Subway is an important means of transportation for urban modernization. It has become an effective means to solve urban traffic congestion and reduce noise and air pollution because of its advantages of not occupying ground space, large volume, high speed, punctuality and convenience. At the same time, however, the vibration and noise caused by subway train operation are also common problems in subways all over the world. Internationally, vibration has been listed as one of the seven environmental hazards.
The high-speed running of subway train is the main source of subway vibration, which not only directly affects the comfort and health of passengers in the train, but also causes the forced vibration of ground buildings along the subway. Because most subway lines pass through buildings and crowded city centers, subway vibration will have a certain impact on the environment and the life and work of residents in surrounding buildings. Residents near Xidan station of Beijing subway complain about the vibration and structural noise caused by subway.
The Labor Health Center of the Ministry of Railways investigated the present situation of environmental vibration pollution caused by train operation in residential areas along the railway through field measurement of railway environmental vibration in several typical cities in China. The test results show that most vibration levels within 30 meters from the track center line are close to 80dB.
(1) When the train is running, the impact of gravity load on the track will cause the vibration of the wheels and track structure;
(2) When the subway vehicle is running, the force generated by the simultaneous action of multiple wheels and rails causes the vibration of the vehicle and track structures (including rails, members, ballast beds, etc.). ) (the measured results show that the vibration level at the vibration source can reach103 db);
(3) Wheel-rail structure vibration caused by wheel-rail interaction when the wheel rolls over the rail joint;
(4) Vibration caused by random excitation such as track irregularity and wheel rough damage;
(5) Vibration caused by periodic excitation such as wheel eccentricity.
2.2 Influencing factors
The factors that affect the vibration source when the subway train is running involve vehicles, tracks, ballast beds, tunnels, geological conditions and so on. See table 1 for specific factors:
(1) In a certain speed range, the vibration level of subway tunnel increases with the increase of train speed. Generally speaking, every time the speed increases 1 time, the vibration level increases by about 6dB.
(2) The wheel-rail surface irregularity will increase the vibration level by 5 ~ 10 dB, and the wheel irregularity will increase the vibration level by10 ~ 22dB;
(3) See Table 2 for the research conclusions of different tunnel structure corrections abroad. Under the same geological conditions and the same tunnel materials, the structural thickness increases by 1 times, and the wall vibration can be reduced by 5 ~ 18 dB, while the vibration of concrete single-hole tunnel is lower than that of cast iron or cast steel single-hole tunnel. The vibration of three-hole tunnel structure is lower than that of double-hole tunnel structure, and the vibration of platform structure is the lowest.
3 the spread of subway vibration
Subway vibration is a complex wave phenomenon composed of longitudinal wave, transverse wave and surface wave, and its propagation form is also very complicated. According to the existing research results, the vibration mode in the near field mainly propagates in the form of bending waves, and the vibration mode in the far field mainly propagates in the form of surface waves. The propagation path of vibration is from the track to the track fastener and ballast bed, and then to the tunnel and rock and soil, thus causing the vibration of nearby ground buildings. The main conclusions about the propagation characteristics of vibration are as follows:
(1) When the subway train is running, the frequency distribution of vibration source is mainly low frequency sensitive to human body reaction, and the vibration intensity of 50 ~ 60 Hz is relatively high.
(2) In the process of vibration propagation, the vibration decreases with the increase of horizontal distance from the track. The high frequency component decays rapidly with distance, while the low frequency component decays slowly. Horizontal vibration decays faster than vertical vibration. Therefore, the impact on the ground and buildings is mainly vertical vibration;
(3) The vibration spectrum changes with distance, and the frequency corresponding to the maximum vibration of subway is in the range of 10 ~ 30 Hz;
(4) The influence range of subway trains on the vibration of adjacent buildings shall not exceed 100m, and the vibration of buildings outside this range can be ignored. The specific influence range will vary with the tunnel structure and geological conditions;
(5) The main factors affecting vibration propagation are train speed, tunnel depth and geological conditions. The higher the subway speed, the greater the vibration response of the building; The greater the buried depth of the tunnel, the smaller the influence range; Different geological conditions have different vibration energy dissipation;
(6) The vibration of ground buildings along the line caused by train vibration is closely related to the structural form, foundation type and the distance from the subway line.
