Lei Mingtang, Jiang Xiaozhen, Li Yu, Meng Yan
(Institute of Karst Geology, Chinese Academy of Geological Sciences, Guilin, Guangxi, 541000)
AbstractChina's soluble rock distribution area reaches 3,650,000km2, which accounts for more than one-third of the country's national territory, and it is one of the most developed countries in the world in terms of karst. In recent years, with the rapid development of urbanization construction in karst areas, the development of land resources, water resources and mineral resources in karst areas is continuously enhanced, and the resulting karst subsidence problem is becoming more and more prominent, and it has become the main geological disaster problem in cities in karst areas, which seriously hampers the urban economic construction and development. Since the generation of karst subsidence is sudden in time, hidden in space and complex in mechanism, it is generally recognized that it is difficult to monitor and forecast the subsidence by means of conventional ground monitoring. On the other hand, experimental studies have shown that changes in karst water and gas pressures have a triggering effect on collapse, which can be used as a critical condition to measure the development of collapse. This means that by observing the dynamic changes of water (gas) pressure in karst pipeline systems, the purpose of forecasting collapse can be achieved. This paper discusses the basic methodology of this technique by taking the karst subsidence monitoring station located in Zhemu Village, Guilin, Guangxi, as an example.
Keywords karst subsidence karst pipe water (gas) pressure critical hydraulic slope monitoring
1 Overview of the study area
The study area is located in Zhemu Village on the west bank of the Lijiang River, about 15km southeast of Guilin City (see Fig. 1), covering an area of about 0.2km2, with 116 existing residents. The area was severely collapsed on November 11, 1997, triggered by the blasting of the Li River channel, resulting in the collapse of 4 residential houses and the cracking of 64 houses. Since the collapse is still developing in recent years, which is always threatening the life and property safety of the people, we take this as the field test site of this project to establish a karst collapse disaster monitoring station and carry out the research work.1.1 Characteristics of karst collapse development
Figure 1 Geographic location of the study area
The Zhemu collapse occurred on November 11, 1997, firstly, the river floodplains 2 collapses occurred, then more than 10 places in Tsuge Village, the ground bubbled up and sandblasted (the water column was more than 3 meters above the ground), and then a large area of collapse and ground cracking occurred, forming 35 collapsed pits, and by the end of 1998, more than 50 collapsed pits were formed **** counted (Fig. 2).
Figure 2 Distribution map of collapse pits in the work area
The majority of the collapse pits in Tsuzuki Village are round and oval in plan form, with only a few irregularly shaped ones; the sectional form is mainly altar-shaped, and except for bedrock collapse pits located in the river floodplain with diameters (or long axes) of up to 30m and depths of more than 14m, the diameter of the collapse pits of the soil layer in the village ranges from 0.5m to 10m, with depths of several tens of centimeters to 5m. The collapse has obvious banding and directionality in planar distribution, and all of the collapses are developed in the NW direction, and most of them are banded along the f1 and f2 faults. The collapse has continuity in time, according to the detailed investigation of the collapse in Guilin City by the project team in 1986 and 1996, there is no record of collapse in the area, and since the first collapse in 1997, there have been collapses every year.
1.2 Geological conditions
The geomorphology of the study area is in the combined part of the first-level terrace of the Lijiang River and the river floodplain, and the thickness of the cover layer is 15-40 m. Among them, the cover layer of the first-level terrace in which Tsuge Village is situated can be summarized as a ternary structure: the upper part of it is a layer of clayey soil, which generally consists of clay, pulverized clay, and miscellaneous fill (containing pebbles and bricks and other hard materials); the middle part of it is a layer of sandy pebbles, which contains no clay grains and is dominated by pebbles, and the middle part is a layer of sandy pebbles. In the middle part, there is a sand and pebble layer, which does not contain clay particles and is dominated by pebbles, and there is a sand layer about 1m thick at the top, and there is a local interlayer of medium-coarse sand; in the lower part, there is a mixed soil, which is dominated by clay pebbles and pebble-clay layer, and the two layers are not clearly demarcated, and they are often alternating with each other, with large variations of the content of clay particles; there is a lenticular body of clay layer near the borehole ZK1, and there is also a soft clay layer about 10m thick at the bottom of the borehole ZK4. The underlying bedrock is Devonian Upper Unified Rongxian Formation tuff (D3r), which is the most developed karst and most collapsed stratum in Guilin.
