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Pressurised purge

Hydrodemolition – high pressure water – is playing a crucial role in repairs to the corroded concrete lining of the Söderled Tunnel in Stockholm.

The 1.55km dual two lane rectangular box Söderled Tunnel and adjoining dual two and three lane Central Bridge are a vital 2.6km long link in the north-south Stockholm traffic artery. Around 85,000 vehicles a day travel through the two adjacent north and southbound box tunnels.

In 2000, a comprehensive inspection revealed water leakage in the tunnel roof and serious corrosion of the concrete reinforcment. The deterioration was caused by de-icing salt spray from passing traffic.

Stockholm City Traffic Office is installing a new fire protecting sprayed concrete lining in the tunnel soffit and sacrificial precast concrete panels along the base of the tunnel walls. The project is Stockholm’s largest concrete repair project, which is estimated to cost over £98M and take five years to complete.

Swedish specialist contractor E-Schakt, is using hydrodemolition to remove all the damaged concrete lining from the tunnel walls and ceiling, using Conjet hydrodemolition robots.

Hydrodemolition is the only concrete removal method allowed in Scandinavia. According to Conjet, conventional methods of removing damaged concrete using percussive tools like jack hammers can damage reinforcement and aggregate and cause cracking in the underlying, sound concrete. Hydrodemolition uses high-pressure water to remove deteriorated concrete, asphalt and grout and provides a bonding surface for repair material and new coating applications.

To minimise traffic disruption Stockholm City Traffic Office is carrying out the repairs in five, 13 week bursts over five years. The work for each year is scheduled between June and September, when traffic flows are down by about 15% to 20% from normal.

During last summer’s phase, which focused on renovating the 850m northern stretch of the tunnel, E-Schakt prepared 10,000m2 of the tunnel’s damaged concrete surfaces. Traffic was temporarily diverted through the southbound tunnel while the northbound tunnel was closed.

E-Schakt worked two, 12 hour shifts, using its smallest Conjet Robot 322 and larger 362MPA and 364MPA. The 322 and 364 had high reach arms fitted with hydraulically driven rotor heads. The 322 was fitted with a single nozzle, modified to reach below road level and into the channel at the base of the tunnel walls. High pressure water was supplied from three separate Conjet Powerpacks to each of E-Schakt’s three robots at a pressure of around 1,000bar and flow of 200l/minute. The larger two robots were reaching up and working directly above on the tunnel ceiling with the rotor heads removing between 5mm and 10mm of the concrete.

At the same time E-Schakt used its smaller robot, with single water jetting nozzle, to selectively remove the salt damaged areas of concrete along the base of the tunnel walls to a variable of depth of between 30mm and 70mm – in some cases exposing and cleaning the rust from the reinforcing steel. "E-Schakt, with its Conjet Robots, has done a very good job and finished all the hydrodemolition by the middle of July," says main contractor NCC’s project manager Hans QvarstrÖm.

With the hydrodemolition complete, NCC followed on spraying concrete lining to complete the soffit restoration. E-Schakt and NCC completed work on the tunnel by September. They will return next summer for the next phase before completing the restoration and upgrade of the two vital structures by the autumn of 2010.

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