation accuracy is not less than 6 mm. After it is put into normal operation, long-term, continuous and reliable measurement data of the coordinates of the monitoring point of the high and steep slope in the resettlement area can be obtained, which serves for the monitoring and early warning decision-making of related geological disasters.

See table 1 for the main technical requirements of the national urban survey code for urban or engineering GPS networks.

Table 1 Technical Requirements for Surveying Accuracy of Urban or Engineering GPS Network in Urban Surveying Code

Comparing the data in table 1 with our results, we can see that although some datum points of monitoring network are blocked by nearby high slopes and buildings, after data processing, the average point error of plane coordinate measurement is 2mm horizontally and 5mm vertically, and the relative accuracy of baseline reaches 10ppm. The measurement results obtained are better than the requirements put forward in engineering design, and also higher than the technical requirements of the above-mentioned "Code for Urban Survey" for the measurement accuracy of GPS networks in cities or projects.

Deformation monitoring is characterized by repeated observation at regular intervals. The external conditions observed in each period are basically the same, and the system errors in each period are basically the same. By analyzing the results of multi-period measurement on the same side and comparing their differences, the repeated positioning accuracy can be tested and the deformation area can be found. After taking appropriate measurement methods and observation data processing measures, the difference between the two periods is basically unaffected by systematic errors.

4 Preliminary results obtained by measurement

The summary and analysis of GPS survey data of Wushan and Wushan are to calculate the vector edges of each period, draw the time variation trend diagram of each vector edge, compare the same vector edges of each period one by one, determine the abnormal vector edges, and find out the deformation area after analysis. Through the analysis and processing of five GPS survey data, it is found that there are 14 vector edges with trend changes, and the trend changes of these 14 vector edges can be divided into two types:

4. 1 vector edge with deformation tendency

The six vector edges with deformation tendency are as follows:

(1) wg021-wg081,and the cumulative change in the fifth period is 16mm. The vector edge is characterized by shortening, and the change trend is shown in figure1.

Figure1wgo21-wg081five-period cumulative change curve

(2)WG0 154-WG048, the cumulative change in five periods is 22mm, and the vector edge is characterized by shortening first and then increasing, and the change trend is shown in Figure 2;

Fig. 2 cumulative change curve of wg0154-wg048 in five periods.

(3)WG048-WG065, the cumulative change in five periods is 1 1mm, and the vector edge shows the growth characteristics, and the change trend is shown in Figure 3;

(4)WG096-WG 149, the cumulative change in five periods is 16mm, and the vector edge is characterized by growth, with the change trend as shown in Figure 4;

(5)WG 100-WG022, the cumulative change in five periods is 1 1mm, and the vector edge is characterized by growth, with the change trend as shown in Figure 5;

(6)WG 177-WG 157, the cumulative change in five periods is 10mm, and the vector edge shows the growth characteristics, and the change trend is shown in Figure 6.

4.2 The trend changes obviously, and we should focus on the vector edge.

The eight baseline edges with obvious trend changes are:

(1) WG177-WG140, and the cumulative change in five periods is 17mm, and the vector edge shows the characteristics of trend growth and acceleration, and the change trend is shown in Figure 7;

(2)WG20 1-WG 157, the cumulative change in five periods is 2 1mm, the vector edge shows the characteristics of trend shortening and acceleration, and the deformation curve is shown in Figure 8;

Fig. 3 cumulative change curve of wgo48-wg065 in five periods.

Fig. 4 cumulative change curve of wg096-WG149 in five periods.

Fig. 5 cumulative change curve of WG100-wg022 in five periods.

Fig. 6 cumulative change curve of WG177-WG157 in five periods.

Fig. 7 cumulative change curve of WG177-WG140 in five periods.

Fig. 8 cumulative change curve of wg201-WG157 in five periods.

(3)WG 162-WG022, the cumulative change in five periods is 12mm, and the vector edge shows the characteristics of trend growth and acceleration, and the deformation curve is shown in Figure 9;

(4)WG 140-WG 157, with the cumulative change of 40mm in five periods, the vector edge is characterized by growth, showing an accelerating trend, and the deformation curve is shown in figure 10;

Fig. 9 cumulative change curve of WG162-wg022 in five periods.

Figure10WG140-WG157 Cumulative Change Curve in Five Periods

(5)WG 162-WG032, the cumulative change in five periods is 8mm, and the vector edge is characterized by growth, showing an accelerating trend. The deformation curve is shown in figure 1 1;

(6)WG 100-WG 149, the cumulative change in five periods is 16mm, the vector edge is characterized by growth, showing an accelerating trend, and the deformation curve is shown in Figure 12;

Figure11WG162-wg032 Cumulative change curve of five periods.

Figure12wg100-WG149 Cumulative change curve of five periods.

(7)WG 157-WG20 1, the cumulative change in the fifth period is-2 1mm, the vector edge is characterized by shortening and accelerating, and the deformation curve is shown in figure 13;

(8)WG048-WG 17 1, with a cumulative change of 25mm in five periods, and the vector edge is characterized by growth. The deformation curve is shown in figure 14.

Figure13WG157-wg201five-period cumulative change curve

Figure14wg048-WG171five-period cumulative change curve

4.3 Deformation points obtained from monitoring data analysis

The ultimate goal of GPS deformation monitoring is to analyze and determine the deformation of the deformation point by comparing the length of the same vector side with the coordinates of the same measuring point in each period. Through the comparative analysis of the baseline edge of Wushan monitoring network, some deformation points are preliminarily determined. From the overall characteristics of deformation, the direction of deformation is towards the Yangtze River.

5 abstract

It has been more than a year since the second phase of water storage in the Three Gorges reservoir area. Under the current high water level, if the precipitation factor is superimposed, the risk of instability of high slopes and high retaining walls will increase. It is suggested that while strengthening professional monitoring, we should do a good job in group monitoring and prevention, and strive to minimize the losses that geological disasters may bring to people's lives and property. GPS monitoring should be strengthened for the deformation areas determined by monitoring data analysis, and monitoring should be actively carried out by various means, especially in rainy season. In rainy season, GPS survey can be carried out once every 1 month, and tilt survey can be carried out once every 15 days, so as to find unsafe hidden dangers, predict them in time and ensure the safety of people's lives and property. At the same time, multi-disciplinary research is carried out on the failure process and deformation law of high slope, and effective ways to correctly predict the deformation, instability and failure of slope are actively explored.

refer to

Ouyang Zuxi and others. Using GPS technology to study the landslide stability in Wanzhou reservoir area of Three Gorges Project. Chinese journal of geological hazard and control, 2003, (6)

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Han Wenxin. GPS deformation monitoring results and comprehensive analysis of Wushan new county in 2003. Institute of Crustal Stress, China Earthquake Administration.

GPS displacement monitoring system of Pudong seawall. Engineering Survey, 2004, (1)

Yue Shun. Study on comprehensive treatment of Shangxiping landslide in Wushan county of Three Gorges reservoir area [master thesis]. Chongqing University, 2002, (12)