Geological disaster stability and hazard

I. Stability Analysis of Geological Hazards

(I) Numerical Method

Numerical method of engineering geology is to use elastic-plastic mechanics theory and numerical computation method to analyze and evaluate the stability state of the geotechnical body under certain environmental conditions from the perspective of studying the stress and displacement field of the geotechnical body. Over the past 30 years, the numerical method has been rapidly developed and widely used in engineering practice. In this paper, FLAC3D (Fast Lagrangian Analysis of Continua in 3 Dimensions) software is used to perform the numerical analysis of slope stability. FLAC3D software is developed by ITASCA Consulting Group of the U.S.A. It is mainly used to simulate geotechnical bodies and other materials. It is used to simulate the deformation and damage behavior of structural bodies composed of geotechnical bodies and other materials after reaching the yield limit. The software successfully uses the Lagrangian method of tracking fluid motion in fluid dynamics to solve rock mechanics problems, and it can not only solve general geotechnical problems, but also carry out complex problems such as high-temperature strain, rheology, or dynamic loading, and water-rock coupling analysis.

1. Model Calculation Methods

FLAC3D software uses the finite difference method to simulate and calculate the deformation and damage behavior of structural bodies composed of geotechnical and other materials after reaching the yield limit, including static calculation and finite difference strength reduction calculation. The results obtained by these two methods are not exactly the same, and these two methods are chosen simultaneously for the calculation and analysis of loess landslides and unstable slopes in this area.

The method of static calculation requires that the model and the selected parameters must make the model converge completely when calculating, and if the calculation process converges quickly, the model is considered to be basically stable. However, when doing landslide stability analysis, due to more factors affecting the stability of landslides, such as slope height, slope, and the different loess body mechanics parameters of different slopes, it is often not possible to get a fast convergence of the computational model, and therefore the safety of the slope can not be fully judged by the way of static calculation. The strength discount method is the only method in FLAC3D that can calculate the safety factor of the slope body. Therefore, this method can be used to find out the safety coefficient of the slope body, and then combined with the results of static calculation to judge the stability of the slope body. According to the Specification for Landslide Prevention and Control Engineering Investigation (DZ/T 0218-2006), the selection of safety coefficient <1.05 is judged to be unstable, safety coefficient 1.05 to 1.15 is more stable, and safety coefficient ≥1.15 is stable, which is used as a criterion for the stability of the main disaster point.

The basic principle of finite-difference strength reduction coefficient method is to divide the cohesion (C) and the angle of internal friction (?) of the strength parameter of the soil body by the value of a reduction factor at the same time. values are simultaneously divided by a reduction factor Ftrial to obtain a new set of Ctrial and ?trial values. The trial calculations are then carried out as new material parameters brought to finite differences. When the calculations converge exactly, i.e., when Ftrial is a little bit larger (the order of magnitude is generally 10 to 3), the calculations do not converge, and the corresponding Ftrial is referred to as the minimum factor of safety of the slope, at which point the soil reaches a critical state and shear damage occurs. The calculation results all refer to the discount factor when the critical state is reached:

Ctrial=C/Ftrial

tan?trial=tan?/Ftrial

2. Model Type and Parameter Selection

Selecting the Moore Coulomb model as the material model, based on the results of the surveys and the tests of the mechanical properties, and taking into account the investigation area The occurrence of the disaster is closely related to rainfall, so the physical and mechanical parameters in the saturated state are selected as the calculation parameters:

Volumetric modulus:

K=4.5MPa

Shear modulus:

G=2.1MPa

Cohesion:

C=3.4×104Pa

Angle of internal friction:

3. Loess slope analysis

(1) Model establishment and grid section

Survey data show that the possibility of deformation and damage is higher for loess linear slopes between 30° and 60°. Considering the convenience of establishing the model, the linear slopes between 30° and 70° are chosen to be analyzed, and at the same time, some stepped slopes for comparative analysis.

