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Come on, feel the noise

The largest road tunnel construction in Sweden faces tough constraints on noise and vibration. Richard Bennett discovers how engineers are keeping the locals happy.

Blasting tunnels under Stockholm concrete apartment buildings founded on hard rock can make you rather unpopular with the residents.

Vibrations are transmitted directly through the foundations and are experienced in the apartments as an alarming noise coming through the concrete frame.

It can be particularly distressing to pets. 'It's the 5% that cause the problem, ' says Goran Manell, civil engineer with contractor NCC. Five per cent of the residents form a vocal minority and frequently lodge complaints.

But tunnel engineers on Stockholm's SEK6,500 (£460M) Sodra Lanken (Southern Link) project have gone to considerable lengths to avoid upsetting the locals. Up to 70 flats are available to temporarily house families living along the tunnel alignment, and 50 pagers have been distributed to local residents which beep a warning 30 minutes before a blast. 'Actually most complaints have been about drilling noise, which is constant, rather than the blasts, which are relatively infrequent, ' says Manell.

Sodra Lanken forms the first phase of the Vagverket (Swedish National Roads Administration) inner ring road scheme, designed to speed up through traffic coming from the south of Stockholm. The population of Stockholm has expanded dramatically in the last few years, and new housing developments are springing up around the archipelago that forms the centre of the city. Many of Stockholm's existing trunk roads were built in the sixties, and are congested and deteriorating.

Over 16km of tunnels are being constructed on the Sodra Lanken project, mostly by drill and blast in the hard (100200MPa), competent Stockholm granite. Depth of cover over the tunnels ranges between 5m and 50m. The project includes three major interchanges and a number of shafts, crossovers and underground concrete structures. A three lane tube runs in each direction, with cross passages every 100m. The Sodra Lanken tunnel construction has been divided up into separate contracts (see diagram), with major packages awarded to NCC, Skanska and Norwegian contractor Selmer.

All the contractors have to deal with stringent requirements for pre-grouting ahead of tunnel construction. 'We spend a lot of time drilling 25m long grout holes ahead of the face, ' says Selmer site manager Mikael Myhre. 'But the rock quality is generally very high and water ingress through the granite is surprisingly low.' This insistence on grouting dates back to the construction of the Stockholm metro, which lowered the water table, leading to a number of settlement claims for buildings founded on the clay that overlays the granite in some areas. Now, the authorities insist that all rock fissures have to be grouted ahead of tunnelling. On Sodra Lanken, no changes to the water table or settlement have been detected so far.

Selmer is using six drilling jumbos for the charge and grout holes on its contract, including Atlas Copco's new Rocket Boomers. After drilling and grouting the long holes around the periphery of the face, the pattern of 5m long charge holes is drilled in robotic mode, using a pre-programmed drill pattern.

The rounds are then charged with ANFO explosive (ammonium nitrate and fuel oil), which is detonated with a non-electric system. Muck from all blasts goes to an on-site crusher, where it is turned into high quality aggregate for use on the project.

The tunnel roof and walls are then scaled for loose rock, and rockbolted and sprayed with a 20mm layer of fibre reinforced shotcrete followed by a 50mm layer of polymer foam and a 40mm layer of unreinforced shotcrete. This sandwich lining provides a drainage channel between the precast concrete roof slab and drains in the floor.

Vibration limits from blasting have been set at 50mm/s for buildings founded on the granite, and vibration and noise sensors have been installed inside apartments along the tunnel alignment. For rock drilling the noise starts to become annoying at a distance of 100m, and allowable levels have been set at a relatively low 55dB during 7am to 10pm weekdays, with 35dB allowed at night and weekends.

'We actually experimented to see if different drill hole diameters affected noise levels, ' says Manell. But there was no discernible difference. With an average advance rate of 4m/d, residents are expected to hear the noise for a period of four to six weeks as the tunnel face passes below.

Design of the tunnels, which was conducted in house by Vagverket, exhibits the famous Swedish emphasis on safety. 'We want drivers to experience a bright, airy and safe environment in the tunnels, ' says Vagverket project director Per Olof Sahlstrom. Light coloured concrete roof slabs and white lighting will be used to increase ambient light, and the asphalt wearing course will be sandblasted to expose the aggregate, producing a brighter surface.

Markings on the walls and roof slabs will give drivers a sense of speed, and colours have been used to provide easily recognisable orientation points, especially near junctions. The tunnels will be ventilated with a 4m/s airflow generated by the motion of the vehicles, assisted during congestion with roof mounted jet fans.

Fire safety systems in the completed tunnel will incorporate features from the Boston Central Artery project, and there will be 300 cameras to detect breakdowns and accidents.

'Tests have shown a big problem is to get people to leave their cars, ' says Sahlstrom. If a fire occurs, drivers should escape through the adjacent tunnel via a cross passage containing telephones and fire extinguishers.

Rock excavation is currently about 60% complete, with completion expected early next year.

Problems encountered so far have included an area in Skanska's contract where low cover depth and soft ground have necessitated ground freezing.

Jet grouting from the surface has been used in other areas of soft ground, and around sheet piles.

With the completion of Sodra Lanken next year, attention will turn to other sections of the inner ring road, along with a surface level outer bypass motorway to the west of the city, which is currently at design stage. The Osterleden eastern section of the inner ring is expected to be let as Sweden's first privately financed road project. All planned sections of the inner ring road will pass under heavily populated areas: now engineers should be well versed in dealing with the vocal 5%.

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