(1) Formation Mechanism of Rainfall-induced Landslide and Debris Flow
In recent 30 years, the study of rainfall landslide is one of the hot spots in landslide research, and its core is to predict the possible landslide state by studying the relationship between rainfall and landslide. According to preliminary statistics, scholars from at least 23 countries around the world have studied rainfall-induced landslides to varying degrees, and scholars from the United States, Italy, Japan, Britain, Australia, New Zealand, China, Hong Kong and mainland China have published more research papers. After 1984, the government of China and Hongkong increased its research on rainfall-induced landslides. In addition to the annual investigation of rainfall landslides, it is particularly important to study the relationship between landslides and rainfall, the distribution and development law of rainfall landslides, the hydrogeological model of rainfall infiltration, and establish a more accurate landslide-rainfall relationship by using mathematical methods such as probability statistics. With the deepening of research, researchers agree that the unique properties of unsaturated soil and residual soil in the weathered layer of igneous rocks in Hong Kong control the formation mechanism of shallow rainfall landslide. The results show that the essence of the formation mechanism of rainfall-type landslide is that the stress balance of the slope is destroyed after rainwater permeates the slope. Therefore, it is the key to explain theoretically the change process of slope stress after rainwater infiltration and the characteristics and process of rainwater infiltration in the slope.
(2) Study on the development mechanism and criterion of karst collapse.
Japanese scholar Noguchi (1970), Soviet scholar Xоменко( 1986), American scholar Ralphj Hodek( 1984) and Thom-As M. Some foreign scholars have tried to use geotechnical centrifuges for collapsibility test, such as Borms and Bennermark( 1967), Marir( 1984), Bertin( 1978), Howell and Jenkins( 1984), Sterling and Ronayne (. Craig( 1990), Abdulla and Goodings( 1996) used centrifuges to simulate the collapse failure mechanism and the critical combination conditions leading to the collapse, focusing on the relationship between the collapse failure of the weakly consolidated sand layer overlying the cave and the size of the cave opening, the strength of the cave itself, the strength and thickness of the weakly consolidated sand layer, the thickness of the overlying sand layer and the surface load.
The risk assessment of geological disasters based on GIS technology in the United States, Italy and Britain includes the risk assessment of karst collapse.
(3) Investigation and risk assessment of regional landslides and debris flows
The early spatial prediction of geological disasters was mainly based on field investigation and aerial photo interpretation, and experts judged and evaluated the sensitivity of geological disasters, so it was called expert evaluation method (Aleotti and Chowdhury, 1999). The accuracy of the evaluation results of this method depends on the detail of field investigation and the knowledge and experience of experts. The hidden rules used in the evaluation make it difficult to analyze and update the results, and the results obtained by different investigators and experts cannot be compared.
In 1970s, taking the landslide sensitivity map of san mateo County in San Francisco, California as an example, the method of obtaining regional landslide disaster prediction map by weighted (or unweighted) superposition of multi-parameter maps was greatly popularized. The advantage of this method is that it overcomes the problem of using implicit rules; The disadvantage is that the determination of weight is still subjective and difficult to popularize and apply.
In 1980s, inspired by the application of statistical regression analysis and discriminant analysis in oil and gas migration and deposit prediction, Carrara( 1983) introduced multivariate statistical analysis and prediction method into regional landslide spatial prediction, and made this technology develop and popularize rapidly all over the world. For example, Haruyama and Kawakami( 1984) used mathematical statistics theory to assess the risk of landslide disasters caused by rainfall in Japan's volcanic active areas. Baeza and Corominas (1996) used statistical discriminant analysis model to evaluate the sensitivity of shallow landslides, and the correct prediction rate of slope instability reached 96.4%, which showed the applicability of statistical prediction. Carrara, Cardinali and Guzzetti (199 1) combined the statistical model with GIS and applied it to the landslide risk assessment of a small watershed in central Italy. The results show that the comprehensive application of statistical analysis and GIS is a fast, feasible and low-cost method for regional landslide risk assessment and mapping.
Since 1990s, with the rapid development of computer technology and information science, geographic information system (GIS) technology with the function of dynamically linking attribute database and graphic database has been developed unprecedentedly, and its combination with quantitative spatial prediction model of geological disasters has also become a new field of geological disaster research.
Mario Megia Navarro and Ellen E. Wall (1994) analyzed the sensitivity of regional geological disasters and the fragility of land and life caused by landslide and debris flow in Medellin, Colombia, and made a risk assessment zoning map with GIS technology. Anbalagan and Bhawani Singh( 1996) put forward a new risk assessment mapping method-risk assessment matrix (RAM) on the basis of Anbalagan's study on landslide risk assessment and zoning mapping in mountainous areas.
