The basic method of landslide monitoring

6.2.1 Stage characteristics and observation of landslide development

The occurrence of landslides has to go through the three stages of creeping slide, sliding and violent slide, and the deformation characteristics of the three stages are different, which show different surface displacements, rates, crack distributions, and a variety of concomitant phenomena of landslides. Therefore, according to the characteristics of different stages of landslide development, the use of targeted observation methods is the key to realize landslide observation, but also the key to effective observation of landslides. Therefore, accurate understanding of the stage of landslide development, and according to the characteristics of different stages of landslide development targeted observation is very important.

6.2.1.1 Stage characteristics of landslide development

The process of landslide development has obvious stage characteristics, which is completely determined by the mechanical properties of the rock (soil) body, reflecting the deformation of different properties of the rock (soil) body under the action of gravity. Therefore, to determine the stage of landslide development can be realized by analyzing the deformation and destruction process of rock (soil) body under stress conditions. Generally speaking landslide deformation can be divided into three stages: creeping slip, sliding and violent slip.

6.2.1.2 Stages of deformation of rock (soil) body

Generally speaking, in natural conditions of rock (soil) in the long-term load, rock (soil) stress, strain will change over time, when the deformation has developed to a certain stage, the rock (soil) damage. This rheological process of rock (soil) can be proved experimentally (Figure 6-6). When a constant load is applied to a geotechnical specimen, the rock (soil) immediately produces an instantaneous elastic strain εe (OA) section. This deformation is so short that it can be considered to be completed at t = 0. The strain is εe = 6/E. εe If the load is kept constant, the deformation of the rock (soil) at this time increases slowly with time. According to the characteristics of creep, the process can be divided into three stages.

The first creep stage (AB section): also known as creep-slip stage. Within this stage, the creep curve has a downward bending shape, showing a rapid decrease in strain rate ε with time. When point B is reached, the strain rate is at the minimum value of this stage. If unloading is carried out in the middle of this stage (a point E on the curve), the strain ε decreases along the curve EFG, showing elastic deformation, and finally the strain is zero.

The second creep stage (BC section): also known as the steady slip stage. The creep curve approximates an inclined straight line, i.e., the creep strain rate ε remains constant and continues until point C. The creep curve is also called the steady slip phase. If unloading is carried out in this stage, the strain is gradually recovered along the curve HIJ, and finally a certain permanent deformation εp is retained.

Third creep stage (CD section): also called accelerated sliding stage. Strain rate increases rapidly from point C, reaching point D, the rock is damaged. This deformation stage is shorter.

6.2.1.3 Influence of the structure and environment of the rock (soil) body on the rheology of the rock (soil)

Since there are a large number of joints, fissures, and other discontinuous structural surfaces in the rock (soil) body, which greatly reduce the integrity of the rock body and the strength of the rock. Therefore, the influence of the structural surface on the strength of the rock body and its damage is very obvious. Tests show that when the direction of force (shear) is perpendicular to the potential structural surfaces in the rock body, the shear strength is close to the experimental strength of the rock. When the direction of force (shear) and the structural surface in the rock body diagonally, the shear strength depends on the degree of cementation of the structural surface of the discontinuous surface and the strength of the cement, because the strength of the cement is generally lower than the strength of the intact rock (soil), so the strength of the rock (soil) body containing the structural surface is lower than that of the intact rock (soil) body, and deformation also has the characteristics of the phased deformation.

Figure 6-6 Creep curve of landslide rock (soil)

The morphology of the structural surface also determines the mechanical properties of the rock body. When the structural surface is in a flat and straight shape. Displacement along the structural surface after the force is applied, the stress-strain curve is a continuous smooth curve. When the structural surface is rough and uneven jagged, the occlusion of the structural surface is larger, the frictional resistance is also larger, and the degree of shear resistance is larger. In the stressing process, the shear stress is first concentrated in the end point of the structural surface or the stressing side of the convex point of the structural surface. When the shear stress reaches the maximum strength value (Cmax) of the structural face endpoint or convex point, rupture occurs first, displacement increases, shear stress decreases gradually, and after reaching a certain level, the C value maintains a certain value, i.e. residual strength. With the development of displacement, stress and in the next structural surface endpoints or bumps to concentrate, and gradually develop, stress and strain curves show jagged oscillations, and finally make the rock (soil) body damage, deformation trend line is still characterized by rock (soil) deformation in stages.

