Vibration Reduction at the source
It is of utmost importance to reduce the vibrations at the source by ensuring and keeping a correct track geometry by advanced measurement and machining technologies, by ensuring smooth rail and wheel surfaces without irregularities by the use of optimal rails with controlled low ‘roughness’ growth and advanced braking systems, by ensuring low dynamic impacts by an optimal vehicle suspension, by an optimal design of the track infrastructure and by optimal vehicle specifications.
Moreover, the vibration mitigation measures developed for rail transit systems, conventional rail and high speed rail are not effective for freight trains. Since freight trains have only a primary (rigid) suspension, large dynamic forces can be introduced into the soil at the first vertical rigid body vehicle resonance frequency (around 10-12 Hz) when this vibration mode is excited at the wheel/rail interface by wheel/rail irregularities or by bumps at frogs of crossovers and turnouts.
The importance of adequate wheel and rail maintenance in controlling levels of ground-borne vibration cannot be overemphasized. Problems with rough wheels or rails can increase vibration levels by as much as 20 dB in extreme cases, negating the effects of even the most effective vibration control measures. It is rare that practical vibration mitigation measures will provide more than 15 to 20 dB attenuation. When there are ground-borne vibration problems within an existing rail transport system, the best vibration control measure often is to implement new or improved maintenance procedures. Grinding rough or corrugated rail and wheel truing to eliminate wheel flats and restore the wheel contour may provide more vibration reduction than would be obtainable from completely replacing the existing track system with floating slabs. For freight trains, smooth wheels and rails are the only practical solution to prevent excessive vibrations.
This the reason why large emphasis is put on having and keeping as long as possible smooth rails, correct rail geometry and smooth wheels. Vibration control at the source is a first priority. It is important to note that vibration problems are situated in the frequency range from 0 Hz to max. 100 Hz. The minimum wavelength of the wheel and rail roughness to be considered is then 10 cm when considering a speed of 10 m/s (36 km/h). For higher speeds this wavelength is becoming higher: a wavelength of 25 m in a high speed rail track can generate important vibrations at 4 Hz at a speed of 100 km/h. The soil vibrations at this low frequency travel very far, they can only be controlled by ensuring proper rail track geometry. The wavelengths to be considered for vibrations are thus 10 cm and higher. This is completely different than for airborne noise problems. For vibrations we therefore speak often about irregularities and long waves rather than about roughness which is often associated with shorter wavelengths as is the case in airborne noise problems.
Following techniques have been used by D2S to reduce vibrations at the source:
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maintenance: As discussed above, effective maintenance programs are essential for controlling ground-borne vibration. When the wheel and rail surfaces are allowed to degrade the vibration levels can increase by as much as 20 dB compared to a new or well-maintained system. Some maintenance procedures that are particularly effective at avoiding increases in ground-borne vibration are:
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Rail grinding on a regular basis. Rail grinding is particularly important for rail that develops rail irregularities which in their turn cause impacts and low frequency excitation.
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Wheel truing to re-contour the wheel, provide a smooth running surface, and remove wheel flats. The most dramatic vibration reduction results from removing wheel flats and out of roundness.
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Implement vehicle reconditioning programs, particularly when components such as suspension system, brakes, wheels, and slip-slide detectors will be involved.
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Install wheel-flat detector systems to identify vehicles which are most in need of wheel truing. These systems are becoming more common on railroads and intercity passenger systems, but are relatively rare on transit systems.
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Install wheel geometry measurement devices (e.g. laser based systems installed at entrance of depot) with possibility of detecting out of roundness, difference of wheel diameter of wheels on the same axle, wheel wear, ... .
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Planning and Design of Special Track work: A large percentage of vibration impact from a new transit facility is often caused by wheel impacts at the special trackwork for turnouts and crossovers. When feasible, the most effective vibration control measure is to relocate the special track work to a less vibration-sensitive area. Sometimes this requires adjusting the location by several hundred meters and will not have a significant adverse impact on the operation plan for the system. Careful review of crossover and turnout locations during the preliminary engineering stage is an important step to minimizing potential for vibration impact. Another approach is to use special devices at turnouts and crossovers, special "frogs," that incorporate mechanisms to close the gaps between running rails. Movable points can significantly reduce vibration levels near crossovers. In the 6th FP, the EC research project TURNOUTS has successfully validated solutions to handle this problem. The findings of the TURNOUTS project are described under the ‘research’ heading.
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Vehicle Design: The ideal rail vehicle, with respect to minimizing ground-borne vibration, should have a low unsprung weight, a soft primary suspension, a minimum of metal-to-metal contact between moving parts of the bogie, and smooth wheels that are perfectly round. A limit for the vertical resonance frequency of the primary suspension should be included in the specifications for any new vehicle.