According to Prof. Zheng Yingren's point of view, while doing the strength reduction method of slope modeling to find the slope safety coefficient, it is required that the distance from the angle of the model to the leftmost side of the slope is 1.5 times the height of the slope, while the distance from the top of the slope to the rightmost side of the slope is 2 times the height of the slope, which will give the most accurate result of the safety coefficient calculated.

Taking the linear slope with a slope height of 40m and a slope degree of 45° as an example, the model was established and the grid dissection was carried out. Although the loess in the investigation area is a laminated structure, and the thickness and geotechnical properties of loess vary in different periods, the data from the investigation tests show that the saturated shear strength does not vary much. Therefore, it is assumed that the loess is homogeneous and the strength parameters are homogeneous throughout the model. The right side and bottom of the model are defined as constrained boundary conditions, and the slope face and top of the slope are defined as automatic boundaries.

(2) Comparative analysis of conventional and simplified models

In the loess slopes in the investigation area, the distribution of slope heights is very uneven, ranging from tens of meters, tens of meters to hundreds of meters, and each slope height corresponds to a different gradient. Therefore, when analyzing the stability of loess slopes, it is necessary to analyze comprehensively and study the safety stability of various slopes with different slope heights and different gradients. In this study, the stability of all slopes from 30° to 70° (every 5°) under 20-50m (every 5m) slope heights was simulated using FLAC3D software. Due to the different number of meshes and nodes in the model, the computation time of the software consists of huge differences. The conventional model proposed by Prof. Zheng Yingren has some validity in the calculation, but it also greatly increases the number of model grids and nodes, so the calculation time for strength reduction is very long. Therefore, it is necessary to compare the calculation results of the conventional model and the simplified model first.

First, the conventional model was used to analyze the stability of all slopes between 30° and 70° of 40m slope height. Using the strength discount factor method to calculate the factor of safety in various slope conditions, the static equilibrium calculation and strength discount calculation can be used to obtain the stability analysis of a certain slope height of a variety of different slopes (Table 3-16). The relationship between the slope and the factor of safety calculated by the conventional model is fitted, and the curve of the relationship between the slope and the factor of safety can be obtained (Figure 3-10).

Figure 3-10 Relationship curve between different slopes of 40m slope height and safety factor of conventional model

Table 3-16 Summary table of slope stability calculation of different slopes of 40m slope height of conventional model

Because of the number of knots and points of the grid of the conventional model is more, the amount of data is excessively complicated in the process of computer processing, and the speed of computation is slow, and the length, width and height of loess slopes tend to be relatively large. is relatively large. In this way, if we use Prof. Zheng Yingren's conventional model to analyze, the efficiency is not very satisfactory. Therefore, the model grid of the slope was simplified and the results were compared with those of the conventional model. When comparing the results, we still take the 40m slope height of 35°~70° as an example, the calculation results are shown in Table 3-17, and the fitting curve of the simplified model is shown in Fig. 3-11.

Fig. 3-11 Simplified model of the relationship between the different slopes of the 40m slope height and the coefficient of safety

Observations of the coefficients of safety obtained by the strength reduction method of the conventional model show that the coefficients of safety calculated by the two models are the same when the slope is unstable. When the slope is unstable, both models calculate the same safety factor; and when the slope is stable, the calculation of the safety factor of the simplified model is a little smaller than the simplified model, but the overall slope stability of the results is not a great influence. In practical engineering applications, we can consider using the simplified model with smaller calculation results for analysis and calculation for safety reasons.

Table 3-17 Simplified model 40m slope height of different slope stability calculation summary

(3) Slope and safety factor of the relationship

Using the simplified model, respectively, combined with static calculation method and strength discount factor method, analyzed and calculated the stability of the slope of various slopes under the case of 20 ~ 50m slope height; and at the same time to get a fixed slope height of the case, the At the same time, the fitted relationship curves of slope and safety factor were obtained for the case of fixed slope height. The fitting curves of slope and safety coefficient show that when the slope is changed at fixed slope height, the safety coefficient decreases with the increase of slope, and the slope is gradually destabilized. And the coefficient of safety shows a logarithmic relationship with the change of slope, and the fitting degree is high.