Rock (soil) body in the environmental conditions (such as water content of the rock) is another factor affecting the rock (soil) body mechanics. Studies have shown that the structural surface of the rock (soil) body in the water-rich, its strength is greatly reduced than when dry. If the structural surface is sandwiched with clay, the shear strength of the structural surface can be reduced by 3 to 5 times after water enrichment, and its deformation trend line still has the characteristics of the rock (soil) phased deformation.

6.2.2 Stages of landslide deformation process

Slope constituted by different rock (soil) deformation damage under gravity, the deformation process is bound to have the deformation characteristics of rock (soil). In general, the deformation process of landslides can also be divided into three stages (Figure 6-7).

6.2.2.1 Creep-slip stage

The first stage of landslide development, that is, the slope of the rock (soil) body under the action of gravity, the stress is first in the slope body of the structural surface (level, joints, cracks, etc.) at the two ends of the bumps and concentration, and creep-slip deformation occurs. As the shear stress on the structural surface increases to reach Cmax, micro-rupture occurs at the end of the structural surface, and gradually develops to the next structural surface end or bump. The slope exhibits slow creep-slip deformation. The deformation characteristics of creep-slip deformation stage are:

(1) surface cracks: transverse tensile cracks appear in the back of the slope body, and the cracks at the back edge of some giant landslides can be pulled apart by tens of meters due to the huge strain accumulation capacity of the landslide body.

(2) sliding zone (surface): in the vertical solid section creep to form shear deformation zone (surface), and cut off the vertical solid section where the stress is first concentrated. The shear strength in the shear band is gradually reduced from the peak strength. Can be seen after the shear activity left behind the scratches, broken cleavage phenomenon.

(3) Deformation of the slide: discontinuous distribution of cracks can be seen in the back of the landslide body and the slide belt (surface), the deformation is almost entirely concentrated in the shear zone, the surface macroscopic phenomenon is not obvious.

Figure 6-7 Stage curve of landslide deformation

6.2.2.2 Sliding stage

The second stage of landslide development. As the shear stress shears each locking segment (point) on the sliding surface one by one, the deformation of the slope body is getting bigger and bigger, showing a slow increase in deformation, at this time, the strength of the potential sliding surface is the residual strength of the sliding surface, and the time-strain curve is a smooth curve or jumping displacement. The deformation characteristics of the sliding stage are:

(1) Macroscopic geomorphological form: revealing the overall contour of the landslide, the disintegration phenomenon is visible in the longitudinal direction. At the same time, the perimeter of the landslide cracks have been basically connected, the trailing edge of the visible tension cracks, part of the visible leading edge bulging cracks.

(2) sliding surface: shear slip zone has been gradually formed, the slip zone can be seen scratches, mirrors and other sliding phenomena.

(3) Developmental time process: this stage of development is longer, and the trigger factor plays a dominant role in accelerating the sliding development process.

(4) Accompanying phenomena: in the process of landslide occurrence, there are often groundwater anomalies, animal anomalies, acoustic emission, geophysical and geomorphological changes, and small avalanches appear on the back wall or the front edge of the landslide.

(5) the movement state of the landslide body: the landslide is a uniform displacement or slow increase, and there is a trend of gradual increase.