1.3 Tectonic conditions
The study area is located in the nuclear part of the Ertang dyke and the intersection of the NW-directed Longjia Fault and the NE-directed Maojia Fault, where the NW-directed Longjia Fault passes right through Zhemu. The Longjia fault (f1) is a large-scale fault, which is compressive-torsional in the early stage and exhibits obvious torsional-tensile characteristics in the late stage, and it controls the course of part of the Lijiang River, and the fault is a water-rich fault.
Shallow seismic measurements show that, in addition to the Longjia fault (f1), another fault (f2) with a nearly NW strike is also developed in Zhemu (Figure 2).
1.4 Hydrogeological Conditions
Based on the conditions of groundwater storage, there are two types of groundwater in the study area: quaternary pore water and karst water. The Quaternary aquifer is a sand and pebble layer, which is the main source of water for local farmers. Karst water is the most abundant tuff water of the Upper Devonian Rongxian Formation in Guilin, which is separated from the quaternary pore water by a layer of mixed soil, which mainly consists of clay pebbles and pebble clay, and its water separating property is average to medium, and karst water has a strong hydraulic connection with the quaternary pore water. The study area is a groundwater discharge area, and the quaternary pore water is mainly recharged by atmospheric precipitation, surface water infiltration, Li River and underground runoff; the karst water is mainly recharged by the quaternary pore water overland flow, rainfall infiltration, underground runoff as well as recharge from the Li River, and the karst water is generally weakly pressurized. Due to the close proximity of the Li River, surface water is abundant, and except for the use of hand-cranked wells or civil wells to extract a small amount of water from the Quaternary sand and pebble aquifer as drinking water, the study area has not carried out any large-scale extraction of groundwater (including karst water and Quaternary water), that is to say, the groundwater in this area is less affected by human activities, and the fluctuation of its water level is basically mainly influenced and controlled by natural conditions.
1.5 Conditions of human activities
There is a single type of human activities in the working area, and each household in the village has a manually-pressurized well to satisfy the water used in daily life, with the depth of the well being less than 10m, and exploits the pore water in the sandy pebble layer of the Quaternary System.
Since the collapse in 1997, blasting activities have been completely stopped in the work area, therefore, the subsequent new collapses are of disturbed soil under the influence of natural conditions.
2 Monitoring and Forecasting Ideas
2.1 Analysis of Development Mechanisms and Influencing Factors of Karst Subsidence
According to the analysis of the field investigation, at that time, blasting of the bedrock of the navigation channel was being carried out in the Lijiang River, which led to the collapse of the bedrock near the outlet of the underground river, and the resultant collapse seismicity was the root cause of the subsequent large-scale subsidence.
Because of the role of high-pressure water flow so that the work area of the fourth system at the bottom of the soil layer has been seriously disturbed, greatly reducing the critical hydraulic gradient to make it infiltration and deformation, and reduce the soil layer's ability to resist collapse. As long as there is a large change in groundwater, it will trigger a new collapse. This is the root cause of the continuous collapse in recent years, which is consistent with the results of the karst collapse physical modeling test (Figure 3).
Figure 3: Illustration of the model test
The generation of new subsidence is mainly affected by several aspects:
One is the change of water and gas pressure in the karst pipe (fissure) system and the soil layer at the bottom of the Quaternary System: when the water and gas pressure of the karst pipe (fissure) system changes or the hydraulic slope acting on the soil layer at the bottom of the Quaternary System reaches a certain value, the soil layer of the Quaternary System will be damaged, which in turn generates the ground subsidence. This in turn produces ground collapse. At present, through the pore water pressure sensor and automatic data acquisition system has been able to record the dynamic change of water and gas pressure of karst pipe (fissure) system, and calculate the hydraulic gradient acting on the soil layer at the bottom of the fourth system.
The second is the composition and nature of the soil layer at the bottom of the Quaternary System: with different compositions and properties, the critical hydraulic gradient for infiltration deformation will be different. At present, through the field borehole sampling and indoor infiltration deformation test method, can determine the critical hydraulic slope value of infiltration deformation occurred in different soil layers.
