“How close is too close to a tunnel or a tunnel portal?”

March 25th, 2017

Now for the last post in this series.

This one relates to the Premier’s leaked announcement that a tunnel and expressway upgrade will be built to ease congestion to and from the northern beaches via the Spit and Military Road – an infrastructure project known as the Beaches Link.

Welcome news undoubtedly for many long suffering northern beaches commuters who have heard the same line for nearly thirty years and are justifiably cynical about the near split second coincidence between this latest formal announcement and two imminent knife edge by elections on the northern beaches.

More than media reports of resumption notices having been sent to 71 properties, what adds credibility to this particular announcement is the fact that unlike previous ones it is linked to Westconnex and other projects whose construction is well underway.

We think the safest assumption for Sydney property buyers to make is that this project  will be built. Although not the subject of any formal announcement and without a local by election in sight, there is a similar conviction amongst residents in the Sutherland Shire regarding the long touted F6/M1 link between Loftus and Wolli Creek.

The impacts in theory

Tunnels and their usually unfiltered portals built at the beginning and end of tunnels have profound environmental and commercial impacts.

For property buyers and owners, the most succinct summary of the environmental impacts we have found is this one starting at page 31 of the 2014 text book written by  Paola Gattinoni et al, entitled “Engineering Geology for Underground Works”:

“The main environmental problems linked to the construction of underground works are listed below:

  • Triggering of surface settlements, structures collapses and slope instabilities
  • Drying up of springs and groundwater alterations
  • Storage and use of excavated materials
  • Noise
  • Vibrations
  • Pollution of groundwater, mainly after the realization of stabilization works by injections
  • Emission of dust and air pollutants.

As far as air pollution is concerned, apart from the obvious increase of pollutants in the construction phase, in theory a road tunnel is built to reduce traffic, noise and pollutant emissions, in particular in urban areas. Actually, the cost–advantage balance also has to take into account a local increase in those pollutants (in particular, carbon monoxide, nitrogen oxides, suspended solids, dusts and benzene) in the area of a few hundred metres surrounding the tunnel portal.

… The opening of underground works causes a deformation of the soils and rocks around the excavation area. Such deformations may trigger sudden collapses, subsidence and sinking that can damage both the work under construction and pre-existing nearby structures, in particular, if the work is being constructed in developed areas. The consequences and the damages depend on the intensity of the phenomenon and on the vulnerability of the elements on the surface (buildings, rivers, industrial settlements, facilities etc.)… Generally, the response to the opening of the underground work and, as a consequence, the extent of settlements depends on the following elements:

  • Excavation technique
  • Dimension and geometry of the excavation
  • Type of excavated material.”

What emerges clearly from the above is that so infinite are the variables involved, a single and reliable answer to the question posed above is impossible.

The closest we have come to a practical answer is found in the following passage from a paper delivered in November 2011 by David Hiller from Arup, a consultancy,  entitled “The prediction and mitigation of vibration impacts of tunnelling”:

“Tunnels are beneficial in minimising many of the impacts normally associated with linear projects, but there is potential for ground borne noise and vibration to affect people and properties above the entire tunnel corridor. This may be from both the construction and operation of the scheme. Vibration during construction arises not only from the excavation method … but also from other associated construction activities, some of which can be as disturbing, or more so, than the main excavation method. These include drilling (e.g. for blast holes), operation of temporary construction railways or compaction of cast in situ linings. The duration for which vibration impacts occur for these can be significantly longer than for the excavation per se, so the impacts may be potentially longer and hence more significant.

In assessing the risks due to a tunnelling project, it is beneficial to be able to define a corridor outside of which it is not necessary to undertake vibration assessments. The US Federal Transit Administration (FTA; 2006) provides guidance on screening distances for operational railways, but similar screening information is not available for construction works.

..[T] here are practical and financial implications to defining an extensive screening corridor because of the potentially very large number of buildings that would be encompassed in an urban area and which would therefore need to be reviewed.

Based on practical experience, a corridor width of 200 to 250m is appropriate in urban environments”.

The caveat to the last statement of course is that the 200 to 250 m relates only to vibrations and none of the other identified potential impacts.

Local impacts in fact

The notion of a 200 to 250 m corridor (plus, say about 30 m for the width of the tunnel itself) certainly seems to be a lesson heeded by several owners along a similar corridor traced by the Westconnex tunnel beneath Camperdown, Newtown and east Erskineville where a swag of properties is selling for prices below initial price guides.

The secondary impacts mentioned in the last extract are also consistent with these reports .

So, regardless of the Premier’s recently professed indifference to the possibility of a tunnel passing directly beneath her Northbridge home, these are far from academic concerns. We are aware for example of litigation still on foot as a result of damage to terraces in East Redfern and elsewhere allegedly caused by tunnelling for the Eastern Distributor in the 1990’s. In some cases, it can take many years for groundwater alterations such as those mentioned above to resolve as they leave sunken structures in their wake.

Possible local impacts of the Beaches Link

While the Beaches Link might be good news for commuters, it could be a very different story for buyers, residential and commercial property owners as well as businesses along the currently proposed route which runs through Seaforth, Balgowlah, Balgowlah Heights, Northbridge, Willoughby, Artarmon and Cammeray.

Seaforth, Artarmon and Cammeray also look set for heritage destroying portals like those in Haberfield and others including Homebush, Concord, Arncliffe and Kingsgrove.

Furthermore, it shouldn’t be thought that the current absence of a final route provides any comfort. As we speak, bulldozers and tunnelling equipment are ploughing through swathes of Sydney while engineers design tunnels and their portals on the run.

Commercial impacts include a possible drop in business for shops and businesses and their landlords along Military Road in Mosman, Cremorne and Neutral Bay which have only recently begun to find some mojo and are likely also to be threatened by the now potentially redundant B Line under construction.

The “movement economics” discipline tells us that whatever your view might be of cars and parking, both bring business to retail strips. As the experience of such strips in Balmain, Leichhardt, Paddington and Redfern has proven, take the cars, bus stops and parking away and the For Lease/For Sale signs as well as roller shutters soon follow.


“Buyer beware!”