Datong, located in the north of Shanxi Province, is an important coal-fired power production base in China, and also a new industrial city with machinery, chemical industry, building materials and medicine as its main production. At the same time, it is a famous historical and cultural city. Since the 1980s, with the rapid development of industrial production, the scale of the city has been expanding, and the city has changed with each passing day. As a rising modern city, it has become an urgent problem to give full play to the potential of land resources, rationally develop and utilize groundwater resources, defend against various geological disasters and protect the environment. Among them, the ground fissure disaster has become one of the important problems in the construction of Datong city and the safety of people's lives and property. Reasonable assessment of the losses caused by ground fissures is an important basis for government departments to rationally plan urban layout and effectively prevent ground fissures.
(1) Geological survey
Datong City is located at the junction of Cailiangshan-Leigongshan fault block in inner mongolian axis, Yungang syncline of Shanxi fault uplift and Sanggan fault depression. The strata in this area are well developed and the geological structure is complex. The city is surrounded by mountains on the east, west and north, and the alluvial-lake plain is in the middle. There is the Yu He River and its tributary-Shili River, which flows into the southeast suburb of Datong from northwest to southeast. Datong city is located on the denudation platform and accumulation terrace between the two rivers. Its topography is generally high in the northwest and low in the southeast, with an altitude of 1030 ~ 1065 m, and the urban area is completely covered by Quaternary loose soil. According to drilling data, the maximum thickness of Quaternary is more than 250 meters ... The basic types of urban landforms are erosion accumulation valley and denudation platform, and ground fissures are developed.
(2) General situation of ground fissures in Datong City
From 1983, ground fissures appeared in the living quarters of Datong Locomotive Works. By 1995, seven ground fissures of different scales had been found in Datong City. The total length of cracks has exceeded 20 kilometers. These ground fissures are concentrated in the northeast of He Shili River, the urban area west of Yu He River and its suburbs. From south to north, it is described as follows:
1. Datong Locomotive Works-Datong Hotel Ground Fracture Zone
The ground fissure zone is located in the southwest suburb of Datong City, starting from the second terrace on the left bank of the He Shili River, passing through the mechanical transport station of the Power Construction Company, the living quarters of the locomotive factory, the coalfield 1 15 geological team, Zhoujiadian and Datong Medical College, and shifting eastward through Datong Hotel, disappearing into Nanguan South Street, with a total length of about 5.0km. The ground fissure is composed of a plurality of single ground fissures that strike NEE, which are arranged in the right direction. Its strike is NE57, inclined to SE, and the dip angle is 70 ~ 80; It is characterized by horizontal stretching (southeast plate descending), accompanied by tensile and torsional ground fissures with left-handed horizontal torsion. The ground fissure was found in 1983, which is the earliest ground fissure in Datong.
2. Xintianbao ground fissure zone
The ground fissure zone is located in the first line of Wenhua Street, about 400m in front of Datong Locomotive Works. It starts from Xintianbao Village in the southwest, passes through 5 buildings of Wenhua Street and 10 in the northeast, and disappears to the east of the Third Ring Road, with a total length of about 1.2km. The ground fissure was discovered late, and 1992 was displayed on the ground, causing strong damage to 5 buildings of Datong Locomotive Works, 10 residential buildings and civil air defence works. Ground fissures have obvious directionality, strike NE53, incline to SE, and the dip angle is about 80; This is a tensional crack, slightly left-handed.
3. The ground fissure zone of Xiahuayansi of the southern suburb sinking team.
The ground fissure started in the family courtyard of Southwest Tobacco Company, passed through the sinking team in the southern suburbs, the city TV station, the northwest corner of children's park, Hepingli, Xiahuayan Temple, and along NE40. Extending from left to right, it is still intermittently distributed in Datong Daily, Cao Mao Lane and Yanbei Foreign Trade Building, with a total length of about 5.5km. The ground fissures strike Ne 30 ~ 45 and tend to se, with an inclination of about 80, and the SE plate drops. Ground fissures developed rapidly after 1984 and 1989, and have been spreading to both sides in recent years.
4. Cultural ground fissure zone
The ground fissure zone starts from the tax bureau, extends northeast through Wenhua Lane, Li Jianshe and Yingzeli, and disappears at the northern end of Yingze Market outside the west gate, about 1.4km long, with the ground fissure strike of Ne 26 ~ 42 and NW dip angle of 82 ~ 85. Northwest disk down, slightly right-handed. The ground fissure was found in 1992.
