Technical methods of land subsidence investigation and monitoring

Since 1960s, Shanghai has systematically carried out land subsidence investigation and monitoring. The main technical methods adopted are drilling, leveling, bedrock plotting and layered measurement, and dynamic monitoring of groundwater level.

At present, the technical methods of land subsidence monitoring in Shanghai include: groundwater monitoring, first-class and second-class leveling, bedrock and layered measurement, GPS measurement, InSAR measurement, automatic monitoring system and so on.

1. Groundwater monitoring

There are 450 groundwater monitoring wells in the city, which are used to monitor the variation law of groundwater level (water quality and water temperature) in six phreatic and confined aquifers with different depths (Figure 1).

Figure 1 Groundwater Level Monitoring Well and Monitoring Data

2. First-class and second-class leveling

Leveling is to measure the height difference between two points by using the "horizontal line of sight" provided by the level, so as to calculate the unknown point elevation from the known point elevation (Figure 2).

Technical characteristics: The advantages of precision leveling are low cost of leveling points, flexible layout of leveling network, and rapid acquisition of settlement information in small areas (even densely populated areas); Its disadvantages are long observation period, large investment in human resources, high labor cost and poor real-time performance when the survey area is large.

Technical indicators: The first-class and second-class leveling shall be carried out according to the Standard for Land Subsidence Leveling (DZ/T0 154-95).

3. The bedrock standard, layered standard measurement

The measurement of bedrock markers and layered markers is an important technical means to monitor land subsidence, and it is the basis to analyze and study land subsidence and formulate corresponding measures.

The bedrock marker is a special observation point buried in the underground intact bedrock, which can be used as the elevation control point for land subsidence measurement. As the benchmark of elevation control survey, bedrock marker can reduce transmission error and improve survey accuracy. Stratification mark is a special observation point buried at different depths in soil layer and sand layer according to the nature of soil layer. They are internationally recognized measures to measure the deformation of loose soil layers and are widely used for accurate deformation measurement of loose soil layers (Figure 3). Technical characteristics: The advantage of bedrock marker is the highest accuracy, which can provide reference points for all land subsidence monitoring and research work; Its main disadvantages are high project cost (usually millions of yuan, even millions of yuan), many construction procedures, high quality requirements and difficult on-site implementation. Therefore, according to the actual needs of land subsidence monitoring, the planning and construction of bedrock targets need detailed demonstration.

Figure 2 Site leveling site

Fig. 3 nanpu bridge hierarchical standard group.

Layered markers are mainly used to monitor the compression deformation of different depths and different soil layers from the ground to the ground. Deformation records are comprehensive and complete, and are generally used in conjunction with bedrock markers, and planning is carried out in the form of bedrock markers and layered markers. Its advantage is that it can monitor the vertical deformation of a certain area or a certain point at different depths, such as the settling funnel, and obtain the deformation of three-dimensional space. If equipped with automatic monitoring system of land subsidence, real-time continuous deformation of soil layer will be obtained. Its main disadvantage is high construction cost.

Technical index: The bedrock standard is the monitoring benchmark of land subsidence, with the highest accuracy level.

Grade scale measurement can be divided into manual measurement and automatic measurement. According to the standard of land subsidence leveling, the accuracy of manual measurement is generally 0.3 mm. ..

4.GPS measurement

GPS measurement is to select or establish a reference station at an appropriate position far away from the deformation zone by using the Global Positioning System (GPS), set several monitoring points in the deformation zone, place GPS receivers on the reference station and the monitoring points respectively for continuous observation, and analyze and process the observation data (Figure 4).

Fig. 4 GPS reference station

Technical characteristics: short observation time, low labor intensity of manual operation, simple observation operation, no visibility between stations, flexible distribution, continuous monitoring at any time, anywhere and all weather conditions, high positioning accuracy and high level of operation automation.

Technical index: According to the requirements of Class B network in Global Positioning System (GPS) Survey Specification (GB/T18314-2001), the mean error of height difference is 34mm, and the actual result is that the accuracy of geodetic height change is about 5 mm.

5.InSAR measurement

Radar interferometry (InSAR) combines the imaging principle of synthetic aperture radar (SAR) with interferometry, and uses the phase information carried by radar echo signals to accurately measure the elevation information of a certain point on the surface and its slight changes. Its principle is to obtain repeated observation data in the same area through simultaneous observation by two antennas (single-track and double-antenna mode) or repeated observation by two antennas (single-antenna and repeated-track mode), that is, single-view double image pair, which is the data source for elevation extraction or deformation monitoring of InSAR.

Technical characteristics: InSAR technology has the characteristics of obtaining the settlement of point, line and surface at the same time and investing less human resources, which shows broad prospects and great potential for land subsidence research. Its shortcomings are also obvious, mainly because InSAR technology is not very mature at present, and it is still in the research stage, and there is still a period of time before it is widely popularized and applied.

Technical index: Shanghai InSAR monitoring test results show that the vertical accuracy of InSAR technology can reach 3.7mm, and it is still under experimental study.

