While tunnels provide benefits, their construction and use can give rise to adverse environmental impacts. It is necessary to understand the potential impacts so that they can be predicted and managed. Failure to do so could compromise the potential benefits of underground construction and may adversely affect the ability of projects to gain consent. Impacts of tunnelling activities can be subdivided into two general groups: those experienced at the tunnel portals or shafts and those remote from the access points. The first group includes airborne noise, fumes and dust, spoil and traffic. The second group may include groundborne noise and vibration, mass ground displacements and changes to the water table. Legal aspects of planning commonly differ between nations, therefore this paper focuses on technical issues that may be useful irrespective of geographical location.
Sources of noise and vibration
Airborne noise is generally the dominant impact from construction activities at the ground surface. Vibration impacts are less common. The impacts of subsurface construction are predominantly from vibration, in many cases it is these effects that impact on the greatest number of people and structures. Principal sources of construction vibration are excavation, dowel and rockbolt installation, and ring erection. Figure 1 illustrates the mechanism of vibration generation from subsurface activities and propagation of groundborne vibration to the surface. Groundborne vibration may affect building structures, interfere with sensitive equipment or disturb occupants. Vibration may also generate noise (sometimes called re-radiated noise) within buildings. Another related effect that occurs during drill and blast is air blast or air overpressure. This is a pressure wave that causes audible effects through its interaction with structures. At very high levels, damage to windows may occur, but more commonly secondary rattling and annoyance (Siskind et al ’80).
Noise and vibration may also arise during the operation of a tunnel. In general, noise and vibration from traffic in road tunnels is of low amplitude and therefore unlikely to be a problem. Exceptions may occur where tunnels are shallow or where sensitive receivers are close by. In particular, if development takes place directly over cut and cover tunnels, consideration needs to be given to the sound insulation properties of the roof slab, structureborne vibration and noise from ventilation systems etc. Airborne noise from traffic on approach roads can be easily predicted, although impulsivity of noise, as traffic emerges from the portals, may increase nuisance. Noise levels close to the portals may be heightened because of the reverberant nature of the tunnel interior. Rail tunnels are the greatest source of operational groundborne noise and vibration effects. However, prediction methods are well developed and numerous options are available for mitigating the impact through track design.
During planning and design it may also be necessary to consider the acoustic environment within the tunnel during operation. For example, in underground stations it will be necessary to ensure that public address and voice alarm systems are audible and inteligible: a review has been given by Anderson and Hiller (’00).
At the earliest stages of a project, numerous considerations are made, starting from the most fundamental: is a tunnel required or sufficiently beneficial to justify the investment? While a tunnel is in many ways a means of mitigating adverse impacts, there will be locations where noise and vibration can have significant influence on the route and design. Predictions of construction noise and vibration at the feasibility stage, are valuable, but only broadbrush due to the limited information available. For example, source locations and potentially sensitive receivers are defined by route options. As the design process progresses the vertical and lateral alignment, construction method and programme become increasingly defined. Noise and vibration assessments can be refined to determine the number and severity of impacts and thereby quantify the need for mitigation. Operational impacts are more easily defined than construction because of the greater certainty of the sources at an early stage.
Noise and vibration criteria
The levels of noise and vibration that cause impact depend on the absolute levels arising, the existing ambient levels and the type of receivers affected. The country in which the works take place also influences the levels considered problematic, both in terms of damage and disturbance (figure 2). Listed in decreasing levels, vibration effects may include:
- damage to buildings and other structures;
- disturbance through perceptible vibration;
- interference with vibration sensitive equipment or processes;
- generation of groundborne noise.
Guidance on acceptable levels of groundborne noise is currently limited. Guidelines by the American Public Transit Association (APTA, ’91), reproduced in Table 1, are commonly used. There is also little guidance on disturbance of sensitive equipment and processes. Work undertaken by Ungar et al (’90) provides possibly the most widely used guidelines as a series of curves of acceptable vibration levels for various activities. These curves have recently been included in the ASHRAE handbook (ASHRAE ’99) and are reproduced in figure 3. Such guidance should be used only for preliminary assessments. Where sensitive equipment is in use and may be affected by vibration, a specialist study should be undertaken to establish criteria appropriate to each case. Figure 3 also includes guidance on levels of vibration that may cause nuisance to people in various environments. Although vibration may be perceptible, it does not necessarily cause disturbance. The nature, duration and number of events in a specified period all influence the public’s tolerance. The vibration dose value (VDV), described in British Standard BS6472 : ’92 is used in the UK to combine the effects of vibration events to establish the probability of complaints. Significant damage to property through vibration from construction works is comparatively rare. Most activities do not give rise to vibration levels of a magnitude that would be damaging. In addition, other factors to consider are the nature of the vibration (transient or continuous), its frequency content and the type and condition of the exposed structures.
