Experimental Investigation of Performance of Base Isolated Structures

This chapter describes alternative vibration control measures that could be directed at the source, in the propagation path or at the receiver. Some options will not be appropriate for an existing railway or building. The order in which measures are presented do not imply any preference, although should where appropriate be considered as alternative or supplementary options to control by base isolation. Appendix 4.1 summarises the relative influences for a range of control measures applicable to railway sources, developed by Krüger (1990).

In certain situations, vibration control implemented at the source can be the most effective and economical option. This may for industrial machinery entail isolating the source, undertaking maintenance on the plant, altering the design of plant, procuring new plant or adopting different work practice. For highway sources, the quality of the running surface is critical, and often the main disturbance is associated with vehicles traversing potholes/loose manhole covers, which may be easily remedied, although there is growing use of road humps which give rise to adverse comments (Watts, 1997). The effect of low frequency airborne noise from engine exhaust may be critical in certain situations. As railway sources are often the main reason for base isolation, some specific options for control measures at the source in the context of a railway are described in more detail.

Reduction in Unsprung Mass: It has been shown by the earliest investigators (Mallock, 1902), that a reduction in unsprung mass can provide a significant reduction in wayside vibration. However, an increase in unsprung mass may be beneficial in some case as it can reduce the resonance frequency of the unsprung mass/track resilience, thereby moving the dominant frequency of the groundborne noise into a less audible region (ORE D151 Report 12, 1989).

Resilient Rail Fasteners: Resilient pads can be introduced directly beneath the rail ‘rail pads’ or beneath the baseplate which supports the rail, see Figure 4.1b(i),(ii). They can provide a moderate degree of vibration isolation relative to standard fasteners (~5dB above 30/40Hz), and can allow better rail-head positioning control. However, at low frequencies (<30Hz) ground vibration levels can be 2/3dB higher relative to standard fasteners, and airborne sound levels can be increased (Nelson,1996). The softness selected for the resilient element is limited by factors such as increased rail stress and gauge widening.

Booted Sleepers (or Sleeper Soffit Pads): Involve a resilient pad between the sleeper and the top of the ballast (see Figure 4.1a(ii)) which can reduce track settlement, or directly onto concrete in a non-ballast track (see Figure 4.1b(iii)). Can provide attenuation in ground vibration above 30Hz. There can be an increase in airborne sound levels. Mainly used in tunnels, particularly with non-ballast track, owing to height restrictions. For at grade situations winter weather can cause problems if water accumulates in the resilient boots and freezes.

Ballast Thickness: Where ballast is used, it introduces a resilience, which is affected by its thickness. The ballast is typically 30cm thick but an increase in thickness has not been found to produce a measurable effect on wayside vibration, whilst decrease of ballast thickness below 30cm can lead to a noticeable deterioration. Ballast which has been tamped after a long period can make an improvement, emphasising a benefit of maintenance (Eisenmann et al, 1985). In contrast, ORE (D151 Report 2, 1982) shows a benefit of increasing ballast thickness on a concrete invert, but at low frequencies (6dB reduction at <10Hz due to ballast thickness from 30cm to 75cm).

Ballast Mats: A resilient mat (typically 25mm to 60mm thick) can be placed beneath the ballast (see Figure 4.1a(iii)). The mat can either bear directly on the soil (or sub-ballast), in which case it can be less or totally ineffective when the support modulus of the soil is comparable to that of the ballast mat. An alternative is to form a concrete slab beneath the ballast mat. The mats can provide attenuation of about 10dB above 30Hz for railways at grade relative to standard ballast-and-tie track, with some magnification around the mass spring resonance frequency (<30Hz). In a tunnel the performance may be better, owing to the higher stiffness of the concrete invert for a tunnel, compared to an at grade situation. An increase of ballast thickness on a ballast mat is however beneficial (~4dB improvement going from 30cm to 60cm thickness of ballast on a ballast mat (Eisenmann et al, 1985)). A beneficial effect in wayside vibration was also seen when an increase in mass of the ballast was achieved by laying steel rails on the ballast mat, above which was laid the ballast. The steel rails were however primarily installed to improve track stability and remove a long wavelength distortion that was seen (ORE D151 Report 12,1989). The ballast mat can introduce additional bending stresses in the rail, and ballast stability needs to be considered. The ballast mat may need some protection, (e.g. layer of sand or hard covering) to protect it from the local effect of ballast stones. Load tests on the mat are usually conducted between flat steel plates, but contact between ballast particles and the mat result in a different load-displacement pattern, warranting more appropriate test condition (ORE report DT 217, 1989).

