Earthquake induced by typical reservoirs in China
-1On March 9th, 962, Guangdong Xinfengjiang Reservoir was hit by an earthquake with a magnitude of 6. 1.
According to relevant historical records, Xinfengjiang Reservoir was built in July 1958. In June 5438+10 of the following year, the sluice was closed for water storage. The dam type is a single-pier big-head dam, which consists of 19 single-pier big-head dams with a spacing of18m, with gravity dam sections on both sides. Maximum dam height 105 m (crest elevation 124 m), crest length of 440 m, design normal high water level 1 18 m, storage capacity11500 million cubic meters (water level/kloc-0. Excavation of spillway tunnel near the left bank (inner diameter10m). The dam-back power plant is equipped with four hydraulic turbines, with a total installed capacity of 290,000 kilowatts. It is the largest reservoir in South China and the largest hydropower enterprise in Guangdong.
After the Xinfengjiang Reservoir (1959+ 10) was filled for one month, seismic activity began to appear, and with the rapid rise of water level, seismic activity was strengthened accordingly. When the water level first approached Man Ku Peak as high as110.5m, 1962 induced an earthquake of magnitude 6. 1 at 04: 00 on March. The earthquake was half a year later than the first high water level, with a focal depth of 5 km, and spread near the dam 1 km. At that time, the earthquake was thundering and the house was shaking. The strong earthquake caused more than 800 houses 1.800 to collapse, seriously damaged houses 1.0500 and damaged houses 1.3400, with 85 casualties. The hydropower plant and high-voltage substation of the affiliated project were also seriously damaged and could not operate. The most typical example is that a pair of stone lions weighing 3,500 Jin twisted counterclockwise 1 1 degree relative to the base in front of Heyuan Leather Factory.
Before the earthquake, due to the first-stage reinforcement measures, the dam as a whole withstood the severe test of the 6. 1 (8 degrees) earthquake. However, there is a 90-meter-long horizontal crack at the elevation of right bank 13- 18 dam section 108.5 meters, which can be seen on both upstream and downstream surfaces. There are also small intermittent horizontal cracks at almost the same elevation of the left bank dam section. At that time, water leakage occurred in many expansion joints of the dam. At the beginning of September of the same year (when the water level was 1 12.7 m), a seepage with a length of about 1 1 m appeared on the downstream dam face of the 14 dam pier. It can be seen that both the upstream dam face and the downstream dam face of this elevation are permeable. It is just where the cross section suddenly changes, and the stress is the easiest to concentrate. /kloc-3m above the elevation of 0/07m and 3m below it. The concrete pouring interval is five and a half months, and the temperature difference of entering the warehouse reaches 6,7℃. The upper part is made of cement of shaft kiln 300. Low strength and instability. In addition, the filling of seismic reinforced concrete below the elevation of 100 meters increases the difference in mass and stiffness between the middle, lower and upper parts, making the whole dam uneven. Therefore, it is not difficult to understand the internal causes of cracks caused by high dynamic stress near108.5m elevation.
Reservoir-induced earthquakes are destructive, which can not only directly cause damage and casualties, but also lead to dam-break floods, secondary disasters and catastrophic losses to people's lives and property in downstream areas. The particularity of this double disaster has aroused widespread concern of experts and scholars at home and abroad; China is one of the countries that studied induced earthquakes earlier.
As early as 1959, scientists and technicians from geophysics, geology, engineering geology, earthquake engineering, water conservancy engineering and other fields. The seismic activity of Xinfengjiang reservoir area was studied, and then the Xinfengjiang dam was strengthened, which resisted the destructive earthquake of magnitude19,65432 March 6. 1.
The characteristics of the earthquake induced by Xinfengjiang Reservoir mainly include:
First, the intensity is high and the earthquake is strong. This is mainly because the source of the reservoir-induced earthquake is shallow. The focal depth of Xinfengjiang Reservoir is about 1— 1 1 km. 1March, 962 19 An earthquake of Ms 6. 1 occurred in the reservoir area with the maximum intensity of 8 degrees. This fully shows that the epicenter intensity of its earthquake far exceeds the intensity of natural tectonic earthquakes of the same magnitude.
Second, the earthquake distribution is concentrated in the reservoir-affected area. Although there are migration phenomena in different stages of reservoir seismic activity, they are basically within the scope of reservoir inundation area and influence area, generally near the reservoir bank corresponding to dam area and deep water area, especially in exposed canyon area. The seismic active area and the strongest earthquake of Xinfengjiang Reservoir are located near the dam with the deepest water level (80 meters).
Thirdly, seismic activity is closely related to the process and operation of reservoir impoundment. Generally, microseismic activity began to be found within 1-2 months after the reservoir was filled, and the main earthquake occurred in 1-2 years. The fluctuation of seismic activity is obviously related to the change of reservoir water level. Guangzhou Seismological Station, located at the southwest of Xinfengjiang Reservoir Dam 160 km, recorded the earthquake in the reservoir area in June of 5438+0959+00, that is, one month after the start of water storage. Due to the rapid rise of water level, seismic activity is very frequent. 196 1 The seismological network established in July recorded the magnitude of1Ms > in February 1972. There were 258,267 earthquakes of magnitude 0.2, of which 23513 ms >; = 1.0.
