In accordance with the new political agreements regarding traffic in the capital, the Southern Link is now under construction. Decisions as to when the other components in the ring road around the city will be implemented have been relegated to the future.
The Southern Link, or Södra Länken, is a 5km long urban motorway running from Årsta Partihallar in the west to Värmdöleden in the east through the southern suburbs situated close to the city centre. Most of this motorway will run in tunnel. It is planned that Södra Länken will be completed by 2004. Most of the tunnel system comprises rock tunnels, the main tunnels consisting of two parallel tubes, one for each traffic direction. In each tubes there are two carriageways for through traffic and one for access and exiting.
Road geometry
In order to promote safety and accessibility, the geometrical design of the tunnel roads includes large curve radii and few gradients. The steepest gradient is 5% on the main carriageways and 6% on the ramps. To avoid noticeable discrepancies in speeds between various vehicle categories, the length of inclines has been limited. This also helps minimise vehicle emissions, especially those discharged from heavy vehicles. On the main carriageways and on ramps with a design speed of 70km/h, the curve radius is at least 300m. On ramps designed for speeds of 50km/h, the minimum radius is 140m.
Tunnel safety
Driving in Stockholm’s future tunnels will be as safe as, or safer than, travelling on a corresponding stretch of the surface road network. In the first place, there will be one-way traffic in all the tunnels, and the tunnel traffic supervision and control systems, as well as the technical installations planned, will also contribute to enhancing road safety.
Traffic supervision, road user information and motorway control systems via the Traffic Management Centre (TMC) will protect drivers against accidents. It is also essential to be able to initiate action immediately when incidents occur. The TMC has a complete television monitoring system and can document incidents and accidents through video recordings. The tunnels will be equipped with automatic incident detectors that signal an alarm when deviations from the normal traffic flow are registered, for example in the event of a vehicle with engine trouble or queue formations. There will be automatic fire alarms in the whole tunnel system and GSM/NMT installations with communication via mobile phone. The fire alarm system will detect a fire larger than 1MW (equal to an engine fire in a private car) within one minute.
There will also be an emergency assistance force to help with anything from engine trouble to evacuation and rescue at the scene of an accident. If an accident occurs that requires evacuation, there are emergency routes every 100m in the main tunnels and every 150m on the ramps. These routes lead to adjoining tunnels. For safety reasons, the main tunnels are designed as independent fire cells that are entirely separate from each other. The evacuation routes are clearly marked by signs and large, distinctively painted door frames which will be labelled and give addresses in the same way as street signs to facilitate orientation for rescue teams. Emergency rooms in the evacuation passages will be equipped with telephones and fire extinguishers.
The TMC, the road assistance rescue team and the motorway control system will provide a smoother traffic rhythm, enhanced road traffic safety and better use of the road network. For traffic control purposes, Södra Länken will be furnished with lane signals and variable route guidance signs. The lane signals can be used to close a traffic lane if there is an incident or if a lower speed is recommended. They are mounted above each lane at 100-225m intervals so that motorists can always see the following signal. In the case of fire or total congestion in the tunnels due to an accident, information can be provided to road users via special signs mounted in the tunnel ceiling, placed at 100-150m intervals.
The electrical power supply is designed so that the vital security systems will function even in the event of a fire in the tunnel. Power is supplied through duplicate cables and transformers. The cable network is divided into sections so that a power failure can only affect a limited part of the tunnel. Fireproof cables are placed under the hard shoulder and cast in concrete. If there is a breakdown in the normal power supply, reserve lighting and guide lights on the safety barriers and emergency exit signs are provided. These are designed with their own special reserve power supply. The reserve lighting in the ceiling functions for 15 minutes after a power failure. The fans in the tunnels are used to control smoke and combustible gases. The wastewater system has a large enough capacity to be able to collect any hazardous liquid spillage quickly.
So that dangerous goods can be transported in the tunnels there must be additional measures to guarantee safety. Different types and amounts of dangerous goods have been classified according to the action to be taken in an emergency. Extra restrictions will be imposed for substances like liquefied petroleum gas. It can, for example, be required that the tunnel be closed to the general public during such transportation or that it can only be permitted outside rush hours. The final choice of system for dealing with dangerous goods will be determined later in consultation with the fire departments in the Stockholm and Näcka municipalities.
Tunnel environment
The tunnels (one in each direction of travel) are ventilated by the movement of vehicles pushing the air ahead, thereby producing longitudinal ventilation. This means that the tunnels in themselves are ventilation channels and that fans will not normally be necessary. It is only when the air circulation in the tunnel tube is insufficient due to congested, slow moving traffic that the fans will be switched on. The fans are installed about every 100m. There are stringent requirements on limiting the noise created inside the tunnel by these fans for environmental reasons as well as out of concern for those working and living in the vicinity.