Guidance on noise levels, whether airborne or groundborne, may be found in relevant national standards or guidelines. Typical levels at which noise may be problematic are provided by the World Health Organisation (WHO, ’99). Two broad approaches may be used to set noise and vibration limits during construction. Either absolute limits are set that must not be exceeded or a series of trigger levels are agreed at which various actions are taken. Whichever approach is used, it is important that the criteria takes into account the existing levels of noise or vibration and the type of receiver. For example, higher noise levels will be acceptable during the working day in a busy urban environment than those permitted in quiet suburban locations. The situation is similar with regard to setting criteria for vibration. There is often concern that equipment or experimental processes are sensitive to vibration and unreasonably low limits may be requested. It is valuable to all parties to establish the levels of vibration to which such installations are exposed during normal activities within and around the building, such as footfalls, road and rail traffic etc. In this way a more informed criteria can be established.
Prediction of noise and vibration
Predictions of airborne noise from construction are usually based on the sound power level of plant, i.e. acoustic energy emitted by a source. Sound power levels for plant are provided in BS 5228: Part 1: ’97. Other information required to predict the noise level (the sound pressure level) at the receiver are the distance and screening effects of the intervening topography or structures. Where criteria are specified in terms of the equivalent continuous sound pressure level (designated the Leq) or statistical noise levels (such as L90), prediction also needs to account for the duration that the plant is operated for.
The highest levels of vibration in construction are associated with blasting. Descriptions for predicting vibration from blasting are given in New (’86, ’89). Hiller and Bowers (’97) compiled vibration data from a variety of tunnel construction methods. The principal factor appears to be the strength of the geology, rather than the excavation method. Recent data from the Ramsgate Harbour Approach Road tunnel in the UK supports this conclusion (Hiller et al ’01).
Groundborne noise can be audible over a wide corridor centred on the tunnel and is commonly the most widespread acoustic impact of tunnelling works. Noise levels can be predicted from vibration data using an empirical technique developed for underground railway operations by Kurzweil (’79). In addition to subsurface tunnelling works, a wide variety of plant may be required for surface activities. Plant may include piling rigs, compaction plant, etc. The most recent guidance on prediction of vibration caused by such plant is available in Hiller and Crabb (’00).
Where perceptible vibration or groundborne noise is of concern, the type of structure in which vibration is to be predicted must also be considered. Vibration passing from the ground into the building foundation is generally attenuated. The amount of attenuation is dependent upon both the frequency of the vibration and the foundation type. Propagation of the vibration modifies the vibration still further. Again, the type of construction affects the amplification of the vibration. Figure 4 provides an example of vibration levels recorded during construction of the Ramsgate tunnel.
During operation, levels of noise and vibration generally only require consideration for railways. Prediction requires a number of parameters relating to the train (speed, length, unsprung mass, etc), the type of track and track support, and the geology (Greer ’99). Airborne impacts of road or rail traffic using tunnel approaches can be calculated using established methods. In the UK predictions are based on the Calculation of Road Traffic Noise and the Calculation of Rail Noise models (Department of Transport ’95, ’88).
Mitigation
Impacts of noise and vibration from the construction and operation of tunnels may be mitigated through the vertical and horizontal alignment, construction method, location of work sites etc. However, these have to be balanced against other issues, such as geotechnical factors, buildability and cost. There is a requirement for works in the UK to use “best practicable means” (bpm) to limit disturbance. This includes considerations such as reasonably quiet equipment and acoustic shrouds and silencers on plant. While modification of the plant restricts noise at source, it may not be enough on its own to be considered bpm and therefore may not achieve requirements. Additional options available to restrict the impacts of airborne noise include:
- restrictions on working hours;
- public relations – i.e. ensuring the public are aware before commencing work and are kept well informed throughout the works;
- noise insulation or temporary rehousing of residents.
There is no practicable equivalent to providing airborne noise insulation that will reduce levels of vibration or noise in existing buildings. Possibly the most effective approach is to ensure the public are fully aware in advance. In particular, reassurance that while perceptible vibration and audible noise may occur, that the structural integrity of their property will not be compromised. Close public liaison and a visible, perhaps independent, monitoring regime can be greatly beneficial in this respect. Mitigation of operational vibration and groundborne noise from railways may be effected by use of modified tracks. A summary has been provided recently by the Association of Noise Consultants (’01). For road traffic, a smooth running surface will minimise vibration at the source. Provision of a structural break between the road deck and adjacent structures will reduce propagation of vibration into adjacent or overlying structures.
Many environmental benefits can accrue through the use of underground space for infrastructure and other uses. However, there are also significant adverse impacts that can arise and these need to be considered, predicted and managed to maximise the benefits and acceptability of tunnels.