(4) Analysis of the change of soil strength parameters

Based on the survey and experimental test data, the cohesion C value of loess in the area as well as the angle of internal friction ? values change a lot (such as Table 3-18), so it is necessary to study the influence of the trend of the strength parameters on the slope safety coefficient.

Table 3-18 Statistical table of physico-mechanical indexes of loess

Taking 20m slope height 60°slope as an example, the cohesion of fixed model:

C=34kPa

Then change the angle of internal friction of the soil body, and use the method of strength discount factor to calculate the safety coefficient in different internal friction angle cases, and get the results as shown in Table 3-19. From the calculation results, it can be seen that with the increase of internal friction angle, the coefficient of safety gradually increases. The smaller the angle of internal friction, the more the potential sliding zone extends outward, the more the dangerous slip arc opens up, and the worse the stability of the slope (Figure 3-12).

Table 3-19 Statistical table of the effect of different internal friction angles on the coefficient of safety

Still take the 20m slope height 60°slope as an example, fixing the internal friction angle of the model:

Then change the cohesion of the soil body, and utilize the method of the strength reduction coefficient to calculate the coefficient of safety of the different cohesion cases, and get the results as Table 3-20. The results are shown in Table 3-20. The calculation results show that the larger the cohesion, the higher the safety coefficient. However, the more the potential sliding surface extends outward, the more open the slip arc is, but the higher the stability is, which is opposite to the effect of internal friction angle (Figure 3-13).

Table 3-20 Statistical table of the effect of different cohesion on the safety factor

Figure 3-12 Trend graph of the change of the slip arc with the angle of internal friction

Figure 3-13 Trend graph of the change of the slip arc with the cohesion

(5) Influence of slope profile morphology

The profile morphology of the loess slopes in the study area is roughly classified into four categories: linear, step type, convex and concave types. The results of the survey found that the number of destabilizing changes in convex and linear slopes was the highest and the possibility was the greatest. Therefore, it is necessary to analyze the effect of slope type changes on slope stability. Here we only make a comparative analysis of linear and stepped slopes.

Taking the slope height of 40m 45° as an example, we establish the linear and stepped slopes respectively, calculate their respective safety coefficients by using the static equilibrium and strength reduction methods, and analyze the stability of these two types of slopes in comparison with the maximum imbalance force curve and the internal shear strain map of the slope body. The results of the calculation found that the linear slope was obviously damaged, and the shear strain inside the slope body was distributed in a band, while the safety factor of the stepped slope increased, and the static calculation converged at 4460 time step, and the slope body was stable (Figure 3-14, Figure 3-15; Table 3-21).

Figure 3-14 Maximum unbalanced force curve under static calculation of linear slope

Figure 3-15 Maximum unbalanced force curve under static calculation of stepped slope

Table 3-21 Comparative analysis table of 40m, 45° linear and stepped slopes

4. Stability analysis of the main disaster sites

According to the above According to the above analysis method, 30 major landslide and unstable slope points in the investigation area are analyzed numerically to find out the safety coefficient of the slope body and judge the stability of the slope body, and the results of the analysis are listed in Table 3-22.

Table 3-22 Table of results of numerical analysis of stability of the major disaster points

(II) Limit equilibrium method

1.Calculation method and software selection

There are more methods to analyze the stability of slopes, and the more mature ones are: Swedish bar division method, Bishop's method, Engineer's Corps method, Lowe's method, Spencer's method, Morganston's method, simplified method, etc. Since these methods make different assumptions about the soil body, the calculation results are different. This time, Geo-Slope software was used to perform stability calculations for 30 selected landslides and unstable slopes.

