1. Rail Transit Noise and its mitigation measures
Noise pollution generated by transport is acknowledged to be a major environmental problem. The use of environmental noise barriers, already widespread in Europe and the USA is now becoming increasingly important, changing the face of our road and railway networks and this in large urban areas is regarded as a growing problem of communities and there are various factors that contribute to increase of noise levels in urban areas.
One of the factors is the increase in urban population, which contributes to high traffic volume combined with increased intensity. In most urban areas, the corridors are developed in a close proximity where people live and work, which led to limited space and thus increase the number of high rise buildings. This type of settlement created a dense environment in urban areas, thus increasing the traffic volume.
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Numerous countries have implemented new technologies to control noise pollution in urban areas. For example, low noise generating engines, changes in quality of vehicle tyres and changes in road material and these technologies have proven to reduce the noise on individual scale and as the overall noise pollution in urban areas is still increasing because of increasing traffic volume.
It is of great importance that noise modelling software on multiple noise scenarios and must be able quickly and reliably to turn these models into noise maps and these maps are used to assess and monitor the influence of the noise effects as well noise maps can be helpful in planning and decision-making processes for reducing the noise pollution.
The primary function of noise barriers is to shield receivers from excessive noise generated by rail road traffic. While the onus of mitigating road traffic noise lies with the road projects, noise barriers are considered the most reasonable noise mitigation measures available.
Many factors need to be considered in the detailed design of noise barriers. First of all, barriers must be acoustically adequate. They must reduce the noise as identified in the EIA studies. A proper design of noise barriers would need due considerations from both acoustic and non-acoustic aspects. Acoustical design considerations include barrier material, barrier locations, dimensions and shapes. However, they are not the only requirements leading to proper design of noise barriers.
Also, non-acoustical design considerations is equally important as is often the case, the solution of one problem (in this case noise), may cause other problems such as unsafe conditions, visual blight, maintenance difficulties, lack of maintenance access due to improper barrier design and air pollution in the case of full enclosures or deck over. With proper attention to maintainability, structural integrity, safety, aesthetics, and other non-acoustical factors, these potential negative effects of noise barriers can be reduced, avoided, or even reversed for aesthetic aspects.
Choice of Material – Road side noise barrier is classified as follows.
- Reflective type- transparent and non-transparent
- Absorptive type- sound absorbent materials and possible finishes of absorptive panels
- Earth landscaped mound and retaining structures
- Mixed type- a combination of the above types
One of the key features in all structures is the material ultimately chosen. Despite the above classification, the materials could largely be classified as reflective and absorptive. The determination whether reflective or absorptive or the combination of both can be chosen through are EIA studies.
Cleaning of material
With the passage of time, barrier surfaces may become stained by contaminants such as water-splash from the road surface, airborne grime, bird droppings, honeydew or sap from overhanging trees. Concrete or masonry noise barriers may not need cleaning in certain locations as the surfaces would be washed by rain water and their textured finish may control staining. Flat surfaces, however, will require regular cleaning as contamination will be more apparent and will detract from the appearance of the barrier. High pressure water jets mounted on purpose-built tankers, or hand washing with brushes and low pressure water are suitable treatments.
To completely killing the noise pollution in Indian scenarios like Europe and the USA, the below check list have to be considered in the design stage of the material
1. The intensity for wind load and calculations for acoustic performance.
2. The quality of the materials proposed to be incorporated in the barrier, particularly those, if any, that are not included in the Material Specifications.
3. That the structural grades of materials used are in accordance with those quoted in the calculations.
4. The supply, transportation and storage of noise barrier materials. Workmanship, particularly any pre-installation treatment required and the method of fixing.
5. That the acoustic properties are maintained by the avoidance of gaps, including gaps due to shrinkage or thermal movement.
6. Easy replacement of parts following accidental or willful damage.
7. Security of components and nature of materials used to discourage willful damage.
8. Maintenance access is provided at appropriate location.
Also, the specific considerations on the particular issues like effectiveness, structural integrity, compatibility with the environment, maintenance, safety, ventilation, lighting and installation.
2. Rail Transit Ground-Borne Vibration Transmission and its Control
With the speedy development of urban mass transit system, more and more environmental concerns are focused on the vibrations from underground trains. Vibrations can arise from the passage of trains inside the tunnel and spread through the tunnel and surrounding soil into nearby buildings.
