HYDRODEMOLITION BEST SOLUTION FOR MUSKÖ TUNNEL REPAIR

2006-01-04 — Hydrodemolition was chosen as the preferred technique to assist in the repair of a sub-sea road tunnel in Sweden joining the island of Muskö to the mainland in the southern Stockholm Archipelago. One of the very latest Conjet Robot 363 MPA high pressure water jetting hydrodemolition machines is playing a key role in the repairs by removing the damaged area of shotcrete lining in the tunnel crown that has been chemically attacked and weakened by water leaking in from above.

The approximate 3,000 m long curved rock tunnel under Stora Skramasö, with its mainland portal at Yxlö, was completed by drill and blast in 1963. The 40 m2 tunnel, which at its deepest point is about 65 m below normal ground level, was strengthened with rock bolts and shotcrete lining predominantly in the crown. In areas of poor quality rock the shotcrete was reinforced with steel mesh. The tunnel was originally intended for military use, but was subsequently transferred for public traffic and operational responsibility passed to the Stockholm branch of the Swedish National Road Authority, Vägverket.

Inspection of the tunnel by Vägverket revealed the shotcrete lining was being attacked and weakened by leaking water. Adhesion was bad and was at risk of collapse and needed replacing. Trials were carried out of removal methods and the hydrodemolition technique of using very high pressure jets of water to cut away the damaged areas of shotcrete was chosen as the process does not cause any damaged to the healthy shotcrete left behind. Hydrodemolition also produces a very rough and uneven surface, which provides a strong bond at the interface for the new shotcrete to key onto.

The contract to restore 480 m of the tunnel’s shotcrete lining, equivalent to an area of 4,800 m2, was awarded to Waterjet Entreprenad AB, Sweden’s largest and one of Europe’s biggest specialist hydrodemolition contractors. To minimize disturbance to traffic the restoration was carried out during night time working. Waterjet used its wheel mounted Conjet Robot 363 MPA running on the road surface with its multi-purpose arm reaching up into the tunnel crown. The 363 MPA Robot was connected to a 550 kW Conjet Powerpack delivering fresh clean water at a pressure of 1050 bar and flow of 234 litres/min to remove a 30mm deep layer of the damaged and weakened shotcrete. The Robot’s hydrodemolition process provided a jagged surface for the new shotcrete lining to key on to and according to Vägverket was a technical and economic success.

For further information please contact:

Lars-Göran Nilsson or Carl Strömdahl

Conjet AB
P.O. Box 507
S-136 25 Haninge
Sweden.

Tel: +46-8-556 522 40
Fax: +46-8- 556 522 60
E-mail: conjet@conjet.se
Internet: www.conjet.com

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