Geo-Slope software is a calculation software integrating limit equilibrium method and finite element method, which is divided into slope stability analysis (Slope/w), seepage analysis (Seep/w), stress analysis (Sigma/w), seismic state analysis (Quake/w), and temperature change analysis (Temp/w). This time, the slope stability analysis (Slope/w) module is mainly used to analyze the safety coefficient of loess slopes, Slope/w can use the limit of force and moment limit equilibrium to calculate the stability coefficient, and its principle of stability analysis mainly adopts the principle of bar division method. That is, the sliding soil block is divided into n vertical strips by slip surface, which can be circular slip surface and various composite slip surfaces. Slope/w synthesizes various methods such as the Swedish strip division method, Bishop's method, Spencer's method, Morgenstern's method, and the simplified method, etc., and Slope/w takes into account the force between strips, which makes the calculation results more reasonable. slope/w, by manually giving the range of the possible changes of the center of the circle. Slope/w automatically searches for the most dangerous sliding surfaces by manually giving a range of possible changes in the center of the circle, and by giving multiple search steps. Slope/w can calculate the stability in the presence of pore water by giving the possible location of the pore water in the soil layer, and it can also calculate the stability in the presence of localized loading conditions.

The Bishop's method is used as an example to briefly introduce the computational principle of the limit equilibrium method.

Bishop's method mainly uses the limit equilibrium of forces to calculate the factor of safety. Bishop's method is used as an example to illustrate the calculation principle of the limit equilibrium method, which is illustrated in Figure 3-16. The loads acting on it are Wi, Ui, Qi, and the reaction and internal forces to be found are Ni, Si and ΔEi. According to the requirements of limit equilibrium on the shear surface, the following equations can be listed:

Yan'an BaoTa District Landslide Failure Geological Hazard

Figure 3-16 Calculation illustration of the Bishop's method

Projecting all of the loads and the reaction and internal forces on the x 'axis, it can be written:

Yan'an Baota District Landslide Failure Geological Hazard

The above equation can be changed to

Yan'an Baota District Landslide Failure Geological Hazard

Iteratively add all the ΔEi of all the subdivisions, and since ∑ΔEi=0, it can be obtained

Yan'an Baota District Landslide Failure Geological Hazard

You can

Yan'an Baota District landslide failure geological disaster

The Ni of the above equation is unknown, we use the equilibrium condition of the vertical force on the slit to get

Yan'an Baota District landslide failure geological disaster

Solve the equation to get:

Yan'an Baota District landslide failure geological disaster

Substituting the equation, we get

Yan'an Baota District landslide failure geological disaster

Yan'an Baota District landslide failure geological disaster

Yan'an Baota District landslide failure geological disaster

This is the most important geological disaster in China. Landslide Failure Geological Hazard

The above formula has k at both ends, so it is necessary to carry out trial calculations when calculating k. Generally, the right side is assumed first: k=1.

Find out the k at the left end, and then substituting into the right end to recalculate the value of k until the value of the assumed k is very close to the calculated k value.

2. Stability analysis of main disaster points

According to the survey results, the occurrence of disasters in the investigation area is closely related to the rainfall factor, so the physical and mechanical parameters of rock and soil bodies in water-saturated state are taken as the calculation parameters in the parameter selection. According to the Specification for Landslide Prevention and Control Engineering Investigation (DZ/T 0218-2006), the coefficient of safety <1.05 is judged to be unstable, the coefficient of safety 1.05～1.15 is more stable, and the coefficient of safety ≥1.15 is stable as the stability criterion of the main disaster sites. Using Geo-Slope software to calculate the coefficient of safety of 30 disaster points and unstable slopes for calculation, the results are shown in Table 3-23.