5.322 ground fissure zone in hospital
322 Hospital Ground Fissure Zone is located about 400m west of Wenhua Ground Fissure Zone, which is basically parallel to Wenhua Ground Fissure Zone. The ground fissure started in the warehouse of Southwest Valve Factory, passed through Xinkaixi No.2 Road, No.322 Hospital, No.59100 Army and their families' area in the northeast, and disappeared in the south gate of Datong Park, with a total length of about 1.3km, and its strike is Ne 30 ~ 40, with an inclination of se above 85. SE disk rotates to the right and descends. The ground fissure was found in 1993.
6. Ground fissure zone of locomotive company
The ground fissure is located in the northeast corner of the city, starting from Tongyan East Road in the southwest, passing through the Second Hospital of the City and the electromechanical company, and then branching off. One branch goes to Tieyi No.1 Middle School, and the other branch goes to Datong No.4 Middle School, ending at the nursing home of Datong General Staff, with a total length of about 1.0km. The ground fissure strikes NE35, tends to NW, and slightly twists to the right.
7. Yongjun North Road-Tongfeng Road Ground Fissure Zone
The ground fissure zone of the railway sub-bureau is located in the north of the city, starting from the army barracks of Yongjun North Road in the west, passing through the railway sub-bureau 14 yard, crossing the railway to Datong Railway Hospital, deflecting northward, passing through the east and west courtyards, hydropower stations, rolling stock depots and material factories, and crossing the Tongfeng Highway, with a total length of about 4. 1km. The ground fissure was found in 1990. It is composed of many oblique ground fissures, with stable overall direction, strike Ne 60 ~ 80, dip SE, and dip angle of about 80. The left-handed twist is obvious, which is a tensile fracture zone.
Judging from the distribution range of the above ground fissures, it has involved a considerable area of Datong city; Judging from the current activities, there is still a trend of continuing to expand to the urban area.
(3) the law of ground fissure activity
1. Ground fissure morphology
Seen from the plane shape, the ground fissures extend linearly and have strong directionality. Generally, the ground fissures strike in the northeast or NEE direction. However, according to the profile of a single ground fissure, its development degree is different, and the ground fissure zone shows obvious lateral zoning. From the profile shape, the shape and occurrence of ground fissures are obviously consistent. All the profiles exposed by artificial exploration trenches confirm that the main cracks are generally inclined to SE, and the dip angle is mostly 70 ~ 80. The secondary cracks in the upper wall generally tend to NW, which is opposite to the main cracks, with an inclination of 60 ~ 70, and the included angle between them is generally 40 ~ 50. Between them, the two are combined into a Y-shaped structure in cross section. The overall tendency of secondary cracks in footwall is se, which is consistent with the occurrence of main cracks. According to the development degree of ground fissures, the main fissure can be taken as the center line on the profile, and the 5 ~ 8 m side of the upper wall is the ground fissure intensive zone; 3 ~ 5m from one side of the footwall is a ground fissure sparse zone. The development width of ground fissures obviously has the characteristics of wide at the top and narrow at the bottom.
2. Ground fissure activity pattern
A large number of data show that the destruction and deformation of soil by ground fissures are carried out in three-dimensional state. Their activities show three-dimensional characteristics: vertical uplift along the strike, horizontal tensile deformation and horizontal torsional deformation. The difference of vertical lifting leads to the cracking of ground buildings; Transverse horizontal tensile deformation intensifies the cracking of ground buildings; The horizontal distortion along the strike accelerated the collapse of the destroyed ground buildings.
3. The activity law of ground fissures
In terms of space, the ground fissures in Datong city have a large settlement in the vertical direction and a small activity in the horizontal direction. From the time point of view, the ground fissures in Datong city show obvious characteristics of intermittent activity, which is positively related to faults and seismic activity. For example, the ground fissures in Datong Locomotive Works are relatively intense due to the acceleration of the activity rate of Kouquan fault from June 30, 1990 to June 30, 1990. Another example is1After the Datong-Yanggao M = 6. 1 earthquake in June, 1989, the rate of ground fissure activity in Datong City was obviously accelerated. From the aspect of human exploitation of groundwater, the activity intensity of ground fissures is also affected by the amount of groundwater exploitation. For example, before 1980, the groundwater level in Datong decreased1.03 ~10 m/a, 1984 ~ 1989, and the groundwater level decreased on average/kloc-0.
Second, the risk assessment of ground fissures in Datong City
It can be seen from the distribution and activity law of the above ground fissures that the ground fissures in Datong basically involve the whole urban area. The active period of ground fissure activity is obviously positively related to tectonic activity. Therefore, when evaluating the risk of ground fissures in Datong city, these characteristics should be fully considered before evaluation.
