Sustainability Issue #3 June 2010

Underground projects for roads and railways are different from other road and rail projects. The great differences are

1. Most of the construction material is already on site
2. The working space is confined
3. This is a relatively new science with a limited number of projects

The fact that most of the construction material is already on site means that the desired properties cannot be selected as in a bridge project. Paradoxically, knowledge of the properties of the construction material in underground projects is often limited. The reason for this is that it is both costly and time consuming to carry out preliminary investigations. But the uncertainties concerning the construction material, the geological uncertainties, may have an impact on both time and cost in the long run.

The confined space imposes special demands on the design of the cross section, services, the safety of persons, ventilation and aerodynamics. It also means that underground construction is a serial system with few points of access, where a disturbance in one activity is easily propagated to several subsequent activities.

The fact that fewer tunnels are built than bridges means that the collected experience is limited. And each individual has the opportunity to participate in only a few tunnel projects during his professional life. Underground construction is also based on sciences that are more recent than for many other civil engineering projects. All in all, this means that all the regulatiions which govern road and rail tunnels are under development and that they are based less on experience than is the case for bridge structures.

Drill and blast

The majority of road and rail tunnels in rock have been, or are being, carried out with the drill and blast method, often with comprehensive grouting prior to blasting. In the Halland project and the City Tunnel in Malmö a tunnel boring machine, TBM, is used. A lining of grout is installed that is used for stabilisation and as a seal, but it is also essential for the progress of the TBM.

The City Tunnel in Malmö consists of a railway line for passenger traffic, and a large proportion of this line comprises two single track tunnels below the central parts of Malmö. The project contains three stations, two of which are  below ground. The City Tunnel is a successful underground project as regaards time and cost, by my experience shows that there is nevertheless a great need of increased knowledge in the areas that characterise underground projects.

Stages in the serial system

There is a need for better and more comprehensive preliminary investigation methods, together with probabilistic calculation methods. This concerns both reinforcement and water disposal.

The great problem is the ability to evaluate how large the uncertainties are and what consequences they may give rise to. Because tunneling is a serial system, it is essential to make an assessment  not only of the consequences of each individual operation, but also to understand how changes impact on all other operations.

I can also state that the confined space has given rise to a lot of questions and revealed a lack of knowledge concerning air quality and aerodynamic effects. In the case of the City Tunnel special measures had to be taken to reduce air velocities on the platforms and to reduce pressure changes. Such issues also occur during the design of the City Line in Stockholm where glass walls will be built between platforms to alleviate these problems.

Safe tunnels

Another constantly recurring discussion is how tunnels and stations are to be designed with respect to the safety of persons. To my mind, this issue is very complex and ranges from technical design to psychological aspects concerning the way people react to accidents in tunnels. For example, the aspiration of the Swedish Rail Administration is that it should be just as safe to travel by train in a tunnel as above the tunnel. In an underground space it is difficult to take countermeasures and to evacuate, while at the samer time a fire in a tunnel causes the tunnel or the station to rapidly become  filled with smoke. More knowledge is needed concerning fire protection of structures, smoke spread, the effects of ventilation, input data for evacuation calculations.

To start with, it was also feared that tunnel boring under the central parts of Malmö could cause disturbance in the form of noise and vibrations which are propagated to the buildings above. In my experience, however, the problem was much less serious than had been envisaged. This suggests that existing forecasting tools can be refined and also used for forecasts regarding rail traffic in the tunnel.

Cost estimates

Apart from the specific underground problems, there is a great need of improved knowledge and to some extent standardisation of cost estimates. Experience regarding the costs of design, preliminary investigations, land purchase, permits, indexation, currency hedge and insurance must  be used as data in future projects.

In spite of the large national investment in underground projects and experiences regarding time and  cost overrruns in several projects, there is very little national finance earmarked for research in this subject. Based on my own experience from projects such as the Bothnia Railway, Ådal Railway, the City Tunnel, the Halland project and the City Railway, I can state that underground projects have special conditions which give rise to unique issues where the level of knowledge ought to be better than it is. In  view of the fact that large investments will be made below ground for both roads and railways in the next few years, the level of knowledge must be enhanced by allocating a larger proportion of national research funds to research in the field of underground construction.

Or is it so that cost and time overruns and disputes are so generally accepted in connection with underground projects that nobody is interested any longer?

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