Table 3-23 List of calculated safety coefficients of major disaster points

Continued

The following is an example of the Zhaojia Bank landslide to illustrate the specific implementation steps of stability analysis using Slope/w:

(1) Introduction of the profile: Slope/w can be introduced directly from the Autocad slope profile can be directly given Scale drawing of the slope profile. In order to calculate the profile accurately, according to the measured profile data, directly enter the data points to draw the profile.

(2) Select the analyzing method: Slope/w can be calculated by choosing limit equilibrium method and finite unit method. In limit equilibrium method, you can choose Bishop's method, Spencer's method, Morganston's method, simplified method and other methods to calculate the factor of safety, and the finite unit should be introduced into the internal stress state function of the slope to calculate. The limit equilibrium method is chosen for this calculation.

(3) Determine the number of chunks and the tolerance of chunks. In order to determine the accuracy of the analysis and calculation, the default number of chunks in the software is 30, and the tolerance is 0.01.

(4) Divide the soil layer and give each soil layer mechanical parameters. Slope/w is mainly used as a demarcation line to distinguish between different geotechnical properties, and different lithologies are divided into different soil zones, and different colors are used to show the distinction. After dividing the soil layers, the mechanical parameters of each soil layer are assigned, which are given according to the survey data of Yan'an part of the area.

(5) Given the location of the center of a potential circular slip surface, the incremental steps in the x and y directions of the center of the circle, and the range of the radius of the arc and the incremental steps of the radius, the program automatically searches for the potentially most hazardous slip surfaces, and calculates their safety coefficients. For Zhaojiashan landslide, the most dangerous slip surface searched is shown in Figure 3-17, from which it can be seen that the back wall of Zhaojiashan landslide is the most unstable.

Figure 3-17 The most dangerous slip surface of Zhaojiakan landslide

(C) Analogy method

Engineering geological analogy method is to apply the existing experience of the stability study of landslides or slopes to the determination of the stability of landslides or slopes of objects with similar conditions. In the analogy, not only to consider the similarity of the structural characteristics of landslides or slopes, but also to consider the evolution of landslides or slopes should be prompted by the similarity of the dominant factors and development stages. Factors affecting the stability of landslides or slopes can be categorized into topography and geomorphology, geological features (stratigraphic lithology, structural surface characteristics of geotechnical bodies, tectonic joints, etc.), rainfall, and human engineering activities (excavation, loading, water storage, etc.). These factors interact and influence each other on the stability of a landslide or slope. Under the interaction of these factors, combined with the deformation characteristics of the slope, the stability of the slope is judged.

1. Topography and geomorphology

Through the statistics of the slope and slope height of the disaster points in the investigation area, it is believed that the landslides in the investigation area mostly occur on the slopes of more than 25 °, with the slope height greater than 30m, and the concentration of slope gradient in 30 ° ~ 50 °, with the slope height of 40-120m on the slope body. In the investigated landslides, the original slope type is convex slope, accounting for 36.52% of the total number of landslides; linear slopes accounted for 52.56% of the total number of landslides; the total accounted for 89.08% of the total number of landslides, i.e., the investigated area of the landslides developed slopes to convex, linear slopes are dominated by slopes with a slope gradient of more than 40 ° of the potential safety hazard slopes, and is concentrated in the slopes with gradient 60 ° to 90 °, the height of slopes is greater than 20 m within the lot in the geomorphology Mostly located on both sides of the gully or the front of the slope body artificial chopping slope, excavation lot.

2. Stratigraphic lithology

The stratigraphic lithology of the investigation area mainly consists of Pleistocene loess, Neoproterozoic mudstone, Jurassic and Triassic sandstone, mudstone and interbedding. Due to the wet disintegrating nature of the Pleistocene loess (mainly Late Pleistocene loess), and the nature of the red clay and mudstone's relative water insulation and softening and strength reduction in the presence of water, it becomes a prone stratum for slope destabilization and the occurrence of landslides and avalanche disasters. Bedrock is the base stratum of the whole area, constituting the slide bed of loess-bedrock contact surface landslide; in the bedrock outcrop is higher, the weathering is strong section or sand and mudstone interbedded section, it is the rocky slope destabilization to form the susceptible area of geological disasters. In the loess slope area, artificial excavation to form high steep slope, become a potential hidden danger of geologic disasters.

