(1 China Geo University (Wuhan), Wuhan, Hubei, 430074;
2 Institute of Hydrogeological Engineering Geological Technology and Method, China Geological Survey, Baoding, Hebei, 07 105 1)
Early warning and monitoring of geological disasters in the Three Gorges reservoir area of the Yangtze River is the main basic work to serve the prevention and control of geological disasters and ensure the construction safety of the Three Gorges Project. In 38 professional monitoring points of landslide disasters in Kaixian, Wanzhou and Wushan counties, comprehensive monitoring methods such as earth deformation monitoring, deep displacement borehole inclinometer monitoring, groundwater monitoring, landslide thrust monitoring, relative displacement monitoring of surface cracks, GPS monitoring, TDR time domain reflection monitoring and macro monitoring have been adopted. At each landslide disaster point, two or more monitoring methods are respectively used to monitor the internal deformation or stress change of the landslide surface; 4 ~ 5 methods are used to monitor important disaster points at the same time, and the comparison and comprehensive analysis are made. According to the statistical analysis of landslide monitoring and monitoring results, the results of various monitoring data have obvious consistency and correlation, reflecting the deformation and characteristics of landslide, which proves that the monitoring method is reasonable and effective. The monitoring results will provide reliable basis for geological disaster early warning engineering and geological disaster prevention engineering.
Keywords: application of monitoring methods of geological disaster early warning project in Three Gorges reservoir area
1 preface
The natural geological conditions in the Three Gorges reservoir area of the Yangtze River are complex, and it is a frequent and hardest hit area for geological disasters. The construction of the Three Gorges Project and the Million Immigrants Project has changed the original geological environment balance to a certain extent and aggravated the occurrence of geological disasters. With the continuous advancement of the Three Gorges Project, geological disasters in the reservoir area have an increasing impact on the Three Gorges Project and the safety of people's lives and property in the reservoir area. Timely and effective prevention and control of geological disasters in the reservoir area has become one of the important tasks of the Three Gorges Project. Geological disaster early warning and monitoring is the main basic work to realize geological disaster prevention and control.
There are 38 professional monitoring points for landslide disasters in the Three Gorges reservoir area, including Kaixian 14, Wanzhou 14 and Wushan 18.
Two monitoring methods
2. 1 deformation monitoring
Total station is used for monitoring. Points with good geological conditions and relatively stable foundation are selected as monitoring reference points outside the landslide body, and representative points are selected as monitoring points on the landslide body. All landmark points are composed of concrete forced centering monitoring piers.
2.2 Deep displacement monitoring
Borehole inclinometer is used for monitoring. Select representative points on the landslide to arrange inclinometers to drill holes, and make positive and negative bottom-up readings in the main sliding direction and vertical main sliding direction respectively, with the distance between monitoring points of 0.5m After the monitoring data is stabilized, it will be automatically recorded with the mobile "CX-0 1 gravity accelerometer drilling inclinometer", and 4 groups of data will be recorded in each monitoring period.
2.3 Landslide thrust monitoring
A series of landslide thrust sensors are preset and connected to the ground by conductive optical fiber. For each monitoring, the data of each point shall be measured and recorded by "BHT-II Landslide Thrust Monitoring System".
2.4 Relative displacement monitoring of surface cracks
Several groups of fracture meters are placed at appropriate positions on both sides of the fracture to monitor the relative displacement of the fracture in situ. Mechanical monitoring has the characteristics of less interference, high reliability and stable performance. The time-displacement curve can be drawn directly from the monitoring data, and the measurement results are intuitive. The general measuring range of the instrument is 25 ~ 100 mm, the resolution of the reader is 0.0 1mm, and the working temperature is -40℃ ~+ 105℃.