Floating Slab Track: These involve mounting a concrete slab, which may be continuous cast in situ (e.g. Figure 4.1b(iv)), or discontinuous precast modular units, placed on isolators. The fundamental vertical resonance frequency of the concrete slab, with vehicle combination tends to be in the range 8Hz to 16Hz. Some systems are as low as 5Hz. With continuous floating slab tracks, confined air in narrow gaps may need to be considered in the natural frequency calculations. The resilient element may be synthetic/natural rubber or steel coil springs. The slabs usually achieve a mass of 4 to 10 tonnes/m. This is usually selected to at least match the train’s mass and three times the unsprung mass, where mass is considered to be distributed along the train length (Wilson et al, 1983). In order to achieve sufficient mass for the floating slabs in as small a volume as possible, to minimise the impact on tunnel size, and therefore construction costs, high-density concrete can be used (compare a density of 2400kg/m³ for normal weight concrete and heavy aggregate concrete: Barytes, 3500; Magnetite, 4000 & Iron Shot, 5900 kg/m³). There will be longitudinal forces induced by braking or run-up, and transverse forces on curved sections. Resilient shear keys can be provided where necessary to accommodate these forces. The support stiffness for the floating slab track can be increased at junctions to standard track forms, to provide a smooth transition. These systems, which are mainly used for underground railways, can offer a high degree of groundborne noise and vibration control (as much as 30-40dB for high frequencies, (VCE, 1997)). However the design should consider all the resonance frequencies of the floating slab. These floating track slab systems can increase the noise within a tunnel and the train. Constrained layer damping has been used in the past to minimise the sounding board effect of the floating slab, and to damp resonance modes of the deck (Grootenhuis, 1967,1968).

Slab Track: 'Modurail' is an example developed by an isolator supplier in Germany known as Getzner. In this case it consists of traditional concrete sleepers which have sleeper soffit pads bearing on a concrete slab. In the area between the concrete sleepers, outboard of the rails, the sleepers are resiliently connected to each other via additional precast concrete units. The system provides some benefit below 30Hz, attributed to the concrete slab, hence the name Slab Track. There is a benefit

between 60 to 200Hz due to passive isolation. An increase arises between 40 to 50Hz due to resonance frequency of the booted sleeper, and increase above 200Hz due to resonances of the entire modular track system. In general, such systems offer benefits at low frequencies due to the higher stiffness of the sub-structure, and the greater control in the precision of rail fixings. Benefits can also be seen at higher frequencies, when resilient elements are used.

Continuous Rail Support and Sleeper Pitch: Discrete rail supports cause low frequency excitation, which is a function of support passage frequency. Various means have been used to achieve continuous rail support. Edilon provide an embedded rail system where the rail is placed in a groove in the concrete, which is then filled with a cork/polymer material. These methods can show benefits in reducing ground vibration and airborne noise (Müller-BBM, 1997b).

Ground Treatments: The soil beneath the track can be stiffened by a variety of measures. Cement based grouts can be injected into granular soils. For clay/organic soils, cement and lime can be mixed in, or mixed in a way to form lime columns. Vibration reduction can be achieved at low frequencies, particularly when the existing soil is very soft to start with. But there can be an increase in levels at higher frequencies, as the resonance is moved upward (Müller-BBM,1997b; SGI, 1995).

The options for attenuation in the propagation path relate to increasing the distance between the alignment and the sensitive buildings at the planning stage. Alternative options may include trenches, or impedance walls. In practice the depth and length of trenches need to be comparable to the wavelength of the disturbing ground vibration, which can be long, making trenches largely impractical. The screening effect also varies as a function of distance from the trench, reducing as distance increases. Levels on the upstream side of trench may be higher due to local reflections. Impedance walls involve a concrete wall being built into the ground in the path between the source and the receiver. They will only be effective for certain soil types, where a sufficient impedance mismatch with concrete for the wall can be achieved. Impedance walls, like trenches are therefore required to be very deep and long, and may prove uneconomical or ineffective. A variation to the impedance wall (typically vertical), entails the use of ground treatments to stiffen the top layer of soil (e.g. by injecting concrete slurry) to create a wave impedance block, as a means of reducing low frequency ground vibration (see Peplow et al, 1997). Absorber blocks have also been tried in the path (see Chapter 2), but without much success. Ng (1995) has presented a method for evaluating the required length of a barrier in the propagation path using a ‘spreading-angle map’ of train induced ground vibration.