3. Structural surface of geotechnical body

The structural surface of geotechnical body in the investigation area is mainly the ancient soil layer draped along the slope within loess, the interface between loess and red clay layer, the interface between loess and sand, mudstone layer, the landslide formed by the landslide joint surface, slip surface and the internal development of tectonic joints, vertical joints, cleavage and so on. Due to the difference in permeability, a water barrier layer is formed on the lithological interface of the strata with large differences in nature, and the converging rainwater makes the overlying loess and mudstone soften and mud, reduces the shear strength, and forms a weak zone, which induces the occurrence of landslides; and the landslide nodal surface and slippery surface developed within the body of the landslide are the main factors inducing the resurrection of landslides or the occurrence of landslides. The existence of these structural surfaces is a potential threat to the stability of the slope, and once the conditions are ripe, it may cause landslides or induce landslide resurrection and cause disasters. The tectonic joints and vertical joints, fissures, etc. developed within loess are an important factor in the destabilization of loess slopes. Loess slopes are often damaged along these internal joints, such as the development of tectonic joints in residential kilns, and kiln collapse accidents often occur along the tectonic joints. High and steep slopes, the soil body is often developed along the vertical joints and the formation of unloading cracks, tensile cracks, the formation of dangerous rocks, dangerous slopes. By tectonic action, the rock body internal development *** yoke joints, the rock body is cut into different sizes, irregular rock, by physical weathering, the development of weathering fissures, making the rock body more broken, in the slope, especially in the high and steep section is prone to collapse fall phenomenon, resulting in disaster. In the sand and mudstone interbedded high steep slope, mudstone shear strength is low, and sandstone strength difference is large, coupled with easy to wind erosion, resulting in the upper sandstone overhanging, bulging tilted, forming dangerous rock body, easy to fall, cracking, bulging and other forms of collapse disaster.

4. Human engineering activities

Human engineering activities are the direct factors inducing the occurrence of geological disasters. Human engineering activities are mainly dominated by unreasonable slope cutting, excavation and construction of water storage reservoirs. Due to the constraints of topographic and geomorphological factors, the residents of the survey area, in order to live, life and economic construction needs, engineering activities are strong, a large number of excavation, chopping slopes, etc., resulting in the foot of the slope stress concentration and sharp increase in the original stress equilibrium state was damaged and out of balance, triggering the slope destabilization and the occurrence of landslide accidents. For example, the landslide in ShangHeNian village, the collapse of MaTa and other disasters, are due to irrational excavation, resulting in the slope is too steep, caused by the foot of the slope stress is too concentrated in other factors under the influence of the landslide accident, resulting in casualties and property losses. Another example is the landslide in the ditch on the east side of the Yan'an City Health School, which was caused by man-made unreasonable slope chopping and excavation of the foot of the slope, resulting in the occurrence of a landslide, which pushed down the stone masonry retaining wall, and the landslide surged to the walls of the residents' houses. At present, the slope has a gradient of about 45° and is in an unstable state, posing a direct threat to the lives and properties of the residents. The artificial construction of water storage reservoirs, caused by the groundwater level elevation, resulting in an increase in the capacity of the slope body, destroying the original state of stress equilibrium, and groundwater led to the slope body inside the soft zone of softening, mud, reduce the shear strength, easy to induce the occurrence of landslides or the resurrection of the old landslides. Zhaojiashan landslide due to the slope after the reservoir bank water storage, resulting in the rise of the water table, the villagers foundation serious seepage, and the water table reached the upper part of the old slip surface, and there are springs exposed, the stability of the landslide body is very poor, there is a danger of resurrection, endangering the lives and properties of the villagers of Zhaojiashan safety.