So, we can monitor the pressure change of groundwater (including karst water-sensor A and soil pore water-sensor B) to achieve the purpose of forecasting ground subsidence.
2.2 Predictive forecasting ideas
Based on the above understanding, the research ideas shown in Figure 4 are adopted to carry out the work, that is:
Figure 4 Research work ideas
(1) Firstly, based on the analysis of the results of the investigation of the existing karst collapse and its influencing factors, and through the penetration deformation test and geotechnical test and other analytical tests, the preliminary determination of the collapse development of the Critical conditions.
(2) Through the sensor and data acquisition system, the dynamic changes of the main inducing (triggering) factors (including karst water-sensor A and soil pore water-sensor B) are directly monitored.
(3) Combined with the anomalous area shown by the geo-radar exploration, the modified model of critical conditions is established, so as to make the results of indoor model test and infiltration and deformation test practical.
(4) When the values of the main inducing (triggering) factors meet the critical conditions, the forecast will be issued directly.
3 Experimental study of critical conditions for collapse development
Based on the results of infiltration and deformation tests of 49 groups of soil samples, the critical conditions for the occurrence of collapse in three types of soils in the work area (expressed by the critical hydraulic gradient I0) were preliminarily determined as shown in Table 1, and the critical rates in the table were obtained according to the results of the previous modeling studies of karst collapse in Guilin by the project team.
The critical conditions of the bottom soil layer of the Quaternary System are mainly used as the criterion in the forecast. The exploration results show that in the whole working area, there exists a clay-pebble layer with good continuity at the bottom of the Quaternary System, and it has been strongly disturbed, therefore, the critical conditions are: I0=0.79,V0=0.06kPa/s.
The judgment is made by two methods in the prediction:
(1) Comparison of the fluctuating rate of the pressure of the karst water, V, with that of V0: When V≥V0, the soil layer near the bedrock surface will be in the vicinity of the bedrock surface. the soil layer near the bedrock surface will be likely to undergo infiltration damage, with the possibility of producing a collapse.
(2) Comparison of hydraulic slope (I) calculated from karst water pressure, soil water pressure and the distance between the two sensors with the critical slope drop (I0): when I≥I0, the soil layer near the bedrock surface will be likely to undergo infiltration damage, and there is a possibility of collapse.
Table 1 Critical conditions for the collapse of Tsuge
4 Monitoring techniques and methods
4.1 Monitoring contents and monitoring methods
Groundwater (gas) pressure: including water (gas) pressure monitoring of karst pipe fissure system and water pressure of soil layer at the bottom of Quaternary System, pore water pressure sensors are used for monitoring, and the data collection methods are computer automatic collection and portable The data collection methods are computer automatic collection and portable meter manual reading two kinds.
Damage of soil layer deformation: fixed measuring line is set up in the working area, and geo-radar is regularly used for monitoring.
Changes in the cracks of private houses: set up monitoring points for the cracks of private houses in the monitoring area, regularly measure the changes in the cracks, and monitor them with a steel ruler.
Water level of civil wells: the two open wells in the working area are regularly measured for water depth, and are monitored manually with measuring ropes.
4.2 Monitoring equipment
4.2.1 Sensors
Vibrating string instruments have been in the spotlight of the engineering community since they were invented in the 1930s due to their unique and excellent characteristics such as simple structure, high accuracy, strong anti-interference ability and low requirements for cables. However, due to historical reasons, the long-term stability of vibrating string instruments has been a topic of controversy. Until the 70's, with the modern electronic readout technology, materials and production process development, vibrating string instrument technology can be improved and can really meet the requirements of engineering applications. At present, the perfect performance of the string instrument has become the trend of the new generation of engineering instruments. For this reason, the monitoring station all use vibrating string pore water pressure sensor, by the Canadian Rock Tester company and the United States Kikang company production.
4.2.2 Data Acquisition System
In order to realize automatic data acquisition and long-distance transmission, MICRO-10X automatic data acquisition system produced by American Kikang Company and Multiloggorl.48, a data acquisition software developed by American Canary Systems, are adopted, in addition to VW-403C, a data acquisition software produced by American Kikang Company. VW-403C portable vibrating string readout meter produced by American Kikon Corporation is also used.