De-Tune Floors: Alter natural frequency to avoid resonance with dominant spectrum from the source. As the human body is sensitive (direction dependent) to certain frequencies, the floor characteristics could be chosen, such that where magnification is unavoidable, it be allowed to occur in a region in which the human body is less sensitive (see Figure 3.5(a) Chapter 3). This generally requires a high floor natural frequency. But it should be recognised that this can be associated with a larger bandwidth (see Chapter 5). This means that a floor with a high natural frequency may be more likely to be excited, if the ground vibration spectrum has a peak within the bandwidth of the floor’s natural frequency. However a low natural frequency floor, although more likely to be of a frequency that is sensitive to the human body, will have a smaller bandwidth, and therefore will only be excited in resonance by frequencies closely tuned to it. There will however remain the possibility of resonances with higher modes of the floor. Low natural frequency floors are becoming common in long span situations, although there is a risk of footfall induced vibration (see ISO 10137:1992). Long span floors typically have low natural frequencies in the region of 4Hz to 7Hz. In some cases a ground floor may be suspended for construction reasons, but solid ground bearing slabs would not result in lightly damped resonance modes. In this way, bungalows with solid floors may be more appropriate at locations close to the source.

Structure Damping: Some structure types offer better damping than others, compare in-situ concrete or load bearing brick in soft mortar to steel construction. Constrained layer damping in floors or structural members may be used where a layer of non-linear material (e.g. products based on bitumen), are interposed in the floor construction. The system operates by inducing shear into the constrained layer material. This shear can only arise when there is motion of the floor. To mobilise this shear damping adequately often requires a high level of motion in the floor, which may be at an unacceptable level, leading to adverse comment. This is therefore only appropriate in certain situations, where perception is not the primary issue (see Grootenhuis, 1970; Nakra and Grootenhuis 1974; Coveney et al 1989).

Decoupling Foundations: The foundations of the building can be taken down into a deeper strata which may be less affected by ground vibration, although the foundations near the surface will need to be decoupled from the soil. This could for example be achieved by mounting the building on deep piles, with the upper shaft of the pile decoupled from the soil, by a sleeve system. This will however reduce the horizontal stiffness of the foundation system, which may require consideration. An alternative may be to site sensitive equipment directly on the ground as opposed to on suspended floors, or place the equipment on a foundation bearing deep in the soil, and decoupled from the building or soil near the surface.

Base Isolation: Where the entire building, or part of it, is mounted on resilient elements (typically rubber bearings, or steel coil springs) placed at or below foundation level, or even higher up in the structure. The building is detailed in a way to avoid any rigid contact with the ground or neighbouring un-isolated structures. The dynamic performance of base isolated structures is not well known, although there are high expectations in Industry (see Chapters 1,5, and Appendix 4.1). Dynamic performance of such structures is the subject of this thesis.

This Chapter has briefly described alternative vibration control measures directed at the Source, in the Propagation Path and at the Receiver, with particular regard to railway sources, as this is the most common reason to adopt base isolation. The most cost effective means of vibration control are likely to be measures implemented at the source. Some measures can be adopted in existing railways, with particular regard to wheel truing and rail grinding. Retrofit of either resilient rail fasteners or sleeper soffit pads are likely to be practical options.

Resilience in train and track form are introduced to provide passenger comfort and reduce wear and tear, but can also provide benefits in reducing groundborne noise and vibration. However the concept cannot be taken too far. Rail-head deflections for operational safety reasons need to be limited, typically 1.5 to 3mm. Alternative measures to introduce resilience in the track form shown in Figure 4.1, cannot be simply added together, as in some cases they may not work (Greer and Gellatley, 1998).

In new alignments, there are clearly opportunities in the planning stage to locate the track in a way that minimises its impact. Consideration of alternative alignments using a basic prediction model may help to identify the most favourable route in the context of minimum environmental impacts, although many other technical and economic factors will come into play.

There is a good deal of scope to implement vibration control measures in the vehicle and track design. Floating Slab Tracks offer a high degree of control, although at a significant cost compared to other options that may be adequate. Given the significant cost implications on a long length of track, extra effort at the design stage may provide a worthwhile economy.

Figure 4.2 shows a comparative view of the insertion loss for some options of control at the source. It relates to the options primarily used for at grade railways and therefore does not include floating slab tracks. There is clearly a broad range to the possible performance that can be achieved, where improvements in one spectral region may be accompanied by disbenefits in another.

Clearly a developer of a site will not have any authority to instigate vibration control measures at the source. Some residents have however formed local action groups and brought enough pressure to bear on Railway Operators, resulting in an investigation of the complaints and in some cases improved maintenance resulting in benefits.

Where existing buildings are subject to an adverse effect from an existing railway, little can be done within the building to reduce the impact. For new buildings on a site exposed to an existing or proposed railway, there are many opportunities for vibration and groundborne noise control, of which Base Isolation should be regarded as one option.

Conclusions

This chapter has highlighted a wide range of control measures available at the source, the limited options in the propagation path, and broad range available at the receiver. Some of the options may be considered to be viable alternatives to Base Isolation or could be considered as supplementary measures to a proposal for Base Isolation.

Chapter 5