Based on the analysis and comparison of the above factors, combined with the signs of deformation and characteristics of the slope, the stability of some of the major disaster sites are judged (Table 324, Table 3-25).

Table 3-24 Stability Analysis of Major Landslide Disaster Points

Continued

Table 3-25 Stability Analysis of Major Unstable Slope Points

(IV) Comprehensive Evaluation of the Stability of Major Geological Hazards

The stability of major disaster points has been analyzed earlier by numerical analysis method, limit balance method and engineering geology analogy method. The focus of the three methods of analysis is not the same. Numerical method mainly adopts elastic-plastic mechanics theory and numerical calculation method, from the perspective of researching the stress and displacement field of rock and soil body, analyzing and evaluating the stability state of rock body under certain environmental conditions; Limit equilibrium method mainly applies the limit equilibrium theory to evaluate the stability of slopes; and engineering geology analogy method is to apply the existing landslide or slope stability research experience to the stability of landslides or slopes with similar conditions. The method of engineering geologic analogy is to apply the experience of existing landslide or slope stability studies to the stability of landslide or slope with similar conditions. The factors affecting slope stability are complex. Therefore, this section will synthesize the calculation results of these three methods to comprehensively judge the stability of the slopes where the major geohazard sites are located.

The results of the comprehensive analysis show that of the 30 landslides and unstable slopes, 3 are stable, accounting for 10% of the total; 7 are more stable, accounting for 23.3% of the total; and 20 are unstable, accounting for 66.7% of the total (Table 3-26).

Table 3-26 Comprehensive Judgment Table of Geological Hazard Stability

II. Geological Hazard Hazard Evaluation

(I) Evaluation Criteria

Threatening objects of geological hazards include population and property. Population can be directly characterized by quantity; property includes land, livestock, houses, roads and so on. Based on the remote sensing interpretation and actual price survey information, the main economic value assessment criteria (Table 3-27) are established, and the calculation is based on the criteria accumulating one by one according to the degree of danger and vulnerability of the threatened objects. The grading standards of geologic disaster and hazard degree are evaluated according to the provisions of Table 3-28.

Table 3-27 Criteria for evaluating the economic value of the disaster-bearing body

Table 3-28 Criteria for grading the geological disaster and the degree of hazard

1) Disaster grading: that is, the degree of disaster grading of the geological disaster that has taken place, using the indexes of the columns of "number of deaths" or "direct economic loss". Economic loss" column indicators to assess; 2) the degree of hazard grading: that is, the possible occurrence of geologic disaster hazard prediction grading, using "the number of people under threat" or "direct economic loss" column indicators to assess.

(2) Assessment of the current situation

1. Landslides

According to the collection of information on past landslides and the results of this field survey, there are 34 landslides*** in the survey area that have been documented in recent years and have caused some economic losses and casualties. Of these 34 landslide disasters, except for one larger grade landslide, the remaining 33 disasters were of general grade, with a total *** resulting in 5 deaths, as well as 1,026,000 dollars in property damage. From the dated landslides, the rate of new landslide disasters was 0.76 landslides/year (Table 3-29).

Table 3-29 Landslide Disaster and Hazard Evaluation Table

2. Avalanches

After the occurrence of avalanches, their remains are not easy to be preserved, and avalanches during the period of geologic history are generally mostly non-existent, and it is difficult to further identify the time of their occurrence yet. According to the time-recorded collapse investigation data, the frequency of collapse in recent years can be given basic data. Since the 1960s, there have been 16 documented avalanche disasters in ****, including 2 larger-level avalanches and 14 general-level avalanches, with 12 deaths and 480,000 yuan in economic losses (Table 3-30). Due to the survey according to the disaster classification, the district geological environment conditions are poor, densely populated, although the annual frequency of occurrence is low, but also should cause special concern, each may bring the loss of life and property.