4.2.3 Geological Radar
Geological radar, which was firstly developed for detecting tunnels in the Vietnam War, began to be used for potential collapse survey in the United States in the early 1980s, and was popularized in China in the 1990s. It is a broad-spectrum (1MHz ~ 1GHz) electromagnetic technology used to determine the distribution of underground media, the basic principle is: through the transmitting antenna to the ground to transmit radar signals (high-frequency electromagnetic waves with a frequency of 80 ~ 1000MHz), and then through the receiving antenna to receive the signals reflected back from the underground interface of different electrical properties. As long as there is a significant difference in the dielectric constant of the underground objects, a reflective interface will be formed, and when the electromagnetic wave propagates in the medium, its path, electromagnetic field strength and waveform will change with the electromagnetic properties and geometric form of the medium through which it passes. Therefore, according to the received wave travel time (also known as two-way walking time), amplitude and waveform information, can be inferred from the structure of the medium. Therefore, geo-radar can detect soil disturbance zones such as underground soil holes. Geo-radar has the advantage of easily, quickly and accurately generating a continuous underground profile.
The monitoring station adopts SIR-10A geo-radar from the United States.
4.3 Sensor calibration
In order to establish the relationship between water pressure and sensor readings, sensor calibration work was carried out indoors, using a 3m high water tank as well as a 20m deep groundwater level observation well, to establish the calibration equations for each sensor.
4.4 Sensor installation method
Two sensors are installed at each measurement point, one of which is installed in the fissure of the karst pipe for monitoring the karst water pressure, and the other is installed in the quaternary aquifer (Fig. 5) for monitoring the change of water pressure in the soil layer.
4.5 Arrangement of Monitoring Points
Based on the characteristics of karst collapse development in the working area, the underlying geological conditions, and the importance and degree of destruction of the buildings in the Tsuge Village, the working area is divided into four monitoring zones, and 16 monitoring points are set up in ****. Among them, there are 8 monitoring points each for water (gas) pressure of karst system and pore water pressure of soil layer at the bottom of the fourth system, 2 monitoring points for water level of civil wells, 4 monitoring points for crack changes in civil houses, and 12 geo-radar monitoring lines for deformation and damage of soil layer of the fourth system, and the location of each monitoring point (line) is given in Fig. 6.
Figure 5 Illustration of sensor installation position
Figure 6 Diagram of monitoring area division and monitoring point layout
4.6 Data Acquisition
***There are 8 sensors using Micro-10X data automatic acquisition system for data acquisition, and the trial operation has been started since February 27, 2002, through the acquisition software ( dataloggorl.48) to set up the channels to which each sensor is connected, and this job will take readings at 10-minute intervals.
The use of portable receivers, the remaining 8 sensors for data collection work, which is installed in 1 ﹟, 2 `, and 3 ` monitoring point of 6 sensors from March 3, 2000 to start monitoring, another 2 buried in 4 `, 5 ` monitoring point of the soil layer of the sensors from February 27, 2002 to start measurements, the sensor monitoring cycle of the rainy season for the first time a day, the usual for the 3 days a Times.
4.7 Soil Disturbance Monitoring
The monitoring of soil disturbance was carried out by geo-radar, and 12 measurement lines were arranged at the site, and the measurements were carried out by geo-radar with the same frequency, and the measurements were carried out once a year.
5 Analysis of monitoring results
Since the beginning of the implementation in 2000, there were 13 times of anomalies on the ground, including 9 times in 2000, 2 times in 2001, and also 2 times in 2002, and Table 2 shows the list of anomalies on the ground in the work area.
Table 2 List of anomalies occurring on the ground in the working area since 2000
Continued
From the table, it can be seen that the anomalies in the past three years mainly occurred in the area located in the I area, followed by the II area and the III area, and the monitoring of the water pressure before the anomalies appeared had a mutation, and this kind of correspondence can be applied to the collapse prediction.
Since manual monitoring was used in 2000 and 2001, the average speed of karst water (gas) pressure change was used, and it was impossible to get the instantaneous speed and use it to forecast the collapse.
