All the Southern Link rock tunnels are being pre-grouted to prevent groundwater seepage into the tunnels. This would otherwise lead to a lowering of the water table and have a negative impact on pipes and building foundations. Although Stockholm’s bedrock is mostly impermeable, an extensive pre-grouting programme is being undertaken to seal the tunnels in accordance with requirements set by the Swedish Water Rights Court.
Holes for grouting are drilled to a depth of approximately 20m, which are then checked for position and orientation. Before grouting, Lugeon tests are carried out by pressurising the holes and recording the rate of water loss to the surrounding rock. This information is used as a basis for the grouting sequence. Grouting, which normally takes three shifts, is followed by three rounds of drilling and blasting, which takes one shift/round. Each round advances 5m. Pre-grouting is repeated for the following three rounds.
Careful and controlled blasting is being carried out, especially close to existing buildings, near intersections with other tunnels and in sensitive environments at tunnel portals. Blasting of the rock tunnels is mainly being performed with Dyno Nobel‘s Site Sensitised Emulsion (SSE) system, with Nobel-Prime as primer and non-electric detonators from Nitro Nobel’s Nonel system. However, conventional explosives such as Gurit, Dynamex, Emulite 100 and Prillit (which is an Anfo explosive) will also be employed. This is the first time the SSE system has been used on a production basis in Sweden, although NCC used it in tests on the Arlanda Rail Link project.
The bedrock, which is generally of good quality granite (Q values ranging from 10-40) with occasional fault zones, is being reinforced with bolts and shotcrete. The line of the tunnel was set to ensure that the best quality of rock was encountered, but, despite this, there are places where the rock cover is not sufficient and here the rock and the over-lying soil is being reinforced from above by installing jet grouted columns. Minimum rock cover should be 5m and columns are being grouted along the line of the tunnel in question to form a soil-concrete cap over the rock. The sub-contractor involved in this work is Hercules Engineering, which is working in a JV with Italian company Trevi.
These columns are being installed in two places along the Southern Link, one at Johanneshov and one at Örsta (both in NCC’s area of responsibility). One of these has already been completed and there the minimum rock cover was only 1m. At the other location, the top of the tunnel will run through the moraine which overlies the rock; this zone will be reinforced by jet grouted columns and the tunnel driven.
An additional complication is that there is a fault in the rock at this point that calls for additional care to be taken during excavation of the tunnel. Spiling will be used and the tunnel will be reinforced with steel arches, shotcrete and a cast in-situ steel reinforced concrete lining. Jet grouting the two areas of columns began in November 1998 and was completed in June 1999.