Ground-borne vibration can be a serious concern for nearby neighbours of a transit system route or maintenance facility, causing buildings to shake and rumbling sounds to be heard. In contrast to airborne noise, ground-borne vibra-tion is not a common environmental problem. It is unusual for vibration from sources such as buses and trucks to be perceptible, even in locations close to major roads. Some common sources of ground-borne vibration (other than train) are buses on rough roads and construction activities such as blasting, pile-driving and operating heavy earth-moving equipment.
Vibrations in buildings associated with rail network operations can cause dis-turbance and complaint in a similar manner to noise. It needs to be considered at the infrastructure planning stage as it is difficult to mitigate retrospectively.
The vibration of the transit structure excites the adjacent ground, creating vibra-tion waves that propagate through the various soil and rock strata to the founda-tions of nearby buildings. The vibration propagates from the foundation through-out the remainder of the building structure. The maximum vibration amplitudes of the floors and walls of a building often will be at the resonance frequencies of various components of the building.
The below figure shows the propagation of ground borne vibration into buildings
Characteristics of Vibration Signal.
Amplitude → Frequency → Phase → Orbit
Types of vibration pick up
Proximity Probe → Velocity pick up → Accelerometer
Criteria for selection of above pick up
-Shaft Vibration Measurement
-Key Phaser Marker
-Shaft Centre Line Position
-Best suited for 1 to 500 Hz.
Velocity Pick up
-For bearing and structural vibration
-Best suited for 10 to 1000 Hz.
– For high frequency range
– Best suited for 1000 Hz onward
Even though the vibration is considered at the infrastructure planning stage and still vibration persists in the underground corridors means, the following may be the reasons
• Pipe pulls
• Shaft catenary / bearing loading
• Unequal flow path clearances
• Shaft bow
• Oil / steam whirl
• Deviated operating parameters
• Defective bearing / assembly of bearing
• Vibration transmittance from other source
• Gear inaccuracies
• Casing distortion
Factors that influence ground-borne vibration
One of the major problems in developing accurate estimates of ground-borne vibration is the large number of factors that can influence the levels at the re-ceiver position. The physical parameters of the transit facility, the geology, and the receiving building all influence the vibration levels. The important physical parameters are as below.
- Operational and Vehicle Factors: This category includes all of the parameters that relate to the vehicle and operation of the trains. Factors such as high speed, stiff primary suspensions on the vehicle, and flat or worn wheels will increase the possibility of problems from ground-borne vibration.
- Guide way: The type and condition of the rails, the type of guide way, the rail support system and the mass and stiffness of the guide way structure will all have an influence on the level of ground-borne vibration. Jointed rail, worn rail, and wheel impacts at special track work can all cause sub-stantial increases in ground-borne vibration.
- Geology: Soil and subsurface conditions are known to have a strong in-fluence on the levels of ground-borne vibration. Among the most im-portant factors are the stiffness and internal damping of the soil and the depth to bedrock. Experience with ground-borne vibration is that vibration propagation is more efficient in stiff clay soils, and shallow rock seems to concentrate the vibration energy close to the surface and can result in ground-borne vibration problems at large distances from the track. Fac-tors such as layering of the soil and depth to water table can have signifi-cant effects on the propagation of ground-borne vibration.
- Receiving Building: The receiving building is a key component in the evaluation of ground-borne vibration since ground-borne vibration prob-lems occur almost exclusively inside buildings. The train vibration may be perceptible to people who are outdoors, but it is very rare for outdoor vi-bration to cause complaints. The vibration levels inside a building are de-pendent on the vibration energy that reaches the building foundation, the coupling of the building foundation to the soil, and the propagation of the vibration through the building. The general guideline is that the heavier a building is, the lower the response will be to the incident vibration energy.
Wayside vibration is important factors in the design of new transit track or retro-fit of existing track. All too often, vibration is ignored until well into the design phase, at which point incorporation of the most cost-effective solutions may not be possible. Successful vibration controls require consideration of both the track and the vehicle as a system, because the interaction of the wheel and the rail is responsible for the bulk of wayside vibration impacts. Hence, the vibration control provisions should be included in track design to avoid impacting wayside communities.