Aleollt(2000) used GIS technology to study the risk and comprehensive risk of landslides, floods, avalanches, taniguchi accumulation and other disasters in Piedmont in northern Italy. Michael-Leiba et al. (2000) studied the danger, vulnerability and risk assessment of slope geological disasters in an urban development planning project in Australia, and studied the danger and risk zoning of slope geological disasters in Keynes region with plane and three-dimensional evaluation systems based on GIS software. Ragozin(2000) theoretically studied the danger, vulnerability and risk of landslide disaster risk assessment. A single landslide risk index considering the validity period of risk assessment target is put forward and expressed by the probability product of its main controlling factors. For the evaluation of regional landslide disaster, a quantitative model is established according to the area of a given area, the area where landslides occur, and the relationship between the number of landslides and time.
10. 1.2 research on monitoring and forecasting technology
(1) Study on Critical Rainfall and Meteorological Warning Caused by Landslide and Debris Flow
In the study of the critical value of rainfall-induced geological disasters, scholars all over the world have adopted various methods to determine the critical value of rainfall-induced landslides, and the difference lies in the different factors considered. Glade( 1997) established three models to determine the critical rainfall value of landslides, which were verified in Wellington, New Zealand. The basic data needed by the three models are: daily rainfall, landslide occurrence date and potential daily evaporation of soil (calculated by Thornthwaite method). The premise of the model is: it is assumed that the evaporation in the area with the largest daily rainfall is the smallest; ② Maximum rainfall induced landslide. These three models basically summarize the current methods to determine the critical value of rainfall that induces landslides.
In the early warning of landslides and debris flows in the San Francisco Bay Area from 1986 to February 12 ~ 2 1, it was first analyzed and determined by the US Geological Survey, and early warning was carried out by special forecasting methods of local radio stations, television stations and the National Meteorological Center of the United States. This landslide and debris flow disaster warning is divided into two stages: the first is the 6-hour disaster danger period on February 14, and the other is the 60-hour disaster danger period from February 17 to June 19. Due to the complexity of geological conditions and the change of topographic conditions, these two forecasts are mainly aimed at the whole San Francisco Bay Area, not a specific landslide disaster site. According to the investigation after the landslide and debris flow disaster, 10 witnesses at the landslide and debris flow disaster site can provide accurate time, and the time of eight landslides and debris flows is consistent with the early warning period.
According to research, when the six-hour rainfall in San Francisco Bay Area reaches 4 inches (i.e. 10 1.6 mm), a large area of debris flow may be triggered. In order to monitor the change of groundwater level during rainfall, they also set up several pore water pressure gauges to observe the change of groundwater level in the slope. The real-time regional landslide early warning system in San Francisco Bay Area includes the experience and analysis relationship between rainfall and landslide, real-time rainfall monitoring data, rainfall forecast of National Meteorological Service Center and sketch of landslide-prone areas.
Since 1984, radar images have been used to explain the small-scale geological structure of Hong Kong to determine the potential landslide area. In addition, the monitoring network of landslide disaster rainfall has been established, and the number of automatic rain gauges 1999 has increased from 48 to 86. Send rainfall data to the management department regularly. If it is predicted that the rainfall will reach 175mm within 24 hours or exceed 70mm within 60 minutes in urban areas, it is considered that the landslide prediction threshold has been reached and the government will issue an announcement. On average, there are about three flash floods and landslides in Hong Kong every year.
(2) Research on monitoring technology and method of landslide and debris flow disaster.
Developed countries such as the United States, Switzerland, Italy, Japan and South Korea have done a lot of work. Especially, a single landslide has reached the stage of real-time monitoring. The monitoring contents include ground displacement, ground fissure, underground displacement, groundwater level (water pressure), water temperature, ground sound, etc. The monitoring technology adopts the combination of routine monitoring, automatic observation, GPS and satellite communication (figure 10. 1, 10.2). In China Hong Kong Special Administrative Region, a relatively complete geological disaster monitoring network based on rainfall monitoring has also been established.
Figure 10. 1 Using solar wireless remote control system (left) and deformer (right).
Figure 10.2 Real-time landslide monitoring in Ticino, southern Switzerland (according to www.geodev.ch)
(3) Research on new technologies and methods of land subsidence monitoring and prediction.
In developed countries such as the United States, the Netherlands and Japan, the groundwater level is often laid in the same hole with bedrock markers and layered markers, and automatic monitoring is carried out at the same time (Figure 10.3). First, it can be judged whether it is caused by excessive pumping of groundwater, that is, the cause of land subsidence; Secondly, it can be dynamically coupled with the monitored land subsidence data to obtain the correlation between groundwater exploitation (water level) and land subsidence; Then the water flow model is used to predict the land subsidence. At the same time, as the real-time monitoring data of groundwater level, it can directly become a reliable basis for groundwater resources management.
