With the increase in demand for residential housing in Globen and towards the E4 motorway the scarcity of land, the City Council recognized the potential benefits of relocating its high voltage power cables underground; redeveloping the corridor of land made available with the removal of the power pylons. Restrictions currently prohibit building within 10 m of the overhead cables and pylons.

The resulting removal of the overhead cables and pylons will open up land for housing up to 30 000 people.

Stockholm Council therefore financed the tunnel at a cost of 335 million Sek ($53 million) to be repaid by land sales to developers. Oden won the contract for the tunnel stretching from the E4 motorway to Skanstull on the outskirts of Globen, including a 500 m extension close the main tunnels midpoint and three 300 m access tunnels.

The GIA Häggloader was considered to be the ideal loading machine for the project's main tunnel 18,2 m? face areas according to project manager, Peter Ahlgren.

The rubber tired digging arm loader has been purpose designed for operations in 3,4 m or 4 m tunnel widths.

A unique digging arm system loads the rock spoils onto a conveyor which fills the haulage trucks at a constant flow of rock.

The conveyor can be raised and lowered to suit the loading height of different haulage vehicles; allowing the 10 HR to operate on ramps as steep as 1:7 going down and 1:5 going up.

Continuous loading directly at the face makes forward and reverse travel unnecessary resulting in reduced tire wear. The units' dozer blades effectively clean the floor area eliminating the need for manual clean up.

The 10 HR offers a loading capacity of 3-4 m?/min and features an electric-powered hydraulic system to ensure good air quality in the tunnel with no diesel exhaust fumes and reduced energy costs.

A Perkins 1104C - 44T rated at 100 hp provides an alternative loading power source for the contractor if required and for travel into the tunnel and on the surface; travelling at speeds of up to 12 km/h.

Tunnel construction
The tunnel, as an elongated-Vee stretches over 6 km from the E4 to Skanstull, and a 500 m extension to Örby, passes through typical Stockholm granite. This according to Peter Ahlgren is "very good with a 'Barton Q-System' factor of up to 40."

Drill and blast techniques are used for tunnelling with 4 m long rods and 48 mm diameter holes on the main tunnel faces. Oden are working to an 8 hour cycle including drill, charge, blast and cleaning with 80 m?/blast.

It takes each GIA loader less than two hours to clear the 80 m? loading two trucks in a five minute cycle to the surface and dumping areas close to each of the three tunnel portals.

Two of the access tunnels at Õstberga and Globen are 300 m long and feature a 1:7 gradient levelling at -40 m to ensure clearance of the numerous road, telecommunication and water tunnels criss-crossing the area.

The third access at Solberga is located close to the E4, with all three providing a total of five working faces for the GIA loaders - one at the Solberga, two from Õstberga and two from Globen.

With staggered deliveries, the first loader was delivered for the Globen access in October 2007, the second in February 2008 for the Solberga face, and the third in April to MIT.

"This is my first experience with GIA loaders" said Peter Ahlgren, adding "with such small cross-sectioned tunnels over a long distance they have proved an excellent solution. Furthermore, the electric power has been a key advantage. Diesel-powered units would have required more air ducts, meaning a bigger tunnel excavation. Additionally, they require just a minimal turning circle every 300 m or so."

Throughout the excavations, Oden Anläggngsentreprenad have been experiencing several sections of very cracked and fragmented granite. Some shotcreting, mostly without steel fibers and a rock bolt per meter has been specified where required.

In one 100 m section between Globen and Õstberga, the rock in the past had been underwater and the contractor is anticipating some problems, which may require a lining.

The project calls for extensive grouting to maintain the specified ground water levels. "We generally aim for four blasts per tunnel and one grouting session every 16 m," said Peter Ahlgren.

Along the tunnel route, five shafts are to be excavated to provide access for the high voltage cables to adjacent transformer houses. "With four located close to highways, residential housing and overhead power cables and pylons, conventional drill and blast methods is not an option," according to Mr. Ahlgren.

Instead the contractor is seriously looking at a new innovative diamond wire cutting technique perfected in Norway. Four holes will be conventionally drilled from the surface to a depth of 22-45 m in the four corners of a 4 m square grid. The diamond saw will then be passed through two of the 76 mm diameter drilled holes cutting through the full 22-45 m before proceeding to the next two holes.

Before the fourth cut - completing the square, the tunnel below will be filled with sand to prevent vibration and provide control of the resulting rock fall.

It is estimated that up to 2 m of rock will fall at a time which is then drilled and blasted, relying on the GIA Häggloaders to clear the resulting rock debris, ready for the next fall.

If successful, the technique will be used for four of the shafts. The fifth, with a more remote location and just 10 m deep will rely on conventional drill and blast.

On completion of Oden's drill and blast contract, the contractor will spend a further six months cleaning the tunnel surfaces, installing cable racks and fitting water and electricity services.

It will then be handed over to the electric company Fortum to install the power cables allowing Oden to finally install small sectional walls and doors to secure the tunnels.

Plans are thought to be currently underway by the Stockholm Council to further extend the tunnel into a ring around the Swedish Capital.

Oden has also recently took delivery of a GIA Charging truck mounted onto a Volvo truck chassis for the Murberg Tunnel project at Härnösand in Northern Sweden. In addition, GIA has also supplied Oden for its ventilation ducts in the Murberg Tunnel.

A second charging truck will also be delivered to Oden in June for a road tunnel project in Stockholm.