Vibration can usually be held to acceptable levels at reasonable cost with ap-propriate design and maintenance provisions, especially if the vehicle and track are considered as a system rather than as separate, independent components. For example, expensive track vibration isolation systems might be avoided where vehicles with low primary suspension vertical stiffness are used, whereas vehicles with high primary suspension stiffness might produce vibration that might require a floating slab to isolate the track- an expensive proposition. The choice of vibration isolation provisions depends on vehicle dynamic characteristics, and the track and vehicle design teams must coordinate their designs during and after the early stages of the project. Mitigation could involve considerable expense, weight, space, or special procurement. Late consideration of vibration isolation may preclude some treatments simply because insufficient time exists to obtain them or to implement design changes.
The following steps are considered to be resolving the vibration Problem
- Accurate measurement of vibration data
- Collection of operating parameters
- Study of history of vibration behaviour / overhauling reports / recurring problem faced
- Interaction with O & M personnel
- Collection of detailed vibration behaviour at various conditions of opera-tion like no load run, run up, run down, with and without excitation, part and full load operation etc.
- Vibration analysis to narrow down the reasons of high vibration
- Formulation of action plan (short term / long term)
- Implementation of action plan
- Response of machine after implementation of action plan
- Fresh vibration analysis if prolong the problem persists
- Implementation of new action plan
The cycle of vibration analysis and implementation of action plan shall be con-tinued till the vibration problem is satisfactorily resolved.
Vibration control provisions
Numerous methods for controlling ground-borne vibration include continuous floating slab track, resiliently supported two-block ties, ballast mats, rubber pad link with wheels, tire-derived aggregate (TDA), resilient direct fixation fasteners, precision rail, alignment modification, low-stiffness vehicle primary suspension systems, and transmission path modification. Achieving the most practical solu-tion at reasonable cost is of great importance in vibration mitigation design with the below factors of maintainability, ease of inspection, and cleanliness.
The main vibration mitigation consideration is as below.
- Mitigation, subject to feasibility, where vibration impact of 25% or more is predicted.
- Mitigation performance required in range of 2-13 dBV minimum attenua-tion, at frequencies as low as 25 Hz.
- Installation of 400 mm deep ballast and 40 mm thick continuous resilient ballast mat expected to achieve required vibration isolation in zones where vibration impact is identified.
Possible vibration mitigation at source
Track measures − minimizing sharp curves to reduce wheel squeal, rail grinding, welding to smooth discontinuities, lubrication, use of soft rail pads, and re-location of signals or turnouts to minimize impacts on sensitive receivers.
Rolling stock measures − wheel truing, on-board wheel lubrication, use of disc brakes, dampening of wheels, use of resilient wheels, wheel vibration absorbers, low-squeal brake blocks and using rolling stock that meets environmentally acceptable vibration.
Besides above measures, the below measures can also be considered for vibra-tion mitigation to the acceptable level.
- Increasing the elasticity of track superstructure
- Eliminating the running surface discontinuitie
- Regular maintenance of the rail running surface
- Regular wheel re-profiling
- Selecting the appropriate type of rail vehicle
- Reducing the speed of rail vehicle
A.Lakshmi Narayanan, (Chief Environmental & Sustainability Expert Representing to MMRDA), Unit No.3 & 4A, The Centrium, 3rd Floor, Phoenix Market City,LBS Road, Kurla(W),Mumbai400070,India. LinkedInProfile He posses very good experience on Environmental Health, Sustainability, Occupational Health, Renewable Energy Technologies in the various overseas countries.
He started his professional career with M/s. Geo-Miller India Private Limited (German Collaboration) where he had an opportunity to carry out the pilot plant bench scale for the chemical factory mother liquor treatment introducing micro-organisms in the reactor and achieved for 99 % efficiency in the treatment.
Subsequently, He was switched over to Sri Ram Institute for Industrial Research where he handled Flue Gas Desulphurization technique in Madras Refineries Limited (MRL).
He has also worked with The Energy and Resources Institute (TERI) and carried out examination of water quality / environmental sanitation and capacity building in Swajal villages of Uttar Pradesh and worked on sensitizing and capacity building of village women on water resources management and environmental sanitation.
While working with Reliance Infrastructure, He was handling with Compliance to Ministry of Environment and Forests to obtain Environmental Clearance (EC).
Presently, He is representing Mumbai Metropolitan Region Development Authority (MMRDA) on system approaches policies of Environmental Policy, Sustainability in Motion Policy, Energy Management Policy, Water Policy, Waste Management Policy, Quality Policy, Solar Policy etc. including CDM initiation with UNFCCC (United Nations Forum Climate Change on Convention).