Servicing Helsinki’s ‘Underground City’

The bedrock under Finland’s capital city already accommodates a vast network of more than 400 tunnels and underground structures. These include everything from utility, water and metro tunnels, to underground shopping malls, parking and storage facilities, leisure complexes and of course Helsinki’s famous Temppeliaukio Church.

With an increasing demand for sustainable development and sound planning of land-use within the area’s red and black granite, the City of Helsinki has decided to capitalise on future construction opportunities by formulating a strategic ‘Underground City Plan’.

This plan is being used to manage the construction of a further 200 underground structures in forthcoming years, including new metro lines and an ambitious road tunnel project, which will connect existing access roads leading into the city.

The first stage of the City’s long-term strategy is to extend and enhance the pedestrian surface area in the centre of the capital, by effectively removing all commercial supply traffic from the streets. This is being achieved via the construction of the US$80M KEHU service tunnel project, which stretches 2.5km across the city from east to west (figure 1).

In June 2006, the City of Helsinki awarded a US$34M lump sum fixed unit-price contract for excavation of the eastern section of the KEHU service tunnel to Helsinki-based contractor Lemminkäinen Infra (formerly Lemcon). Located in the commercial centre of the city, the contract includes a total of 1035m of drill and blast excavation and reinforcement works at depths of up to 35m, under some of the oldest buildings in the capital. Once completed, the tunnel will be used to service all the shops, restaurants, offices and hotels in the area. The contract forms part of a larger project for Lemminkäinen, who are also separately undertaking the construction of a new City Center service and parking parking cavern, as well as other storage structures and tunnel connections for the Kämp Hotel.

Kalliorakennus Corp is constructing the new Stockmann parking cavern, with SRV-Viitoset undertaking the remainder of the KEHU excavation works, including the new Kamppi parking cavern and the City of Helsinki’s other 1.5km long western tunnel contract, which will provide access to the new parking facilities.

Drill and blast works

Mobilising in July 2006, Lemminkäinen began by enlarging an existing underground civil defence shelter, on Vuorikatu, to create an entrance and point of attack at the eastern end of the tunnel alignment. Working two shifts, 6am-2pm and 2pm-10pm, good progress was achieved and at the beginning of 2007 a second face was opened up from another existing tunnel at Mikonkatu, midway along the project.

Automated drilling has been undertaken using a maximum of two Sandvik (Tamrock Axera) T11 3-boom rigs, with the number of holes drilled per round between 80 and 150. Round lengths vary from 2m to 6m depending upon the particular blast limits of each area. “The drill plan is continuously refined as we progress the drives,” explains Marrti Keskinen, development manager for Lemminkäinen. However, it is the rigs’ computer that provides drilling accuracy. “Today operators have a great view of the face, but they don’t really need it with automatic drilling, says Keskinen.

In order to reduce blast vibrations, Lemminkäinen has adopted a new method of controlling the delay times of the Nonel system. The previous system used by the contractor allowed just 25 different delays. Now, by combining the surface delays, 150 different delays can be created with the system. “Therefore we can blast more than 100 holes within the vibration limits,” says Keskinen. In some of the more sensitive areas, where the settlement limit is just a few millimetres, the advance is also split into two faces.

At the time of T&TI’s site visit, at the end of last year, drill and blast operations were progressing well, with the contractor advancing one round per day on each of four faces. In the main, rock conditions had been pretty much as expected, in the area’s red and black granite. Due to the quantity of abrasive quartz in the rock, the Sandvik drill bits used were being changed about every 70m-80m.

Excavation challenges

Amongst the most significant challenges on the project, has been construction through the Kluuvi Cleft, a poor quality zone of gravels and clays that infringes upon the central section of the tunnel alignment.

“Here the tunnel splits into two parallel tubes at that point, to avoid the worst of the zone,” explains Keskinen. However, significant amounts of grouting, along with systematic rockbolting and wire mesh reinforced shotcrete have still been required in the area.