Table 3-30 Evaluation Table of Disaster and Hazard Degree of Avalanche Disaster

(C) Predictive Evaluation

Predictive evaluation of geologic hazard is to make an assessment of the hazards of geologic hazards that may jeopardize the safety of the residents' lives and properties, and the construction of the project. This assessment is divided into three types of landslides, landslides and unstable slopes to predict and evaluate their hazards. The assessment content is mainly the number of people threatened and the potential economic losses due to property damage.

1. Landslides

Landslides in the area can be categorized into 3 types: ancient landslides, old landslides and new landslides, which have the possibility of resurrection under the dual triggering of both natural and man-made factors. The total number of landslides investigated in the field*** is 293, which can be categorized into active landslides and inactive landslides. In this section, 39 active landslides, accounting for 13% of the total number of investigated landslides, are screened to predict and assess their hazards.

By predicting and assessing the hazards of these 39 landslides, there are 8 with high hazards, 25 with medium hazards and 6 with low hazards. In total*** there are approximately 2,098 people threatened by the landslides with a potential economic loss of approximately 28.63 million dollars (Table 3-31).

Table 3-31 Landslide Hazard Prediction Assessment

Continued

2. Avalanche

Geological hazards in the survey area are dominated by loess landslides, with avalanches taking the second place; the avalanches referred to in the survey are of two kinds: avalanche hazards and already-occurring avalanches, and what is referred to here is the prediction of potential hazards of the already-occurring avalanches. According to the results of the field survey and previous data investigation, there are 14 out of 52 landslide hazards occurring in the district which are still in an unstable state and potentially hazardous, accounting for 27% of the total number of investigated landslides. The slopes on which the avalanches occurred have also been modified by precipitation-dominated weathering and are highly susceptible to vegetation growth, which is also not easy to detect. The fact that there are few established avalanches does not mean that the avalanches are less hazardous. Avalanche formation conditions prevail in the survey area, deep loess, good uprightness, vertical joint development, Yanhe River and its tributaries on both sides of the loess steep cliffs abound, most of the kilns are to choose a very steep slope (> 65 °) horizontal digging, kilns before the bungalows and yards are placed in the high steep loess cliffs under the threat of avalanches.

The 14 collapse disaster, the hazard of moderate 6, the hazard of small 8, the hazard of large temporary, which is related to the scale of the collapse disaster, the impact of the scope of the smaller. 14 collapse **** threat 240 people, potential economic losses of 560,000 yuan (Table 3-32).

Table 3-32 Predictive Assessment of Avalanche Disaster Hazards

3. Unstable Slopes

Unstable slopes are a kind of potential geologic hazards, both bedrock slopes, loess slopes, and loess-bedrock slopes, which are widely distributed in the investigation area. There are mostly residents living under the slopes, or they are the office and production bases of enterprises and institutions, which are the main places of production and construction and people's life in the whole area, thus constituting potential hazards. Unstable slopes only make the basic judgment of instability on the stability of slopes, but do not give a definite conclusion on the change pattern of their instability. This is due to the potential changes in the existence of many uncertain factors, it is not yet possible to make an accurate prediction of its future changes.

Of the 51 unstable slopes investigated in detail, 11 have a large potential threat, accounting for 22% of the total number of unstable slopes. Estimated statistics on their threatened population and potential economic losses show that there are 3 unstable slopes with large hazards, 8 with medium hazards, and the other 40 with small hazards (not included). The total **** threatened 909 people and the potential economic loss was $6.52 million (Table 3-33). The survey only selectively chose some unstable slopes in different areas as survey points to reflect the basic characteristics of unstable slopes. In fact, the hazards of unstable loess slopes that have not experienced avalanche-slip disasters are the most difficult to assess, and the prediction and assessment of unstable slopes need to be further researched and explored.

Table 3-33 Hazard prediction and assessment of unstable slopes

Continued table