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Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
above, the whole line will be underground. More than half a
million commuters are expected daily on this line.
The three stages will have a staggered opening with stage
one (DTL1) to be completed by 2013 followed by DTL2 in
2015 and DTL3 in 2017. Stage One incorporates the stations in
the existing Central Business District through the New
Downtown to the Bugis district and will have four interchange
stations linking into three different underground lines: North
East Line at Chinatown; the East West Line at Bugis; the Circle
Line at Bayfront and Promenade Stations. Construction of this
line started early in 2008.
2.2.
DTL1 C903 Bugis Station and Associated Tunnels
Contract 903, Bugis Station and Associated Tunnels, was
awarded in November 2008 to Soletanche Bachy as main
contractor, and leader of the Joint Venture with local company
Koh Brothers. It was the last contract awarded on DTL1 and is a
Construct Only contract with the design being undertaken by
Arup on behalf of the LTA. At the time, it was the largest rail
project ever awarded by the LTA at S$582 million (360m euros
at current exchange rates). The new Bugis Station will be the
terminus for Stage One of Downtown Line (DTL1) and was
therefore a critical section as the crossover and overrun was
integrated within this contract.
The Bugis area is an important commercial centre,
historically linked to seaborne trading but now a thriving area
with private and public housing, major shopping and
commercial complexes along with a large private hospital.
Bugis is surrounded by the tourist areas of Little India and
Kampong Glam. The new station was to be build partially
below Rochor Road and adjacent open land, whilst the cut and
cover tunnels were built below the heavily traffic Rochor Road.
Indeed Rochor Road is one of two principal arteries into central
Singapore from the South and the first when coming from the
airport via the busy East Coast Parkway.
The project extends over 600m with four major road
crossings at Beach Road, North Bridge Road, Victoria Street
and Queen Street. The Victoria Street crossing is further
complicated due to the existing Bugis MRT Station running
below the street and across the alignment of the new tunnels. At
Beach Road and Queen Street numerous utilities ran below the
surface.
Figure 2. Schematic plan and cross section of the overall works.
Due to the number of technical and physical constraints it
was necessary to construct nearly all of this entire section as a
cut and cover. At the Southern end the crossover precluded
bored tunneling and beyond the station the physical constraints
going below the existing Bugis Station and the narrow
horizontal footprint meant a cut and cover tunnel was the only
solution until after Queen Street. Furthermore, the connection
tunnels from the new to the existing station would also mean
significant works below Rochor Road.
At either end of the project, bored tunnels would make the
connection to the adjacent stations. At the Southern end a
temporary access shaft was constructed by the neighbouring
contractor to launch their TBM’s towards Promenade Station
and at the North end a receiving shaft was built to reception the
TBM’s coming from Rochor Station, the first DTL2 station.
The excavation depth for the station and tunnels varies along
the alignment but was generally 27m below ground level with
the deepest section at the Queen Street receiving shaft.
The geology in this area of Singapore is essentially 3 to 5m
of fill/sand overlaying 20 to 30m of Marine Clay above Old
Alluvium.
All the major retaining walls were constructed using the
diaphragm wall technique and barrette piles were used for the
foundations. The excavation depth and the stiffness required to
avoid settlement and movement to the surrounding assets meant
that a robust earth retention system was required over the full
length of the project. Due to the very soft clays overlying the
founding Old Alluvium, additional strengthening of the ground
was required prior to bulk excavation to minimise deflection of
the retaining walls. This was carried out using two different
techniques. In areas where utilities and obstructions were
expected, one or two jet grouted slabs were installed to improve
horizontal restraint. In more open areas such as the station, cross
walls were constructed between the two retaining walls built
using the same equipment as the diaphragm walling. Soletanche
Bachy were able to propose a value engineering scheme where a
significant section of jet grouting was replaced by cross walls.
In addition to being a more robust solution, there was a benefit
to the programme as the same equipment could be used
reducing congestion on the working platforms and as
importantly no additional materials testing regime was required
such as the lengthy coring of the jet grouted slab to ensure
compliance with the specifications.
As explained below a three principal construction methods
were necessary to overcome the environment that the physical
constraints imposed over the length of the site. These were
using the Bottom Up and Top Down Methods for cut and cover
construction and mined tunnels where existing infrastructure
precluded open excavation.
3 CONSTRUCTION BY BOTTOM UP METHOD
The Bottom Up method was applied in the central area of the
site between Beach Road and North Bridge Road. This area had
less physical constraints and the Contractor had the space to
move Rochor Road to either side of the excavation during the
construction.
Once the diaphragm wall was constructed the bulk
excavation could commence. Up to 9 layers of steel struts were
installed to support the retaining walls as the excavation
proceeded to a depth of 27m. A total of 20,000 tonnes of steel
struts were installed for the whole site (nearly three times the
Eiffel Tower’s 7,300 tonnes!). Once the excavation reached the
formation level, the base slab is poured and the structure built
from the bottom back up to the roof slab. As the reinforced
concrete structure is built and the walls braced by the inner
structure, the temporary steel struts can be removed.