6.2.2.3 Accelerated stage

The accelerated stage is the most obvious characteristics of landslide development, the fastest deformation rate, the most likely to occur in the destruction of the stage. When the sliding surface has been basically through, the residual strength of the sliding surface close to the landslide body of the downward force, the rock body is in a rapid displacement state, displacement of the ephemeral curve rapidly upward. This trend continues to develop, will eventually lead to landslides. Landslide accelerated deformation stage is characterized by:

(1) surface cracks: all types of cracks on the body of the landslide may appear, but the change is rapid. Slip cans appear on both sides of the trailing edge and side edge cracks, and small avalanches often occur on the back wall. Many tension cracks in the middle section. Fan cracks occur in the front section.

(2) Sliding surface: the sliding surface has been completely penetrated, forming a complete sliding surface.

(3) Motion state of the landslide: the landslide body slides under the action of gravity, which is manifested as a one-time or intermittent multiple completion of the sliding process.

(4) the role of the trigger factor: the trigger factor continues to play a role, especially intermittent occurrence of sliding landslides, the role of the trigger factor is very obvious.

(5) Accompanying phenomena: groundwater anomalies, animal anomalies, acoustic emission and other phenomena continue to occur, and there is a marked increase in small avalanches on the back wall or leading edge.

(6) Developmental duration: short or very short.

6.2.3 Main contents of deformation observation of landslide

6.2.3.1 Surface displacement monitoring

The deformation observation on the surface of the landslide body is to find out the rate and direction of the horizontal and vertical displacement of the landslide and the rate and direction of the slope inclination by setting up observation points on the surface of the landslide body. At the same time the deformation of the slope surface crack observation, crack observation is to identify the development of the landslide state, displacement rate, nature (tension, shear, pressure, production), the length of the crack, width, extension direction, with or without filler, filler water content and the relative displacement of the two walls of the crack (Figure 6-8).

Feature deformation observation is mainly to identify the deformation of the building on the landslide body, the shape and characteristics of the cracks, horizontal displacement, vertical displacement rate and direction, and the time of deformation.

Figure 6-8 Landslide observation content

6.2.3.2 Underground deformation observation

Underground deformation monitoring is also known as deep deformation observation, the monitoring content includes: monitoring the location, depth and number of groups of weak structural surfaces or sliding surfaces within the slope, and determining the rate and direction of the relative displacement of the upper and lower sliding surfaces (zones). Through the analysis of the monitoring data, you can determine the connectivity between the sliding surface (belt), the physical properties of the slip belt soil, to provide a basis for the stability analysis of the landslide.

6.2.3.3 Observation of influencing factors

Monitoring of influencing factors is mainly to observe the factors inducing landslides, and the common observations of landslide-inducing factors include: observation of precipitation, observation of groundwater dynamics, observation of surface water, observation of geoacoustics, geothermal temperature, geo-stress, earthquakes, etc., and observation of human engineering activities. Among these triggering factors, precipitation observation, groundwater observation, and observation of human activities are the most common.

Precipitation observation: It is through the establishment of a rain gauge in the landslide area, to observe the size of the rainfall in the area, paying special attention to the observation process rainfall, 24-hour rainfall, the maximum hourly rainfall.

Groundwater observation: observation of the number and location of groundwater outflow points, the source of groundwater (recharge), the state of groundwater seepage in the body of the landslide, to identify the type of outflow points, flow. Where possible, the physical and chemical properties of groundwater can be observed, such as water temperature, turbidity, hardness, and p H value.

Human activity observation: mainly including blasting and seismic operation observation; engineering excavation and engineering landfill observation.

6.2.3.4 Macro-geological monitoring

Macro-geological monitoring is to observe the landslide by using methods such as geological patrols and simple measurements, and the observations include:

(1) Distribution of surface cracks.

(2) Expansion of cracks, nature of cracks.

(3) Slope falling blocks, rolling stones.

(4) Surface animal anomalies.

(5) Changes in the number of groundwater outflow points, changes in groundwater level, groundwater flow.