2.5 Groundwater monitoring
Drilling holes are arranged at representative points on the landslide, and the parameters such as groundwater level, pore water pressure, soil water content and temperature are monitored by automatic water level recorder and pore water pressure monitor. The pore water pressure of the pore water pressure monitor ranges from-80 kPa to 200 kPa, with a resolution of 0. 1kPa and an accuracy of 0.5% f·s；; The range of soil water content is 0 to saturated water content, and the resolution is 1%. The temperature range is 0 ~ 70℃, the resolution is 0.65438 0℃, and the accuracy is 65438 0% f s.
2.6 GPS global positioning system monitoring
Select the point with good geological conditions and relatively stable foundation outside the landslide as the monitoring reference point; The representative points on the landslide are selected as monitoring points, and all the landmark points are forced to be centered by concrete monitoring piers, and multi-point joint measurement is adopted during observation. The GPS monitoring method can be used for all-weather monitoring, and is not limited by visibility conditions. It is convenient and flexible to monitor the displacement in X, Y and Z directions at the same time, and it can also monitor the regional crustal deformation in the zone where the disaster body is located. UZ CGRS GPS produced by American Ashtech Company is adopted, with minimum sampling interval of 1s, and at least 12 satellites are tracked and received. Using Ashtech Solution 2.6 software, the accuracy level can reach 3mm+ 1ppm, and the vertical accuracy can reach 6mm+2ppm.
2.7 TDR monitoring in time domain
That is, the "radar" test technology in the cable is used to transmit pulse signals in the cable and monitor the reflected signals at the same time. Select representative points on the landslide to arrange monitoring boreholes, bury coaxial cables in the monitoring boreholes, connect the ground surface with TDR monitor, compare the test signals with the reflected signals, judge the open circuit, short circuit and deformation state of coaxial cables according to their abnormal conditions, infer the deformation position of cables, and then calculate the deformation position and displacement of landslide strata. TDR monitoring adopts fixed preset coaxial cable, which has low cost, can be used for continuous monitoring of the whole section from top to bottom, and has a wide measuring range.
2.8 Macro monitoring
On the basis of regular inspection method, according to the deformation characteristics of landslide with large deformation, a certain number of simple observation points are arranged for regular observation to grasp its deformation dynamics in time.
For each landslide disaster point, more than two monitoring methods are used to monitor the surface deformation and internal deformation or stress change of landslide, and 4 ~ 5 methods are used to monitor important disaster points for comparison and comprehensive analysis. The layout of monitoring points should be highlighted to control the key parts of landslide; Comprehensive treatment, try to reflect the overall deformation of landslide. Concrete is poured around the borehole orifice, and accurate monitoring points are arranged.
3 monitoring effect analysis
According to the professional monitoring data of landslide disaster from July to June, 2003, the monitoring method and application effect of geological disaster early warning project in the Three Gorges reservoir area are preliminarily analyzed.
3. 1 Ground Deformation Monitoring
Ground deformation monitoring, including the monitoring of Jiushe landslide in Daqiu, Kaixian County, Jiushe landslide in Ju Ping, Gouzibao landslide in Wushan County, Banbitang landslide and four landslides. Taking Jiushe landslide in Daqiu County of Kaixian County as an example, the monitoring effect is briefly described.
Daqiu Jiushe landslide is located on the slope of Daqiu Jiushe in Zhendong Town, Kaixian County. The plane shape of landslide is approximately rectangular, and the section is concave. The distribution elevation is 205~300m, the length of the sliding body is about 250m, the width is about 300m, the area is 765,438+10,000m2, the estimated thickness is 20m, and the volume is about1.40,000m3. Landslide developed in a gentle layered slope composed of purple mudstone and sandstone interbedded in Jurassic Shaximiao Formation (J2s). The landslide body is mainly composed of sandstone and sandstone clastic soil, with loose surface and sandstone debris exposed locally, which is a landslide accumulation body.