Another key issue has been the strict limitations on blast vibrations. As mentioned, there are numerous buildings located above the alignment. Many are extremely sensitive structures, such as the national art museum, the national library and the university. In addition, there are a number of sub-surface concerns, including the Helsinki Metro, which runs extremely close to the service tunnel (as near as 10m at one point).

During project planning, a risk analysis was undertaken for each individual structure, with vibration limits set for each accordingly (typically in the region of 10m/sec). In some areas acceleration values have also been specified, the smallest of which is 0.25g (or 5mm/second) close to an underground electrical equipment facility. The project team report that so far all of these limits have been met.

Under normal conditions Lemminkäinen is allowed to blast until 10pm. However, despite the late hour for such a residential area, this curfew is somewhat frustrating for the team. With the metro closed in the evenings higher vibration limits can be tolerated, offering a greater opportunity for advance. However, as the team gets near the closest point to the metro tunnels, it will be allowed to blast later into the night. “Most people who live in Helsinki are used to drill and blast works,” says Keskinen. “For the last 30 years one contractor or another has been excavating here, so they are fairly used to the noise.”

Groundwater lowering has been an additional concern during the works. Aged, and often rotten, wooden piles support many of the older buildings in the city, so it is vital that groundwater levels (generally 1m-2m from surface) are not affected during excavation. This has required extensive pre-grouting throughout the duration of the project.

Extension rods have been used to drill three rounds ahead of the face (approximately 24m) and cement grout, supplied by Rescon Mapei, injected. The leakage limits vary depending upon location, but are typically 2.5l/100m. The pre-grouting is reported to have been highly successful so far, with the project team noting very little seepage.

Tunnel support & muck removal

Ranging from 1-7, the tunnel support installed is determined depending upon the rock class encountered. A minimum of 60mm of steel fibre reinforced shotcrete is specified for Type 1, with a maximum of 250mm of steel mesh reinforced shotcrete and systematic rockbolting (3m-4m long, 25mm diameter) for higher rock classes.

When T&TI visited the project, Lemminkäinen reported shotcrete use had been slightly higher than expected, mainly due to the weakness of the rock around the Cleft. However, following scaling and installation of permanent drainage, typically a 80mm thick steel fibre reinforced shotcrete layer was being applied using a Normet rig.

Thanks to a centrally located custom underground concrete plant, with horizontal silos, material transports can be made outside of the city’s rush hours, therefore it is not important if a transport is delayed. Another advantage of this set-up is that the plant is well away from the entrance of the tunnel during the harsh temperatures of the winter months.

By the end of the drill and blast works, 170,000m3 of rock will have been excavated and transported to Helsinki’s Vuosaari harbour redevelopment project, where it will be used mainly for building works. By the end of the project a total of 28,000 trips through the city centre to the harbour are expected, with about 6-10 trucks required per advance. Both mechanical scaling and rock transport is sub-contracted to a local firm.

Drill and blast works on Lemminkäinen’s contract are now almost complete, well ahead of the July 2008 schedule. Other contractors will shortly commence casting of the concrete road deck, and installation of ventilation and M&E systems. The service tunnel is due to open to commercial trade in 2010.

PROJECT FACTFILE

KEHU Service Tunnel
Client: City of Helsinki
Design: Saanio & Riekkola Oy
Contractor: Lemminkäinen Infra (east section)
SRV Viitoset (west section)
Project duration: 2006-2010

ACKNOWLEDGMENTS

T&TI would like to extend thanks to Sandvik Mining & Construction for organising this visit, and Lemminkäinen Infra’s project team for its hospitality and assistance in preparing this article.

A Sandvik Tamrock T11 3-boom rig at work under Helsinki Lemminkainen's Axera T11 is used within the red granite of the area A drill rig operator at work on the service tunnel Pregrouting was adopted to maintain the existing groundwater level, protecting the old wooden piles of buildings above An Axera T11 rig is moved between the project's multiple headings Map showing the current drill and blast works under Helsinki's city centre