6. Automatic monitoring system

Automatic equipment is installed on layered signs and water level holes to realize the functions of soil deformation, automatic observation, recording, transmission and database entry of layered signs, which further improves the automation degree of layered signs and water level measurement (Figures 5 and 6).

Fig. 5 Schematic diagram of automatic monitoring facilities for land subsidence

Schematic diagram of data acquisition and transmission system for land subsidence monitoring.

Technical features: The automatic monitoring system for land subsidence has the advantages of high precision, continuity, real-time, automatic recording, automatic transmission, unattended operation, etc. The data collection time can be set at will, and the settlement of different soil layers can be monitored at the same time, which is beneficial to separate the deformation of each soil layer from the deformation, calculate the contribution of different soil layers to the total settlement, and study the causes, mechanisms and mechanisms of land subsidence. The main disadvantage of the automatic monitoring system for land subsidence is the high one-time construction cost, which is more suitable for selecting representative and typical areas such as the center of subsidence funnel and the edge of funnel. Because of its high cost, it is mainly used to monitor point objects at present.

Technical indicators: the average absolute error of automatic monitoring accuracy of layered standards should not be greater than 65438±0mm；; The monitoring accuracy of groundwater level should be 0.01m.

Second, the scope of application and application examples

(1) Scope of application

This result is widely used in land subsidence monitoring.

(2) Application examples

1. First class and second class leveling

According to the requirements of Land Subsidence Leveling Standard (DZ/T0 154-95) and National First-class and Second-class Leveling Standard (GB/T 12897-2006), Shanghai Geological Survey and Research Institute established the first-class and second-class elevation control network in the whole city. Based on bedrock markers, a large-scale elevation control network is formed from one bedrock marker to another.

According to the coverage area and repetition frequency, the elevation control network is divided into suburban elevation control network and central city (within the outer ring road) elevation control network. Suburban elevation control network covers the whole Shanghai area, including Chongming Island, hengsha island Island and Changxing Island. The repetition frequency is once every five years, which is used to update and publish the elevation control data of the whole city. A first-class road map is drawn in the map. The second and third categories cover the whole suburb, and there is a lack of route information at present; The central city elevation control network (the area within the outer ring road) is distributed in the central city, and the repetition frequency is 1 year, covering an area of about 1000km2, which is used for land subsidence analysis and research. Both sets of elevation control networks take bedrock as nodes, which realizes the organic unity and connection of suburban elevation control network and central urban elevation control network.

2. Bedrock marking and layered marking

Since the study of land subsidence, the datum point of elevation control network has been a key problem, and the effective solution is to choose stable bedrock to establish bedrock markers. Shanghai Geological Survey and Research Institute has been engaged in this work for a long time, especially through the practice test of large-scale network construction projects, which are important topics of the municipal government in the Ninth Five-Year Plan, the Tenth Five-Year Plan and the Eleventh Five-Year Plan. He has gained rich experience in construction and management, formed a set of strict operating rules, mastered the construction technology of bedrock standard, and obtained the patent for the implementation technology of bedrock standard (patent number: ZL 0 12394556, certificate number:). At present, a relatively complete land subsidence monitoring network has been built in Shanghai, especially since the implementation of the "Eleventh Five-Year Plan" land subsidence prevention and control project, 35 land subsidence monitoring stations have been built in the city (/.

Fig. 7 Ground subsidence monitoring station of Shanghai World Expo Park.

Fig. 8 Automatic monitoring facilities of land subsidence monitoring station

3.GPS measurement

In 200 1, 1 ~ 20 10, 12 years, Shanghai Geological Survey and Research Institute * * * organized GPS primary network monitoring 13 times. The period from 200 1 to June 2002 is 3h or 6h. Since June 2002, the cycle has been changed from 65438+ 12h to 24h. The observation scheme and data processing scheme are further optimized, and the accuracy and reliability of GPS monitoring land subsidence are gradually improved.

4.InSAR measurement

The map 10 is the map of rate of land subsidence from 2003 to 2007 measured by Shanghai InSAR.

Fig. 9 Data curve of land subsidence standard group

Figure10d-InSAR monitoring rate chart of land subsidence in Shanghai work area in 2007.

Third, promote the transformation mode.

The research, development, maturity and perfection of land subsidence monitoring technology provide strong technical support for patent applications, regulations and norms, and also play a leading and exemplary role in land subsidence monitoring and prevention in the Yangtze River Delta region, even in the North China Plain and the Weifen Basin in the northwest.

Through years of research on standard design, construction convenience, cost, applicability, reliability and stability of bedrock, a set of mature standard construction technology of bedrock has been formed, and a patent has been applied for (ZL J 2 39455.6, certificate number 478596).

The compiled rules and specifications include Technical Specification for Monitoring and Prevention of Land Subsidence (DG/TJO8-205 1-2008, Shanghai), Technical Specification for Monitoring of Land Subsidence (China Geological Survey) and Standard for Measuring Land Subsidence (Ministry of Land and Resources), which have made positive contributions to further standardizing the monitoring and prevention of land subsidence in China.

Technical support unit: Shanghai Geological Survey and Research Institute.

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