Figure 1 Cumulative displacement curve of Jiushe landslide in Daqiu, Kaixian County
(a)X direction (b)Y direction (c)H direction d 1- Monitoring point number
There are three rows of monitoring points of Jiushe landslide in Daqiu, with 9 monitoring points in each row. Two reference points are arranged on the slope opposite to the landslide, and monitoring is carried out at the two reference points respectively. The layout of monitoring network not only controls the whole landslide but also highlights the key points, and the forward intersection method is used for measurement.
The first measurement was carried out on August 5th, and the second measurement of D 1 was carried out on September 20th, 2 1. The results show that the deformation trend is obvious and the sliding body slides in the direction of NEE. 10 year124 October, the monitoring results show that the deformation of each monitoring point tends to ease. The monitoring results in June 1 1 and February 1 show that there is no obvious change at each monitoring point (see figure 1). The monitoring data are consistent with the qualitative analysis of macro-investigation.
Using total station to monitor soil deformation is characterized by convenient monitoring, which can monitor some dangerous landslides at any time. Both permanent monitoring piles and temporary monitoring piles can be set on the landslides. The monitoring accuracy is high, and the measuring point error can reach 3.5 mm; ; Not only the relative displacement can be measured, but also the absolute displacement can be monitored. Under the condition of meeting the measurement conditions, continuous monitoring can be carried out to monitor the whole process of landslide sliding, and there is no scope limit. However, the monitoring of this instrument is limited by weather factors and lighting conditions, so it is difficult to monitor in rain and fog conditions and at night, and it is also limited by terrain and visibility conditions, so it is difficult to achieve automatic monitoring mainly by manual operation.
3.2 Deep displacement borehole inclinometer monitoring
The monitoring points of deep displacement borehole inclinometer are 6 landslides and 16 boreholes in Kaixian county, 5 landslides and 19 boreholes in Wushan county, 8 landslides and 24 boreholes in Wanzhou district, accounting for 19 landslides and 59 boreholes. Taking the landslide of Hucheng Village in Kaixian County as an example, the monitoring effect is briefly described.
Huchengcun landslide is an accumulative landslide, which is located on the slope of Huchengcun, Changsha Town, Kaixian County. The plane of the landslide is approximately rectangular, and the profile is concave. The distribution elevation is 330 ~ 400 m, the longitudinal length is about 300m, and the lateral width is about 500m m. The estimated average thickness of the landslide is 12m, the area is150,000 m2, and the volume is180,000 m3. The landslide developed on the horizontal layered rock slope composed of purple mudstone and argillaceous siltstone in Shaximiao Formation (J2s) of Middle Jurassic, and the upper part of the landslide was colluvial purple gravel soil. Landslides threaten the safety of more than 400 residents and their property. The landslide is equipped with three deep displacement borehole inclinometers to monitor the borehole.
Borehole Kx- 162 is located in the middle of sliding body. In June, 2004, the depth of 10 and 9.5 ~ 10.5 m was obviously displaced and deformed. The deformation amount this month was 5.56mm, and the deformation direction was 247. The month of 1 1 has no increasing trend, and the cumulative deformation is 4.58mm, slightly less than the month of 1, and the deformation direction is 253 (see Figure 2).
The Kx- 165 drilling hole is located at the lower part of the sliding body. In June, 2004, 10, obvious displacement and deformation occurred at the test depth of 15.0 ~ 16.5 m (see Figure 3). The deformation this month is 5.45mm, and the deformation direction is 24 1. 1 1 month, with no obvious increasing trend. The cumulative deformation is 5.39mm, which is similar to that of 1 month, and the deformation direction is 240.
Essays on Geological Disaster Investigation and Monitoring Techniques and Methods
Fig. 2 Variation curve of borehole displacement with depth of Kx- 162 landslide in Hucheng Village, Kaixian County.
(a) East-West direction (b) North-South direction
Fig. 3 Variation curve of borehole displacement with depth of Kx- 165 landslide in Hucheng Village, Kaixian County.