Figure 10.3 automatic monitoring system for layered labels and its schematic diagram (according to Amelung et al., 1999)
In Sacramento, California, USA, GPS survey has replaced the leveling of regional ground elevation. 1986, 38 GPS monitoring stations were built in this area. After 1989, the number reached 68. Through strict surveying procedures, the accuracy of geodetic elevation can reach millimeter level. After the application of Ashtech Z 12 dual-frequency GPS signal receiver in China and Shanghai for nearly two years, the high accuracy of 1999 can reach 3mm. Its advantage is that it can get twice the result with half the effort for large-scale regional land subsidence monitoring.
According to the data of the US Geological Survey, the interferometric synthetic aperture radar (InSAR) technology used to detect land subsidence in the United States is still under development and testing (Figure 10.4). Gabriel et al. first published the paper Mapping Large Area and Small Elevation Change: Radar Interference Measurement Method in 1989. 1993, Massonet et al. drew the ground deformation field of the lander earthquake through radar interferometry. Van der Kooij et al. used the satellite aperture radar data interfered by spacecraft to investigate the land subsidence in the natural gas exploitation area of Gloning, the Netherlands. Marco et al. made a detailed study on the land subsidence of Bellich Oilfield 1992 ~ 1996 by using the interferometric satellite aperture radar data of the American Society for Experimental Research. Due to the use of this detection technology, the accuracy of land subsidence measurement has reached millimeter level, and its detection results can be well processed into two-dimensional settlement isoline map. Moreover, this method can save a lot of manpower and material resources for conventional leveling stone measurement. Therefore, we can't underestimate the development and application prospect of this new technology. At present, we can carry out experiments in China with reference to successful foreign experiences.
(4) Research on monitoring technology of karst collapse.
Benson (1987), an American scholar, put forward a method of monitoring and forecasting by using geological radar, which was tested on a military railway southwest of Wilmington, North Carolina, USA. The monitoring period is half a year and good results have been achieved. In 2002, with the support of the national land and resources survey project, the Institute of Karst Geology of Chinese Academy of Geological Sciences established the first karst collapse disaster monitoring station in Zhemu Town, Guilin, Guangxi, which provided a good condition for in-depth and systematic study of karst collapse prediction methods.
Figure 10.4 Las Vegas Valley in Nevada obtained by synthetic aperture radar interferometry.
(5) Research on the transmission, processing and publishing system of geological disaster monitoring and early warning information.
Developed countries and regions pay more and more attention to the informatization of geological disaster monitoring. For example, the United States, Japan, Italy, France and South Korea have established real-time monitoring systems for geological disasters, which can realize real-time early warning in practical applications. There are many researches on the monitoring and early warning of a single geological disaster, but few on the integrated system of multiple disasters.
10. 1.3 Study on engineering technology of geological disaster control
(1) geological disaster prevention theory
Attach importance to the theoretical study of geological disaster prevention and control based on the formation mechanism of geological disasters. For example, in view of the characteristics of landslides in hot spring areas, Japan has studied the comprehensive protection of landslides by using exhaust engineering and groundwater interception engineering; France adopts siphon drainage technology for clay landslide induced by rainfall; The United States and Japan are studying shallow landslides in areas with good vegetation coverage and taking biological measures to adjust plant types. In the prevention and control of geological disasters, biological protection systems are widely used, and attention is paid to the protection of ecological environment. In landslide control in Japan, anti-slide piles are combined with building foundations to realize the combination of prevention and control projects and land development and utilization.
(2) Technical methods of geological disaster prevention and control engineering design.
Foreign countries have done a lot of research in the design technology of complex envelope, groundwater drainage technology, technical regulations based on environment and landscape design and practical computer software development, and formed a relatively complete theoretical method of design and calculation and industrial software. For example, the United States has developed a fast Lagrangian analysis software for three-dimensional continuum-FLAC3D, and a three-dimensional simulation discrete element program-3DEC; Geo-Slope Office (GEO-Slope Office 5.0 for Windows), a software package developed by Canada, has been widely used in the design of landslide prevention projects in many countries in the world, forming modular design software and methods.
(3) Geological disaster control engineering technology
Geotextile, prestressed composite supporting structure and groundwater drainage technology are widely used treatment technologies. In particular, the United States, Western Europe, Japan and China SAR have invested heavily in geological disaster management and made remarkable achievements. For example, the landslide control project in Japan costs 654.38+0.5 billion yen (about 654.38+0.5 billion RMB), which can be regarded as a museum of geological disaster prevention and control projects.