6.2.4 Landslide Monitoring Technology Methods and Means

Landslide monitoring technology and so on in the land and resources, railroads and water conservancy and hydropower and other departments have more in-depth application, according to the monitoring object and content is different, the monitoring methods, methods and means are also different (Table 6-11). Common monitoring methods are:

Table 6-11 Conventional methods of landslide monitoring

6.2.4.1 Surface displacement monitoring

1) Geodetic method

The advantages of geodetic method are mature technology, high precision, reliable data and large amount of information; the disadvantage is that it is affected by the topography of visual access conditions and climate. Geodetic method using instruments are:

(1) latitude and longitude, level, rangefinder, which is characterized by fast input, high precision, wide monitoring surface, intuitive, safe, easy to determine the direction of displacement and deformation rate of landslides, applicable to different stages of deformation of the horizontal and vertical displacement, subject to the limitations of the terrain and the climate of the conditions, can not be observed continuously;

(2) full-station type electronic distance meter, electronic latitude and longitude meter: it is characterized by high precision, high speed, high automation, easy operation, labor saving, tracking automatic continuous observation, large amount of monitoring information, and is applicable to the monitoring of horizontal and vertical displacements in the stage of accelerated deformation to drastic change and destruction. This method has been commonly used on more than 10 monitoring bodies in the Three Gorges reservoir area of the Yangtze River, and the monitoring results are directly used to guide the construction of prevention and control works.

2) Global Positioning System (GPS) Observation

The Global Positioning System (GPS) method is highly accurate, quick to put in, easy to operate, and can be observed in all-weather, and simultaneously measure three-dimensional displacements X, Y, and Z, and can accurately measure the rate of points in motion, and is not subject to the conditions, and can be continuously monitored. The disadvantage is the high cost. It is suitable for monitoring horizontal and vertical displacements in different deformation stages. China has already established GPS observation network in Beijing-Tianjin-Tangshan crustal activity area and the dam area of the Three Gorges Project of the Yangtze River, and applied GPS technology in the monitoring of landslides in the Three Gorges Reservoir area, the deformation monitoring of the dangerous rock body of the Chained Cliffs, as well as the monitoring of the effect of the management of the Chuankou landslides in Tongchuan City.

3) Remote sensing RS method and close-up photography method

Remote sensing RS method and close-up photography method are suitable for large-scale and regional landslide monitoring. According to the remote sensing pictures, landslide judgment, according to different periods of image changes to understand the changes of landslides; the use of high-resolution remote sensing images on the dynamic monitoring of geologic hazards: with the continuous development of remote sensing sensor technology, the remote sensing image of the ground is increasingly high resolution. For example, the resolution of the TM remote sensing image of the American LANDSAT satellite is 29m, the resolution of the full-band image of the French SPOT satellite is 10m, and the resolution of the American IKNOS satellite image is as high as 1m. By utilizing the satellite remote sensing image to reflect the richness of the ground information, and by obtaining the same image periodically, the remote sensing image of the same geologic disaster point can be compared with the remote sensing image of the same geologic disaster point in different periods. Utilizing the feature that satellite remote sensing images reflect rich ground information and can periodically obtain images of the same place, it is possible to compare remote sensing images of the same geologic disaster point in different periods, thus achieving the purpose of dynamic monitoring of geologic disasters. The near-view photography method uses land photography and latitude and longitude instruments to carry out monitoring, which is characterized by a large amount of monitoring information, manpower saving, fast input and safety; however, the accuracy is relatively low, mainly applicable to the monitoring of horizontal displacement of landslides with large deformation rate and changes in cracks of dangerous rock steep walls, and is greatly affected by climatic conditions. Such as for the Three Gorges reservoir area of large landslide prone to the division and prediction of the section as well as Tibet Bomi Yigong high-speed giant landslide analysis and prediction.