Fourth, there are abundant foreshock sequence. The earthquake sequence of Xinfengjiang Reservoir consists of foreshock, main shock and aftershock. There are many foreshocks and slow attenuation of aftershocks. The B value in the relationship between frequency and magnitude is similar to other reservoir-induced earthquake sequences, but different from normal tectonic earthquakes. The b value of the foreshock (1. 12) is higher than that of the aftershock (1.04). These values are all1.4-1.5 times larger than the tectonic earthquake (0.72). In addition, the magnitude ratio of aftershocks to main earthquakes is high (0.87). The attenuation coefficient of other earthquakes is 0.9, and the attenuation is slow.
The research shows that there is a close relationship between reservoir storage and earthquake in time and space. Seismic activity is affected by water level. The rapid rise of water level to high water level is often accompanied by the increase of seismic activity. Therefore, deep circulating groundwater channels can be formed in the fractured zone. Accordingly, with the increase of seepage pressure, the groundwater can circulate deeper, which causes the reduction of normal stress on the section and the mud on the weak rock structure surface, thus reducing its shear strength. At Xinfengjiang Dam, because the water level rose by 20 meters, water seepage occurred in the fault zone on the northwest side of the dam at first, and as a result, microseisms were induced. When the water level rises by 50-60 meters, the seepage pressure increases, forming a series of small earthquakes and strong earthquakes with Ms > 3.0. When the water level reaches the peak, the seepage pressure becomes the largest, and the strongest earthquake occurs with the climax of seismic activity.
The seismic fortification measures and countermeasures of Xinfengjiang Reservoir mainly include:
1. Prevention measures for induced earthquake after reservoir impoundment.
At the beginning of the design of Xinfengjiang dam project, according to the records of four felt earthquakes (V-VI degree) in the history of Xinfengjiang, the basic earthquake intensity standard at the dam site was set at 6 degree. 1959+00 After the reservoir was closed for storage in June, with the rising of water level, the seismic activity intensified. 1960 In May, there were continuous felt earthquakes near the dam (about intensity V), and an earthquake of magnitude 4.3 (intensity 6) occurred in July of the same year. In view of the great earthquake endangering the dam safety, Premier Zhou personally cared about the safety and earthquake prevention work of Xinfengjiang Reservoir and instructed to take measures to solve it. In the absence of precedent, the vast number of scientific and technological workers have worked hard to tackle key problems and make comprehensive analysis. It is decided that the seismic fortification intensity standard of the dam is 8 degrees. Considering the normal high water level of the reservoir (118m), it is required to carry out seismic reinforcement at 9 degrees. After comparing the schemes, the reinforcement countermeasures of strengthening lateral stability with herringbone concrete retaining wall are adopted. Its advantages are: ⑦ enhancing the integrity. A box structure consisting of front and rear supports and dam piers, each of which is connected with each other through a supporting wall; (9) The gravity center of the buttress wall is close to the pier, and the seismic inertia force of the pier can be evenly distributed on the buttress wall and transmitted to the gravity dam sections on both sides, thus reducing the influence of pier torsion; ② The upstream retaining wall is close to the big head, which is beneficial to improve the stress of the big head foundation.
When the first phase of dam reinforcement project was about to be completed, a 7-degree destructive earthquake occurred. However, the dam as a whole is as stable as Mount Tai and has withstood the severe test of this strong earthquake. The reinforcement effect was tested. This is unique in the history of world hydraulic architecture.
2. Anti-seismic measures for dam reinforcement after earthquake.
6. After the main earthquake of magnitude1,the aftershocks were strong, the activities were frequent, the observation data of ground spread were few, the trend was unknown, the experience was insufficient, and the dam safety was not lifted, which worried the downstream people. The Central Committee and the Provincial Party Committee attached great importance to it and decided to carry out the second-phase seismic reinforcement of the dam at all costs. After research and discussion by all parties concerned, with the approval of the Ministry of Water and Electricity, it is decided to adopt the combination of 6.5-degree seismic fortification and water level load116m for construction reinforcement; In order to solve the longitudinal stability of each pier and the anti-sliding ability of dam heel stress, the construction task was completed on 1965. After testing and strengthening, the corresponding stability has reached the requirements and the seismic capacity has been greatly improved.
3.108.5m elevation crack prevention measures.
The penetrating cracks before and after the elevation of dam 108.5m are undoubtedly caused by the instantaneous pulse action of earthquake force of magnitude 6. 1, which is a big problem left over and must be taken as preventive measures. To this end, two problems need to be solved first. First of all, the peak part refers to the elevation of this range. Although it is strengthened with reinforced concrete slabs, it has been tested by the earthquake of magnitude 6 in August, and the effect of 1962 is not good, which shows that the elevation of108.5m is a weakness. The elevation of 103m is the changing part of the angle (cantilever) of the dam section, so the elevation of the dam top should be determined as 100- 124m. Secondly, when checking the cracks at elevation 108.5m, it is assumed that the tensile strength of concrete is 6kg/cm2 (allowed
Value is not more than 6.0kg/ bungalow cm), according to the general specification, the maximum dynamic coefficient (a) is 5, so a = 5 is used for calculation. According to the above ideas and scheme comparison, the prevention and control are strengthened by using skewers, with 27 holes for each dam pier and 5Q32 steel beams for each hole, and backfilling and grouting. After treatment, the running state test shows that it meets the previous design requirements and the seismic design code of hydraulic structures in China, and the effect is good.