57
january-february 2014
international
construction
DRILLING
Unconventional drilling
Environmental impact
Environmental concerns are also a big issue in the drilling sector,
and manufacturers are coming up with new technology to
reduce a drill’s impact on the ground around it – an issue that is
particularly pertinent when drilling in urban areas. New water-
powered drilling technology has been developed by manufacturer
Wassara, for instance, and is in use on a rail project in Stockholm,
Sweden.
A 6 km commuter rail tunnel is being
built beneath central Stockholm – a € 1.7
billion (US$ 2.3 billion) development
dubbed the Citybanan project.
Since the project involves tunnelling
under the historic heart of the capital in
challenging ground conditions, authorities
stipulated that environmentally sensitive
construction techniques must be used.
To limit interference from vibrations
and noise, no tunnel boring machines
could be deployed along the route. In
addition, instead of conventional air-
powered drilling, which could pressurise
the rock formations and put surrounding
buildings and infrastructure at risk,
contractors must use water powered
down-the-hole drills (DTH), which were
supplied by manufacturer Wassara, across
the project.
However, subsidence is a big issue in
central Stockholm as the foundations of
many buildings sit just above the water table. The ground beneath
Stockholm consists mostly of solid bedrock, but in some places
the rock is deeper and covered by a layer of clay – these areas can
be particularly sensitive to groundwater fluctuations.
Contractors therefore have to keep careful track of water
levels and dewatering pumps from Grindex are being employed
together with the Wassara drills to remove waste water.
Citybanan at a glance
Central Stockholm rail project
T
he Citybanan (city line) scheme under central Stockholm, Sweden, is designed to
relieve a long-standing commuter rail bottleneck in and out of the city.
The project, which began in 2009 and is scheduled for completion in 2017, runs
between Tomteboda and Stockholm South and involves construction of two new stations –
City and Odenplan – as well as a 1.4 km rail bridge at Årsta and a 300 m immersed tunnel
under the waters of Riddarfjärden between the city’s central islands of Riddarholmen and
Södermalm.
Citybanan is being built by Swedish Transport Administration in co-operation with the City
of Stockholm, Stockholm County Council and Stockholm Transport.
Six contractors were awarded work along various stages of the project – Peab, Bilfinger,
Strabag, NCC as well as Züblin and Pihl, which formed a joint venture to complete the
immersed Söderström tunnel. However, this is now being completed solely by Züblin after
Pihl filed for bankruptcy in 2013.
Atlas Copco’s new DM 30 II down-the-hole (DTH) rotary
blasthole model, manufactured in Nanjing, China, and
targeted at all non-Tier 4 emissions countries.
Close to the existing Stockholm South railway station, the new
commuter rail tunnel will be driven beneath an existing park and
also beneath a building that is listed as a National Heritage site.
Challenges
This shallow, 331 m part of the tunnel is being built using the
cut and cover method, which involves driving retaining wall
foundations then excavating the tunnel and casting the base slab
before the roof of the tunnel is put in place. When completed, the
building will actually stand on the tunnel’s roof.
Contractor Bilfinger Spezialtiefbau is using Wassara DTH
drills and jet grouting hammers to support foundations work.
Dewatering pumps from Grindex are also on site to keep water
levels in balance.
Wassara drills are powered by high pressure water (up to 180
bar), and use up to 300 litres per minute. No power is lost
through the drill string during operation, and when leaving the
Dando has launched a new cable percussion rig for borehole
drilling – the Dando 1000 MK2. Designed for operation in tight
and hard-to-reach areas, the rig features a collapsible mast
derrick and can drill to depths of 46 m, using 4 in (102 mm)
casing and tooling.
The two approaching tracks to Stockholm
Central Station are overloaded with
freight and commuter trains
The 300 m immersed rail tunnel is
nearing completion under the waters
of Riddarfjärden between the islands
of Riddarholmen and Södermalm.
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