The air pollution level in the tunnels will be measured continuously. Goals have been set high: during normal operating conditions, the highest level of nitrogen dioxide must always be less than 400 micrograms/m³. While it is the nitrogen dioxide values that are the basic design parameter – i.e. the pollutant trigger factor, carbon monoxide levels will also be monitored. Most air pollution is caused by lorries; the percentage created by private vehicles has decreased in direct relation with the increase in catalytic converters.
In order to avoid a concentration of vehicle emissions outside the tunnel portals, ventilation shafts with fans will be constructed to force air out through ventilation towers. Thanks to the height of the towers, an effective mixture with the surrounding air will occur, thereby reducing emission levels on streets. Calculations have been made regarding the dispersion of the contaminated air and the pollution levels around all tunnel portals and ventilation towers, leading to an optimum height for the ventilation towers. The two towers on Södra Länken will be 20m high.
At certain tunnel portals near buildings, measures will be taken to reduce noise from traffic and fans inside the tunnel to eliminate their impact on the surroundings. Close to the portals, sound absorbing acoustic material will be mounted high on the tunnel walls and in the tunnel crown adjacent to the false inner ceiling.
The environment during the construction period
The Swedish National Road Administration imposes extensive demands on Södra Länken contractors regarding the environmental impact permitted during the construction period. Contractors are required to have an environmental management system for supervising and documenting that these demands are being met. During the construction period, the Swedish National Road Administration will inspect and monitor the environmental undertakings of the contractors.
A fair amount of noise is unavoidable at road construction works.Excavation for access and exits to the Södra Länken tunnels are often located in housing developments, and everything possible is being done to reduce disturbance. For example, sheet-piling is being forced down into the ground under pressure and hammering is only resorted to when it becomes impossible to push the sheet piling all the way down.
Drilling in the tunnels causes two types of noise – that transmitted through the air from the drilling machinery itself and noise transmitted through the bedrock as a result of the steel bit striking the rock. The latter is transmitted to the structural framework of nearby buildings and is experienced as so-called framework noise. The noise level is highest in buildings with foundations directly on the bedrock. Unfortunately, there are few practical means available for reducing framework noise.
The capacity to feel vibrations is a particularly well developed sense in human beings. For our ancestors, this was a question of survival – being able to detect danger in the form of earthquakes or wild animals far enough ahead to protect themselves. Vibration emanating from rock blasting can be very tangible, making people feel ill at ease. The human body can feel vibrations of 0.1mm/s. The limit for blasting in densely populated areas has been set at about 50mm/s in nearby buildings founded on rock. A maximum vibration figure has been given for each building close to Södra Länken. This figure varies depending on the foundation for the specific building and its condition in general.
During the course of the works, vibration levels at the surrounding buildings are checked continuously and the blasting schedule will be changed as required so that permissible vibration values are not exceeded. If the building foundation is on rock, drilling operations will be more apparent. The noise is also magnified if the rock is of high quality – which is generally true in the Stockholm region. The stronger the rock and the sturdier the building foundation, the louder the noise.
The noise caused by drilling in rock can be heard 50m away and sometimes even up to 100m from the work site. The tunnels are driven forward at a speed of about 4m a day. Thus, someone living above a tunnel will hear the noise for two to three weeks before the operations are right underneath. Similarly, it will take another two to three weeks before excavation will have progressed far enough for the noise not to be heard.
The requirements contain rules on the maximum acceptable noise and vibration levels emanating from the construction works as well as on the roads that the contractor may use for transport to and from the work site. The handling of fuels and chemicals as well as any potential water or ground contamination will also be regulated. There are also requirements for staff education in environmental issues and information dissemination to those living in the surrounding areas is to be implemented.
In accordance with the water rights decree, work must be carried out so as to prevent any damage or permanent changes. This is primarily achieved through maintaining a stable groundwater table. Lowering it could potentially affect the natural vegetation and buildings in the tunnel vicinity. In sections where the groundwater table cannot be held stable through normal sealing methods, infiltration is implemented. This means that the groundwater is kept at a constant level by pumping water into the rock through holes drilled from the tunnel level below or from the surface above. Specifications for carrying out the construction works also stipulate that stormwater and groundwater drained from the work sites must pass through a purification plant equipped with an oil separator.
Work on the tunnels is performed in two shifts on weekdays, form 7am to 10pm and can also be carried out in a limited way on Saturdays.
Related Files
Figure 1: Stockholm’s inner city ring road
Figure 2: Route of Stockholm’s Southern Link