(a) East-West direction (b) North-South direction
The monitoring method of deep displacement borehole inclinometer can monitor the vertical sliding direction and relative sliding displacement of shallow, middle, deep and sliding zones in landslide bodies in boreholes arranged at certain positions. However, when the landslide occurs large or rapidly accelerated displacement deformation, the inclinometer probe cannot be sent into the borehole due to the deformation and damage of the borehole and the inclinometer tube in the borehole, which may make the borehole lose its monitoring value.
3.3 Landslide thrust monitoring
There are 2 measuring points and 4 boreholes for landslide thrust monitoring: 2 boreholes for Li Tang landslide in Wushan County and 2 boreholes for Caojiatuo landslide. Taking Li Tang landslide as an example, the monitoring method and effect are briefly described.
Li Tang landslide is located on the left bank slope of the main stream of the Yangtze River in Quchi Township, Wushan County. Landslides are irregular round-backed chairs on the plane. The leading edge is 90m high, the trailing edge is 400m high, the average slope is about 30 ~ 40, the longitudinal length is about 800m, the lateral width is 150~250m ~ 250m, the sliding body is 20m thick, with an area of 240,000m2 and a volume of 4.9 million m3. Landslides developed in limestone, marl and mudstone of Triassic Badong Formation (T2b). The main materials of landslide are marl and mudstone fragments, the surface layer is mostly loose soil, and the structure of the lower fragments is dense.
The thrust hole Ws-t-tzk 1 is located in the lower part of the sliding body, and the thrust hole Ws-t-tzk2 is located in the middle part of the sliding body. See Figure 4 and Figure 5 for the data of landslide thrust monitoring results. The thrust monitoring curve shows that the monitoring data have strong regularity and are basically consistent, and no obvious numerical change is found by the sensor. The monitoring results of landslide thrust are consistent with the macro monitoring results and the simultaneous monitoring results of borehole inclinometer, indicating that the landslide is in a relatively stable micro-deformation state at this stage.
Fig. 4 Monitoring curve of Ws-t-tzk 1 borehole landslide thrust in Li Tang, Wushan County.
Fig. 5 thrust monitoring curve of borehole Ws-t-tzk2 landslide in Li Tang, Wushan County.
The landslide thrust monitoring method belongs to fixed-point monitoring. Sensors are preset in the borehole, connected by sensing optical fibers, and the sensing information is collected by the ground landslide thrust monitoring system. Drill holes are arranged at a certain position on the landslide body, and the vertical landslide thrust changes in the shallow, middle, deep and slip zones in the landslide body are monitored from top to bottom, and the monitoring data are collected regularly. On the basis of perfecting the acquisition, transmission and processing system, unattended automatic continuous monitoring can be realized.
4 conclusion
(1) Through comprehensive monitoring by various means, the deformation and stress of the surface and internal sliding zone of the monitored landslide are mastered. The comprehensive analysis of the data shows that it reflects the displacement change and dynamic characteristics of landslide, provides important basic data for disaster early warning, and shows that the monitoring method adopted is reasonable and effective.
(2) The deep displacement monitoring method of borehole inclinometer, when a certain amount of slowly changing displacement occurs in landslide, some boreholes can no longer be measured in the whole hole, resulting in the waste of investigation and monitoring funds and the reduction of landslide monitoring points and monitoring parts.
(3) At present, once a month monitoring cycle, it is difficult to ensure effective monitoring when landslides are in danger of slipping. Therefore, in addition to professional monitoring, group monitoring and preventive monitoring should be carried out. Under special circumstances, for dangerous landslide disaster points, adjust the monitoring scheme, and carry out encrypted monitoring or continuous monitoring to make the monitoring meet the requirements of early warning and forecasting.
(4) Considering the long-term development, monitoring should combine unattended, easy to maintain, low-cost, fixed, automatic, rapid and continuous acquisition and transmission, semi-automatic monitoring and manual monitoring to establish an efficient geological disaster monitoring network and geological disaster early warning system.
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