Vibration Mitigation
The purpose of vibration mitigation is to minimize the adverse effects that the project ground-borne vibration will have on residents living nearby. Because ground-borne vibration is not as common a problem as environmental noise, the mitigation approaches are not always well defined. In many cases, very expensive floating slabs (which require maintenance) are used in order not to take any risk: over-dimensioning of the mitigation measures is very common. In order to come to sustainable mitigation measures with a high performance to cost ratio it is necessary to develop innovative approaches to control the impact.
Given that the track and vehicles are in good condition, the options for further reductions in the vibration levels (mitigation measures) fit into one of four categories: (1) changes in the track support system, (2) building modifications, (3) adjustments to the vibration transmission path, and (4) operational changes.
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Special Track Support Systems: When the vibration assessment indicates that vibration levels will be excessive, it is usually the track support system that is changed to reduce the vibration levels. Floating slabs, resiliently supported sleepers, high-resilience fasteners, and ballast mats have all been used to reduce the levels of ground-borne vibration. To be effective, all of these measures must be optimized for the frequency spectrum of the vibration. Most of these relatively standard procedures have been successfully used on several subway projects. Applications on at-grade and elevated track are less common. This is because vibration problems are less common for at-grade and elevated track; cost of the vibration control measures is a higher percentage of the construction costs of at-grade and elevated track; and exposure to the elements can require significant design modifications.
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Resilient Fasteners: Resilient fasteners are used to fasten the rail to concrete track slabs. Standard resilient fasteners are rather stiff in the vertical direction, usually in the range of 40 kN/mm (dynamic stiffness), although they do provide vibration reduction compared to classical rigid fastening systems. Special fasteners with vertical dynamic stiffness in the range of 8 kN/mm will reduce vibration by as much as 15 dB at frequencies above 30 Hz.
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Ballast Mats: A ballast mat consists of a rubber or other type of elastomer pad that is placed under the ballast. The mat generally must be placed on a concrete or asphalt layer to be most effective. They will not be as effective if placed directly on the soil or the sub-ballast. Consequently, most ballast mat applications are in subway or elevated structures. Ballast mats can provide 10 to 15 dB attenuation at frequencies above 40 Hz. Ballast mats are often a good retrofit measure for existing sleeper-and-ballast track where there are groundborne noise problems.
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Resiliently Supported Sleepers: The resiliently supported sleeper system consists of concrete or wooden sleepers supported by an elastomer pad. The rails are fastened directly to the sleepers using standard rail clips. They can be used in concrete trackbeds and in ballasted trackbeds. Existing measurement data indicate that resiliently supported sleepers may be very effective in reducing low-frequency vibration in the 25 to 40 Hz range.
This has been validated in the EC CORRUGATION research project where resiliently supported sleepers for ballasted track have been developed for ballasted trackbeds, consisting of an elastomer pad under the sleeper and a rigid interface plate between the elastomer pad and the ballast. This rigid layer protects the elastomer pad and gives a better load distribution over the complete surface of the elastomer pad (the elastomer is no longer in direct contact with the ballast).
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Floating Slabs: Floating slabs can be very effective at controlling ground-borne vibration and noise. They basically consist of a concrete slab supported on resilient elements, usually rubber or a similar elastomer. Floating slab come in long slabs (semi continuous slabs) and in small slabs variants that e.g. consist of 1.5 m long slabs with 4 or more rubber pads under each slab. Floating slabs are effective at frequencies greater than their first vertical resonance frequency. Floating slabs are designed to have a vertical resonance in the 7 to 20 Hz range. The primary disadvantage of floating slabs is that they tend to be the most expensive of the vibration control treatments and that they require inspection and maintenance. Its performance at higher frequencies is reduced by secondary resonances of the floating slab (which is not a single degree of freedom system).
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Other Marginal Treatments: Changing any feature of the track support system can change the levels of ground-borne vibration. Approaches such as using heavier rail, thicker ballast, or heavier ties can slightly reduce the vibration levels.
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Building Modifications: In some circumstances, it is practical to modify the impacted building to reduce the vibration levels. Vibration isolation of buildings basically consists of supporting the building foundation on elastomer pads similar to bridge bearing pads. Vibration isolation of buildings is seldom an option for existing buildings; normal applications are possible only for new construction. This approach is particularly important for shared-use facilities such as office space above a transit station or terminal. When vibration-sensitive equipment such as electron microscopes will be affected by transit vibration, specific modifications to the building structure may be the most cost-effective method of controlling the impact. For example, the floor upon which the vibration sensitive equipment is located can be stiffened and isolated from the remainder of the building to reduce the vibration. Alternatively, the equipment could be isolated from the building at far less cost.
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