Suomenlinna sea fortress island has a ca. 1,3 km long maintenance tunnel which was built in the early 80's. The tunnel is small with a cross-section of 11,5 m2. The tunnel is used to bring district heating and electricity to the island. Also, water and waste water pipes are located in the tunnel. In addition to technical equipment the tunnel is used as route for emergency units.

After nearly four decades, the tunnel needed renovation. The problems in the tunnel were various. The rock surface of the tunnel was damaged in the 90´s due to breaking of the heating pipe. The heat from the water caused spalling in the near surface of the rock. This resulted in a few cave-ins.

Water leakage was high, ca. 400 m3/d, ca. 20l/min/100m. In addition to high amount of water leakage, there was a high concentration of radon gas in the tunnel air, in the average 8000 Bq/m3. Due to radon gas the tunnel was in a risk to be closed as the concentrations were too high for maintenance work.

An alliance was formed between the client Suomenlinna, contractor YIT and designer Pöyry to renovate the tunnel. The aim of the project was to replace most of the installations, only the heating pipes, water and waste water lines were left intact. After the equipment was dismantled an extensive post-grouting campaign was commenced.

The grouting had two targets. One target was to reduce water ingress in to the tunnel thus decreasing also the radon values. The other target point was to remove local ingress points from the technical sector of the tunnel. The sector includes heating pipes and other HVAC equipment.

The grouting was done mostly by microcement with some chemical grouting. The grouting was designed so that it was doable efficiently in the small tunnel. Also, much thought was given to the sections where grouting was needed. Water samples were taken and air and radon gas in the air was monitored to determine the areas where radon was present in high concentrations. This information guided the design of the grouting order. Due to a limited time frame for execution and the size of the tunnel the order of grouting design was streamlined. Grouting started with a full profile to create a boundary between dry un-grouted section and a section where grouting was needed. After this, systematic grouting was done to the other end of the section. The grouting profile was kept the same throughout the tunnel. At some points only the direction of the drilling was changed due to joint directions.

The result of the grouting was somewhat successful. We were able to reduce the amount of water ingress on the equipment but could not drastically reduce the overall amount of water in the tunnel. However, the reduction of radon gas was very successful. After one year since the tunnel was reopened, the concentration of the radon gas has been ca. 1000 Bq/m3. The result is much better than originally predicted at the start of the project. The reduction is due to efficient ventilation.

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