The anomaly that appeared in Tangzhaoxie's house on May 16, 2002, had an obvious relationship with the pressure change of the measuring points ZK2 and ZK5, especially ZK5 was buried with the sensor of automatic collection, and it was detected that the pressure change speed of the point was 0.0057, and the pressure change speed was 0.0057. The rate of pressure change is 0.0057kPa/s, which is quite different from the critical value obtained from the indoor experiment, but the permeability slope drop is about 0.5, which is close to the critical value obtained from the indoor experiment.
From the monitoring results, basically no changes were monitored in the wall crack deformation monitoring, indicating the suddenness of the collapse development, which is difficult to achieve the purpose of forecasting through crack monitoring.
6 Geological radar exploration results
The 100MHz geological radar antenna and continuous scanning method were used for monitoring. 2000 completed the first round of geological radar exploration 1 time, 13 survey lines were arranged, and in 2001, the survey lines were optimized, and 8 original survey lines and 4 new survey lines were retained.
Table 3 lists the locations of the anomalies shown by the three surveys. As can be seen from the table, there were 43 anomalies in 2001, 2002, down to 36, located in Zone I, Line 1, 2. 2002 anomalies were increased by 5, 8 than in 2001, which corresponds well to the monitoring of Zone I, ZK, ZK7, ZK8 has more than the critical rate of water (gas) pressure fluctuations in the speed.
The detection results in 2002 showed that in the position of 30-40m in line 1 ', 41-47m in line 2 ' and 0-3m in line 10 ', the soil disturbance has been close to the ground, and it is very likely to produce a new collapse.
Table 3 Comparison table of geo-radar detection results
Continued
Continued
Continued
7 Conclusion
Through the present study, the first karst subsidence disaster monitoring station with automatic data collection was built in China, and the technical system and method of karst subsidence prediction and forecasting were established initially, and the monitoring work of the last three years showed that: <
(1) Under the current technical conditions, the karst ground subsidence triggered by groundwater activities can be predicted and forecasted.
(2) The determination of critical conditions for the development of karst subsidence, the selection of monitoring factors, the installation and burial of sensors, and the application of automatic data acquisition system are the keys to carry out subsidence prediction and forecasting work.
(3) This project adopts the method of monitoring the water (gas) pressure in the fracture system of karst pipes and the change of the pore water pressure of the fourth system as the main method, supplemented by the monitoring of geo-radar, which is an effective method of prediction and forecasting. Twelve ground anomalies occurred during the monitoring period, all of which were related to sudden changes in karst water/gas pressures at the monitoring sites near the anomalies and the higher permeability of the soil layer at the bottom of the Quaternary system.
(4) From the results of sensor monitoring:
a. Area Ⅰ is the most dangerous area with the strongest change of karst water/gas pressure, and the rate of change of karst water/gas pressure in 2002 reached 0.47kPa/s, while the infiltration slope drop of groundwater acting on the soil layer at the bottom of the Quaternary System is 0.17-3.12, which is more than the critical condition, so that this area is the most dangerous area.
b. In 2000 and 2001, the infiltration slope drop of groundwater acting in the soil layer at the bottom of the Quaternary System in Area II was 0.55 and 0.79, and in 2002, the infiltration slope drop reached 0.78, and the rate of change of the pressure of karst water/gas was 0.085kPa/s, which has also reached the critical damage condition.
c.The groundwater infiltration slope drop in Zone III is smaller, with -0.3, 0.52 and 0.44 in 2000, 2001 and 2002, respectively.
d.The groundwater infiltration slope drop in Zone IV is smaller, with -0.44, and the rate of change of karst water/gas pressure is 0.012kPa/s. The critical damage condition is also reached.
(5) Geo-radar can find the shallow anomalies of soil layer in time and effectively, the detection results of three consecutive years show that there are 43 anomalies in 2001, decreased to 36 in 2002, and the anomalies of Line 1 and Line 2 located in Zone I in 2002 are increased by 5 and 8 respectively compared with 2001, which is related to the monitoring of Zone I of the ZK, ZK7, and ZK8 have more than the critical speed of the water ( gas) pressure fluctuation velocity has a good correspondence. The soil layers of the other lines are still in the process of adjustment.
(6) Geological radar has found 3 locations, 30-40m in line 1, 41-47m in line 2 and 0-3m in line 10***, where the soil disturbance is close to the ground, and it is very likely to produce a new collapse.
(7) The monitoring results show that no significant changes have appeared through the manual monitoring of the wall cracks.