Through the analysis and study of Datong ground fissures, we think that the risk analysis of Datong ground fissures includes the following contents:
(A) the classification of ground fissure disasters
Different scales of ground fissures cause different losses. The longer the length, the wider the ground fissure, and the greater the loss.
Three of the seven ground fissure zones in Datong City are exposed continuously from south to north as follows: the fissure from Datong Locomotive Works to Datong Hotel is about 5.0km long; The ground fissure from the sinking team in the southern suburbs to Xiahuayan Temple is about 2.5km long. The ground fissure from Yongjun North Road to Tongfeng Road is about 3.0km long. All belong to extra-large ground fissures.
(2) Mechanism analysis of ground fissure disaster.
The ground fissure disaster in Datong city is mainly caused by the surface soil rupture caused by the fault activity in Fangzicun. Among them, fracture activities exert tensile stress and shear stress on soil, underground engineering and surface buildings through stress concentration, transmission and release. At the same time, combined with the additional load imposed by the building's own weight on the foundation, the building is deformed and destroyed, leading to disasters. That is to say, the disaster-causing force of the ground fissure comes from the structural stress of the creeping movement of the fault at the lower part of the ground fissure, which acts on the soil with tensile stress and shear stress, thus destroying the surface soil structure. When the stress is transmitted to the medium (soil) around the building foundation, underground structures and underground pipeline projects, the underground structures, foundations and soil will be deformed and destroyed under the combined action of tensile stress, shear stress and building load, thus forming a geological disaster of ground fissures. On the one hand, the deformation and damage of surface buildings focus on the transfer of stress from the lower structure to the upper structure; On the other hand, due to the destruction of foundation soil, the bearing capacity is significantly reduced, and uneven settlement occurs under the weight of buildings, resulting in horizontal tensile-shear failure of foundation and superstructure. See figure 15-5 for the disaster mechanism of ground fissures.
(3) Ground fissure risk index and distribution map
From the disaster mechanism of ground fissures, it can be seen that the ground fissure activity in Datong is closely related to the local tectonic activity. The disaster process of ground fissure is formed by the destruction of soil in its active area, which leads to the destruction of underground engineering and surface buildings. Therefore, when calculating the risk index of ground fissures, factors such as ground fissure activity, tectonic movement, groundwater exploitation and surface water (mainly rainfall) are mainly considered, and the risk of ground fissures to underground engineering and surface buildings is divided into five grades, namely, A, B, C, D and E, and the corresponding indexes are 9, 7, 5, 3 and 1 respectively, and ground fissures are calculated according to standards.
According to the specific standard of grading index, we divide Datong urban area into 300 grids with the same size according to the grid size of 30″× 15″ (longitude× latitude). Get the risk index in each grid, and draw the contour map of Datong ground fissure risk index (figure 15-6). As can be seen from the figure, the three banded high isoline distribution zones are completely consistent with the three ground fissures in the urban area. This objectively reflects the risk distribution of ground fissures in Datong city. The destructive effects of these three ground fissures on the ground tend to extend to both ends, and the destructive effects of ground fissures in Datong Locomotive Works-Datong Hotel, Southern Suburb Drilling Team-Xiahuayan Temple tend to be linked together.
3. Vulnerability analysis of ground fissures in Datong City
The danger of ground fissure disaster reflects the natural attribute of ground fissure disaster; The vulnerability of ground fissure disaster reflects its socio-economic attributes, that is, the destruction of social wealth and its influence on human socio-economic activities, including the psychological impact on people. Obviously, the ground fissures with the same intensity occur in rural areas and cities, and the losses caused are completely different. The reason is that the affected people are different. The more concentrated the social wealth, the more developed the economy, and the greater the vulnerability of ground fissure disasters, and vice versa. For the vulnerability evaluation of ground fissures in Datong, we have carried out the following work:
Figure 15-5 Block diagram of ground fissure disaster mechanism system
Material fragility
* * * includes two aspects: first, land resources (mainly refers to land use types) and social assets (including buildings above ground, underground facilities in cities, the number of enterprises and institutions, services, finance, transportation, mineral resources, etc. ) in Datong; The second is the relationship between the intensity of ground fissures and the degree of destruction of land resources and social assets. For the first aspect, we have made statistics on the distribution of land resources and the statistical summary of social assets in Datong (table 15- 14). As for the second aspect, through the previous analysis, we already know that ground cracks, ground cracks, ground buildings and underground facilities are all destroyed. With the development of ground fissures, ground buildings and underground facilities with a certain distance from ground fissures are gradually destroyed.