The common technical projects to control the collapse and landslide disasters abroad are as follows: ① scour protection projects: scour dam, sediment dam, bank protection, wave dam and groin; ② Weight reduction and back pressure engineering; ③ Surface drainage works: surface drainage ditch and seepage control works; ④ Underground drainage works: underground drainage ditch, drainage tunnel, horizontal drilling, water collecting well and siphon drainage works; ⑤ Underground water interception project: water interception by impervious core, water interception by grouting and water interception by chemical solidification; ⑥ enclosure engineering: retaining wall, grating wall, anti-slide pile and anchor rod; ⑦ Exhaust project: used to control landslides in hot spring areas; ⑧ Using biological slope protection technology and light net protection system to control collapse and small-scale landslide disasters.
Because water is an important inducing factor of landslide, surface drainage engineering and underground drainage engineering are always the first treatment technologies to be considered, and they are also the first treatment technologies used in large-scale landslide prevention and control. The United States, Japan, New Zealand and other countries widely use underground drainage engineering technology in landslide control, and use horizontal drilling drainage and drainage well and drainage tunnel combined drainage technology to control landslides. Siphon drainage technology was adopted in France to control clay landslides induced by more than 100 rainfall. This is a sealed PVC piping system. The biggest advantage of this technology is that it can drain water by itself and reduce the groundwater level of landslide.
In the application of supporting engineering technology, the research and application of various supporting structures such as large-section anti-slide pile, anchor cable anti-slide pile, anchor cable, small steel frame pile with anchor cable, micro-pile group and so on. Remarkable progress has been made in anchor cable anticorrosion technology, general calculation method, design software and technical standards. Weight reduction and back pressure engineering are economic and effective engineering measures to prevent landslides. The "neutral line" method proposed by Huchinson in Britain provides a theoretical basis for the calculation of weightlessness and back pressure.
In recent years, in the practice of geological disaster prevention and control engineering in developed countries, the supporting technologies of lightweight net protection system and biological slope protection system have been applied in the treatment of collapse and small landslide disasters, which has further developed the prevention and control engineering in a light and beautiful direction. Such as SNS flexible support system and biological slope protection system, have been widely used in many European countries.
10.10.4 analysis of the development trend of international scientific and technological research on geological disaster prevention and control
The overall development trend of geological disaster prevention and control technology in the future is to attach importance to the early prediction and early warning capacity building of geological disasters, improve the scientific and technological level and disaster prevention and mitigation capacity in the field of geological disasters, establish 3S (RS- remote sensing, GPS- global positioning system and GIS- geographic information system) technical platforms, develop and establish regional real-time monitoring websites and prediction and early warning information systems, establish geological disaster information system platforms and * * * shared channels, and improve the disaster mitigation capacity of geological disasters.
In-depth study on the formation mechanism of geological disasters has always been a difficult point in international geological disaster research, and the study of rainfall landslide is one of the hot spots in landslide research. The key point is to study the change of critical rainfall induced by debris flow and shallow landslide with regional and climate changes, reveal the relationship between rainfall and landslide, and predict the possible landslide state.
Using GIS technology to analyze the regional characteristics of geological disasters and draw the spatial map of disasters is becoming a research hotspot. It will become an important trend of disaster geology research to organically combine disaster analysis with risk evaluation and risk prediction by using computer high-tech means (GIS, GPS, RS, etc.). ) form a real-time early warning decision-making system.
In terms of various monitoring technologies, developed countries attach great importance to the application of high technology while strengthening the construction of real-time monitoring stations for various geological disasters. The application research of high-tech space earth observation technology in geological disasters is also an important research direction in developed countries. The application of various more advanced remote sensing detection systems is gradually deepening. The United States, France, Italy and Japan all focus on the application of GPS and interferometric radar remote sensing in the dynamic investigation and monitoring of landslides and land subsidence.
In recent years, developed countries have the following characteristics and development trends in geological disaster control engineering technology.
In the theory of prevention and control: pay attention to the theoretical research of prevention and control of geological disasters based on the formation mechanism of geological disasters; Pay attention to the combination of prevention and control projects with ecological environment protection and land use; Form modular design software and methods, and develop new governance technologies and methods.
In the aspect of disaster information processing: various high-speed numerical forecasts have been gradually realized; With the development of high-speed, intelligent and comprehensive communication network technology, distributed database technology and massive data operation technology, disaster communication, computer network and information development and processing are integrated to form a comprehensive disaster information network system, so that all kinds of scattered disaster information can really enjoy resources; The development of artificial intelligence, multimedia and 3D simulation technology promotes the application and reprocessing of disaster information products.