Lerum-Partille Wastewater Tunnel in SwedenGRYAAB, the Gothenburg regional sewage agency, is responsible for wastewater purification in the region. The company is owned by the Ale, Gothenburg, Härryda, Kungälv, Lerum, Mölndal and Partille municipalities. It treats the wastewater, both discharge water and rainwater, which is purified in Ryaverket, the regional wastewater treatment plant. In December 2006, the municipality of Lerum decided to be connected to the Ryaverket sewage treatment works outside Gothenburg by a tunnel from Hulan in Lerum to the existing tunnel system between Björndammen and Partille. Project backgroundThe 8 km Lerum-Partille wastewater tunnel is part of the comprehensive wastewater tunnel system of the Gothenburg area on the Swedish west coast. Since the early seventies, Gothenburg has already approximately 130 kilometres of such wastewater tunnels, which transport wastewater from the city and the surrounding communities to the treatment plant in Ryaverket. Ryaverket treats wastewater from approximately 800,000 people. The tunnel features a minimum horse shoe-shaped cross-section.GRYAAB, the project's owner, signed a contract in January 2007 with Beton- und Monierbau (BeMo) from Innsbruck, Austria after a 3-month tendering phase. Site supervision is done by Bergab and Karlander Konsult. BeMo's Swedish branch office was founded in spring 2007 purposedly for the project. Construction works started in March 2007 and will be completed by the end of 2010. Visit www.gryaab.se, www.bemo.net, www.bergab.se and www.karlanderkonsult.se Geology and hydrogeologyAccording to the geological prognosis, mainly granite and gneiss of good to very good quality can be expected. Single disturbance zones with more fractured rock are possible. The Q-value system is used by the owner to classify the rock and to determine the final support measures.The tough Swedish regulations regarding possible influence of the original groundwater level are to be respected. The tolerable water egress during tunnel excavation and for the final stage of the tunnel is limited to 9 l/sec, which equals approximately 6.4 l/min and 100 m of tunnel. Not only a significant lowering of the groundwater table is unacceptable because of environmental reasons but also due to the fact that some houses at the surface have so-called energy wells to operate heat pumps and/or are dependent on the water supply through drilled wells. Site installationThe site installation for the project is located in Jonsered, a small village between the two cities of Lerum and Partille, at about half way along the wastewater tunnel. From there, a 320 m 25 sq m access tunnel was driven with a descending gradient of 14%. At the intersection area between the access tunnel and the actual wastewater tunnel, a 286 m-long and approximately 8 m-wide cavern was designed and built, which serves as a railway station (two 900 mm tracks side by side) and as the area where rock material gets tipped into tipping pits. ConstructionBasically, the main sequences of the construction consist of drill-and-blast tunnelling to drive the 2.8 m-wide, 4 m-high tunnel (11 sq m in cross-section), systematic grouting of the surrounding rock to ensure water tightness, mucking of blasted rock by rail-driven equipment, transportation of blasted rock material to the surface by conveyor belts, installation of a V-shaped concrete slab in the tunnel, connection of the tunnel to an existing wastewater tunnel on the Partille side, construction of a concrete portal at the end of the wastewater tunnel in Lerum and a concrete portal at the entrance of the access tunnel in Jonsered, and construction of a measuring station in Partille.Drilling at the face is performed using two new Atlas Copco M2C rigs (equipped with ABC Regular for automated operation of the rig). Site Sensitized Emulsion (SSE) from Orica is used as explosive and gets initiated by non-electric detonators (Nonel). Mini-SSE is used for charging. The underground ventilation SwedVent is supplied by GIA. Visit www.atlascopco.com, www.orica.com and www.gia.seAt the beginning of May 2008 the wastewater tunnel has been driven over a total length of approximately 900 metres in addition to the complete length of the access tunnel (320 m). The main challenge of the project is to deal with the small cross-section in combination with the long tunnel length.Mucking-outThe blasted rock material gets hauled to specially formed mucking niches. This heavy duty job is done using LF6 and LF7 haulers from GHH Fahrzeuge, Germany. In the meantime, a smaller wheel loader (type Liebherr LH510) starts to load the material within the niche into railbound Mühlhäuser wagons. Schöma locomotives are used to haul the full wagons to the tipping pits. A Hydrema M1100 excavator hoists the material then into a crusher. Visit www.ghh-fahrzeuge.de, www.liebherr.com, www.tunnelling-equipment.com, www.schoema-locos.de and www.hydrema.comCrushed material with a maximum corn size of approximately 250 mm is then transported to the surface with a 470 m-long 800 mm-wide conveyor belt supplied by Marti Technik, powered at 160 kW and with capacity for 200 tonnes/hour. The system will transport 210,000 tonnes of rock altogether. Visit www.martitechnik.chSupportGrouting at the face is done systematically after four blasts (maximum distance between two grouting umbrellas is 15.40 m). A grouting umbrella consists of between five and 25 holes with a single length of 20 metres, drilled in a special pattern around the perimeter. The permitted deviation is 4%. The actual grouting is done using Atlas Copco Unigrout pumps which got built on platform wagons. Grouting pressures are between 25 and 30 bar depending on the overburden.Due to good rock conditions, the permanent support measures (consisting of SN bolts and steel fibre-reinforced shotcrete with 30-80 mm thickness) will be installed at a very late stage of the project. If necessary, the contractor has to install initial support measures in case the tunnel drive enters geologically disturbed zones. Click here to view pictures. 20/08.