4) landslide deformation (displacement) observer

Landslide deformation (displacement) observer (also known as landslide crack meter, landslide deformation observer): this type of observation instrument is a lot of structural types of mechanical, electronic or mechanical-electronic type of instrumentation, is mainly used for landslides, surface cracks, building cracks deformation displacement observation, can be obtained directly from the continuous changes in displacement -Time curve, can meet the long-term, stability, reliability, robustness requirements of the work under field conditions. The landslide deformation (displacement) observer is suitable for long-term work in the field, and the recorded data curve is intuitive, less interference, and high credibility, so it is very widely used. As there are more landslide cracks and they are widely distributed on the landslide, therefore, the number of instruments required is larger and the arrangement is scattered, and each observation instrument reflects the displacement and deformation of only one observation crack, which also causes some difficulties in the integration and transmission of observation information, and generally requires a person to operate the instrument directly. In the sliding of the emergence of dangerous situations, there are personnel should not be close to the shortcomings.

5) row pile observation

Row pile observation is a simple observation method. The method is to start from the stable rock at the back edge of the landslide, and set a series of rows of piles at equal distances along the axial direction of the landslide (Figure 6-4). Row piles are generally laid in the axis of the most obvious landslide deformation. If the width of the landslide is large, multiple rows of observation piles can be laid side by side. The starting point of the row of piles (point 0) is buried in the stable rock outside the back edge of the landslide, and they will be the starting point of the measurement, and then the No. 1 and No. 2 piles are buried along the axial direction in turn. The spacing of each pile is about 10m. The number of piles depends on the width of the distribution of the pull seam at the back edge of the landslide.

Measurement, respectively, the length of N0 → N1, N1 → N2-→Ni-1, Ni and the ground inclination angle αi between the corresponding piles, the change in the length between the piles that reflects the change in the control crack between the two piles.

6.2.4.2 Underground deformation monitoring

1) Borehole inclinometer

Monitoring using borehole inclinometer and multi-point inverted whacking instrument is mainly applicable to the monitoring of the initial stage of slide deformation, i.e., determining the deformation characteristics of the different depths within the slide body and location of the slide zone within the boreholes and shafts. Borehole tilt method is one of the best ways to monitor the deep displacement. It is highly accurate, effective, easy to protect, less disturbed by external factors, and reliable information, but the range is limited and the relative cost is high. The borehole inclinometer can be divided into two categories, mobile and fixed, according to the installation and use of the probe, and is widely used in landslide monitoring.

In recent years, with the development of fiber optic sensing technology, geotechnical deformation observation also appeared on the fiber optic, grating sensors, and made some application tests on landslide observation. Fiber optic, grating sensor observation of high precision, reliable operation, maintenance, in landslide observation is a very promising means of observation. At the same time, due to the small size of fiber optic and grating sensors, the sensor needs an intermediate medium to realize the observation of landslides. Therefore, the selection of intermediate media, the structure of the design of the style is very important, there is no such specialized molding design, are based on the requirements of landslide observation and implementation of the conditions for the design, the use of the effect of the existence of a greater degree of randomness, the installation is also very complex. Currently in the landslide observation on the use of less.

2) Measuring seam method (shaft method)

Using multi-point statistics, well wall displacement meter, dislocation meter, convergence meter, TDR and so on for observation. The observation method is generally carried out through boreholes, flat caves, and vertical shafts to observe the relative displacement of deep cracks, slip zones, or weak zones of landslides. It is characterized by higher accuracy, small range and easy protection, but the investment is large and costly, and the instruments and sensors are easily affected by groundwater, climate and other environments. At present, limited by the performance of the instrument, range, mainly applicable to the initial deformation stage of the landslide, that is, the measurement of small deformation, low rate, observation time is relatively not very long monitoring.

6.2.4.3 Landslide inducing factor monitoring

1) Groundwater dynamic monitoring

Groundwater dynamic monitoring includes groundwater level and interstitial water pressure monitoring. The water level is measured by automatic water level recorder, and this method is effective for long-distance telemetry, multi-point measurement and small-diameter drilling (only 30mm). Automatic water level recorders are being commonly used in China. Gap water manometer: in foreign countries, the application of gap water manometer for landslide monitoring has been more common, but not yet popularized the use of domestic. The key to the technology is how to measure the real pore water pressure value in the sliding zone, which involves a lot of installation and burial of technical problems. Over the past few decades, countries have developed various forms of interstitial water pressure measurement instruments, such as open-ended riser type, Caron Grande type, pneumatic type, hydraulic type and electric type of probe, etc..