Economic fragility
The economic vulnerability of ground fissures includes the following contents: first, the direct economic losses that ground fissures may cause, including land value, house maintenance or relocation costs, lifeline engineering repair or diversion costs, etc. The second is the indirect economic losses that may be caused by ground fissures, including the losses caused by production suspension, the impact on human livelihood activities and other secondary impacts. According to statistics, the direct economic loss caused by ground fissures in Datong City is 40 million yuan. The indirect economic loss is calculated as 3∶ 1, which should be1.200 million yuan.
Social fragility
The social vulnerability of ground fissures is different from other geological disasters. Generally, it will not cause people's psychological panic. As far as Datong is concerned, so far, there have been no reports of casualties caused by ground fissures.
Figure 15-6 Isogram of Ground Fissure Risk Index in Datong City
Table 15- 14 Summary of Social Assets Statistics of Datong City (Urban Area)
(4) Damage rate and loss rate
Ground fissures directly destroy land, buildings and underground engineering facilities. The destruction rate of ground buildings and underground engineering is defined as: the destruction area of ground buildings caused by ground fissures/the total area of buildings. Its loss rate can be defined as: the value of the damaged area of ground buildings caused by ground fissures/the total value of buildings. The above two definitions can also be analogized to the destruction of land resources by ground fissures. According to the damage rate and structural condition, the damage degree of ground fissures to the structure is divided into five grades: micro, light, medium, large, heavy and heavy. At the same time, we give the percentage of maintenance cost to construction cost, so as to get the maintenance cost for reference.
(5) Isograms of vulnerability index and comprehensive vulnerability index.
Through the analysis of the material vulnerability and economic vulnerability of ground fissures, it can be known that the vulnerability of ground fissures mainly depends on the richness of material wealth and the degree of social and economic development in ground fissures. In order to reflect this influence intuitively and reasonably, we select three factors that have great influence on the vulnerability of ground fissures: population density, land type and structure. According to the different States of these factors, a certain vulnerability index is selected; Then compare the importance of these three factors to the vulnerability of ground fissures. According to the comparison results, the analytic hierarchy process is used to calculate the weight of each factor to the vulnerability of ground fissures, and the comprehensive index of ground fissure vulnerability is obtained. Its specific practices are as follows:
Datong urban area is divided into 300 grids with the same size according to the grid size of 30″× 15″ (longitude× latitude). According to the access criteria of evaluation factors in table 15- 15, the indexes Ep, EL and es (table 15-5) of each factor P (population density), L (land type) and S (structure type) in each grid are obtained, and then P, L and S are formulated into. The importance indexes are 9, 7, 5, 3 and 1 respectively. Finally, the importance index is substituted into the analytic hierarchy process calculation program to calculate the weight WP of P, the weight WL of L and the weight WS of S, and the comprehensive vulnerability index VI is obtained according to the following formula:
Theory and practice of geological hazard assessment
Table 15- 15 Standard Table of Vulnerability Assessment Elements and Factor Indicators
Point the comprehensive vulnerability index on the map and draw the contour map of the comprehensive vulnerability index of ground fissures (Figure 15-7).
From the isoline distribution map of vulnerability index, it can be seen that the greater vulnerability of ground fissures in Datong is mainly concentrated in the urban area (see the line of isoline 6 in the figure), and the greatest vulnerability is mainly concentrated in the commercial and financial district in the city center.
Four, Datong city ground fissure disaster damage loss assessment
The direct objects of ground fissure damage are land and various facilities built on the ground and underground. Therefore, the evaluation of ground fissure damage loss considers these two aspects. Each aspect can also consider the actual loss assessment and future loss assessment. Its specific contents are as follows:
(A) accounting for the loss of land value
There are two kinds of damage and loss to land caused by ground fissures: one is that the land is completely unusable due to the damage of ground fissures. For example, where the main ground fissures pass directly, the ground is destroyed, making the land lose its use value. The other is the land degradation and utilization caused by the destruction of ground fissures. For example, the area affected by major ground fissures used to be a type of land, but now it has been reduced to a type of land of three or four. The following is considered from the actual loss assessment and future loss assessment.
1. Real land loss assessment
The evaluation of real land loss is to calculate the loss of the area where the ground fissure disaster has occurred. The purpose of evaluation is to obtain the total loss of land resources caused by disasters. The specific evaluation method is:
According to the use category of the area where the ground fissures pass, combined with the damage degree of the ground fissures to the land, the actual total land loss DT0 is calculated according to the following formula:
Theory and practice of geological hazard assessment
In which: n 1- the number of blocks that completely destroyed the land area;
N2—— the number of degraded land area blocks;
Esti-the land area of the I-th plot that was completely destroyed;
Esj-the land area used for the demotion of Block J;
Pei-the unit price of the I-th piece of land;
Pej- unit price difference of degraded land in the j th block.