4 subway vibration control measures
Vibration problems caused by subway operation can be controlled to some extent through some measures in the initial planning, design and construction stages of subway projects.
4. 1 Control measures in the planning stage
In the initial planning stage, the literature.
(1) light vehicle.
(2) The wheels are smooth. The vibration intensity of vehicles can be effectively reduced by adopting wheel smoothing measures such as elastic wheels, damping wheels and wheel tread grinding.
(3) Use heavy rails and jointless tracks.
(4) Use a disc brake.
(5) Adopt linear motor. Linear motor has many advantages, such as low cost, low vibration, low noise, low energy consumption, low pollution and good safety performance, and it is the development direction of urban rail transit in 2 1 century.
(6) Properly control the running speed of subway trains.
(7) Adopt appropriate elastic fasteners or track dampers. At present, the fasteners commonly used in domestic subways mainly include DTI ~ DTVII, WJ2 and single-toe spring fasteners. These fasteners are mainly used in road sections with general vibration reduction requirements, and most fasteners can reduce vibration by 2 ~ 9 dB. Track dampers are usually used in areas with high vibration reduction requirements. At present, the new shock absorbers commonly used in track shock absorbers include Kelong egg shock absorber, improved Kelong egg shock absorber, sleeper boots and so on. Among them, sleeper shoes have the best vibration reduction effect, which can reach19db; Secondly, the improved Kelong egg reduces vibration by 7 ~ 8db；; The vibration reduction value of Kelong egg is 3 ~ 5dB.
(8) Choose a reasonable track structure to reduce the excitation intensity of vibration source. At present, in addition to the traditional ballasted track structure, there are two kinds of vibration reduction track structures: floating slab track structure and elastic short sleeper track structure (LVT, namely Sony Weir low vibration track). According to the measured data in Germany, the vibration reduction effect of floating slab track structure can be as high as 30dB, but its disadvantage is high cost. Floating slab track structure and elastic short sleeper track structure are adopted in different sections of Hong Kong West Railway, which has achieved good vibration reduction effect and made Hong Kong West Railway one of the quietest rail transit lines in the world. The domestic Guangzhou Metro 1 Line and Line 2 also adopted these two track structures reasonably, and achieved the expected results. However, the cost of these two track structures is relatively high.
4.2.2 Vibration Propagation Path Control
Through the analysis of vibration propagation path and its influencing factors, some vibration isolation or other measures can be taken to reduce the influence of vibration.
The elasticity of (1) foundation greatly affects the sensitivity of buildings to vibration. The greater the stiffness of the foundation, the lower the vibration response of the building. Therefore, we should strengthen the foundation of buildings within the influence range of subway vibration.
(2) The building vibration is related to the building structure, and the vibration attenuation is zero for the light structural frame or foundation; For heavy structural frames or foundations, the vibration attenuation is (15 5) db; For heavy structure buildings, the vibration decreases with the increase of stories, and each story decreases by 1 ~ 4 dB, while the vibration of light structure does not decrease with the increase of building height.
(3) By adjusting the stiffness of the building structure system, the natural vibration frequency of the structure is changed to avoid the phenomenon of * * * vibration caused by the low-frequency coupling between the main vibration source and the building structure.
(4) The control device can be installed on the building structure to reduce the vibration response of the building structure caused by subway vibration.
5 Problems to be studied
Although scholars at home and abroad have done a lot of research work on the vibration caused by subway operation, there are still many problems to be solved. For example:
(1) In the mechanism of subway vibration, the main influencing factors of subway vibration sources are the conclusions obtained through actual measurement, and there is no mature and accurate expression for the relationship between parameters.
(2) At present, due to the limitation of time and space, when studying the influence of subway train vibration, this three-dimensional space problem is generally simplified into a two-dimensional plane problem, and the accuracy of the conclusion cannot be determined.
(3) In the aspect of vibration control measures, the vibration control measures with frequency higher than 20Hz are mature, but the low-frequency vibration is still an unsolved problem. In recent years, the research on urban rail transit in Toronto, San Francisco and Philadelphia shows that the design of vehicle bogies (mainly low-stiffness suspension system and elastic wheels) has a great influence on low-frequency vibration, which deserves further study.
(4) The research on the influence of subway vibration on ground buildings and the response control of buildings will have important engineering and social significance.
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