2) Meteorological observation

The technical aspect of meteorological observation is to observe the meteorological factors through rain gauge, evaporation meter, etc., and analyze the relationship between rainfall and landslide sliding. Most of the landslides in China are related to rainfall, so the study of the relationship between the critical value of rainfall and landslides has a very important significance to the landslide problem.

3) Geoacoustic monitoring

Geoacoustic monitoring technology method is to use the determination of the intensity and signal characteristics of the stress wave released during the stress damage process of the landslide rock body to discern the stability of the rock body. The earliest application in mine stress measurement, the last decade or so gradually been applied to the monitoring of landslides. Instruments are geophones, geophones detectors. The use of instruments to collect the deformation of the rock body rupture or destruction of the release of stress wave intensity and frequency and other signal information, analyze and judge the deformation of the landslide. The instrument should be set in the stress concentration part of the avalanche and slide body, with high sensitivity and continuous monitoring, which is only suitable for the development of the deformation monitoring of the avalanche and slide body or slope, and is not suitable for the stage of uniform deformation of the avalanche and slide body. The probe is placed at different depths of the borehole or crack to monitor the damage of the rock body (especially the sliding surface). Acoustic emission technology can be used as a means of early monitoring and forecasting in the extrusion stage of landslides, where ground cracks are not obvious and ground displacements are difficult to measure, and it has a high prospect of application to avalanche landslides, but the possibility of applying it to other types of landslides has yet to be studied in depth.

4) Ground temperature observation

The technical method of ground temperature monitoring is to measure the ground temperature by using thermometers, to analyze the relationship between the temperature change and the rock deformation, and to indirectly understand the deformation characteristics of the hazardous rock body.

5)Seismic monitoring

Since seismic force is one of the special loads acting on the body, it plays an important role in the stability of the body. Seismographs should be used to monitor the intensity, time of occurrence, location of epicenter, and depth of epicenters of earthquakes occurring in the area and periphery of the area to analyze the seismic intensity in the area and to evaluate the effect of seismic action on the stability of the body.

6) Observation of human-related activities

Since human activities such as cave digging, slope cutting, blasting, loading and operation of water conservancy facilities often result in artificial-type geologic hazards or induce the generation of geologic hazards, when the above situation occurs, it should be monitored and an activity should be stopped. Human activities monitoring, should monitor the project that has an impact on the avalanche and slide, monitor its scope, intensity, speed, etc..

6.2.4.4 Macro-geological survey observation

Using the conventional geological survey method, the macroscopic deformation signs appearing in the avalanche-slip body (such as the occurrence and development of cracks, ground subsidence, subsidence, slumping, swelling, upliftment, and deformation of the building, etc.) and the anomalies related to deformation (such as the ground sound, groundwater anomalies, and faunal anomalies) shall be investigated and recorded periodically.

In summary, at present, domestic and foreign landslide observation technology and method has been developed to a higher level. Mainly manifested in:

(1) from the past artificial surface measurement with a ruler and other simple monitoring, the development of instrumentation can be used to observe the disaster, and is gradually realizing the automation, high-precision telemetry system.

(2) the development of monitoring technology and methods, broaden the monitoring content, from the surface monitoring to underground monitoring, underwater monitoring, etc., from the displacement monitoring to strain monitoring, related dynamic factors and environmental factors monitoring.

(3) The development of monitoring technology and methods depends largely on the development of monitoring instruments. With the development of electronic camera laser technology, GPS technology, remote sensing telemetry technology, automation technology and computer technology, monitoring instruments are to high precision, good performance, wide range of adaptability, high degree of automation in the direction of development.