According to the above formula, we get that the actual total land loss DT0 caused by ground fissures in Datong is about 6,543,800,000 yuan.
Figure 15-7 Isogram of Ground Fissure Vulnerability Index in Datong City
2. Future loss assessment
The future loss assessment of land is to calculate the possible loss of land resources in the area where the ground fissure disaster will occur. The purpose of evaluation is to obtain the total loss of land resources DT 1 caused by future disasters. The specific evaluation formula is:
Theory and practice of geological hazard assessment
Where: k is the proportional coefficient, and the specific solution of k is obtained according to the isoline map of ground fissure risk index made earlier, where the isoline is the highest (that is, where ground fissure has been found), k is 1, and the value of k in other places can be (VI/VMAX) 2;
Vi—— Ground fissure risk index value of the selected location;
Vmax—— the maximum risk index of the evaluation site; Other quantities have the same meaning as formula (2). According to this calculation, the loss of land value in Datong in 2000 was about 20 million yuan.
(2) Loss accounting of buildings such as houses.
The damage of ground fissures to the structure is manifested in the cracking or even collapse of houses. The losses caused by it can be considered in two situations. One case is that the house is damaged to a certain extent and can continue to be used after maintenance; Another situation is that the house is completely destroyed and can no longer be used. In the first case, the loss can be estimated directly through the maintenance fee. In the second case, we still consider it from two aspects: actual loss evaluation and future loss evaluation.
1. Actual loss assessment
The evaluation of the actual loss of buildings is to calculate the loss caused by ground fissures to buildings. The purpose of the assessment is to obtain the total structural loss DB0 caused by the disaster. The mathematical model of its evaluation is:
Theory and practice of geological hazard assessment
Where: HSI-the area of damaged structure in the first section;
Phi-the cost per unit area of the structure in Section I;
N—— The number of buildings damaged by disasters.
According to the above model, we find that the actual loss of buildings caused by ground fissures in Datong is about 30 million yuan.
2. Future loss assessment
The future loss evaluation of structures is to calculate the possible loss of structures that will have ground fissures. The purpose of the assessment is to obtain the structural loss DB 1 caused by future disasters. The evaluation formula is:
Theory and practice of geological hazard assessment
Wherein: k has the same meaning as formula (3); The meanings of other quantities are the same as those in Formula (4). If effective preventive measures are not taken, it is estimated that by the year 2000, the loss of buildings in Datong will reach about 40 million yuan.
Through the above analysis, we can get that the actual total loss of ground fissure disaster is (DT0+DB0), that is, 40 million yuan; The total loss in the future (2000) is (DT 1+DB 1), that is, 60 million yuan.
Five, ground fissure disaster prevention and control engineering evaluation
The ground fissures in Datong belong to tectonic ground fissures, and their remarkable characteristics are irresistible. Therefore, there is no effective method to prevent and control the ground fissure disaster at present. Let's briefly analyze the feasibility and cost of the prevention and control project.
(A) the feasibility analysis of prevention and control projects
The feasibility analysis of the ground fissure prevention project should include the following two aspects:
1. Engineering geological evaluation of ground fissure site. Based on the analysis of the main factors controlling and influencing ground fissures, combined with the physical and mechanical properties of soil, the affected areas of ground fissures are classified and divided.
2. Analysis of prevention and control engineering technology. Mainly analyze whether the prevention and control scheme is reasonable and feasible.
(2) Control benefit analysis
1. Direct economic index of disaster reduction benefit (z)
Z=J-T
Where: j is the reduction of disaster loss after adopting specific disaster reduction technology for disaster prevention and control; T is the input of prevention and control costs.
2. Benefit ratio of disaster reduction (b)
B=Z/T
3. Input-effect ratio
Bt=J/T
Generally speaking, z only reflects the net value of direct economic benefits obtained after a disaster prevention and control. It fails to reflect the benefits of disaster reduction activities, that is, to obtain the same net value of direct economic benefits, the cost of prevention and control T may be quite different. In order to reflect the benefits of disaster reduction activities, it can be measured by index (B). B reflects the net value of the direct economic benefits obtained by unit investment, that is, the benefits of disaster reduction activities. When the value of B is greater than or equal to the discount rate of the society in that year, it can be considered that the investment in disaster reduction activities is feasible.
According to the actual investigation of Datong and Xi, after taking effective preventive measures, all kinds of losses caused by ground fissures can be reduced by more than two thirds.