There are six transit projects currently under way in the small island state of Singapore, the two largest of which – the North East Line (NEL) and Changi Airport Line – involve tunnelling (Nov ’99,and Mar ’00).

The US$2.9bn North East MRT Line is 20km long with 16 stations (all underground) and a 27 ha. depot. The work has been let as 12 separate design+construct contracts and completion in 2002 will increase Singapore’s MRT network from 83km and 48 stations to 103km with 64 stations. A total of 11.5km is being bored by TBM, with 8.5km in cut+cover.

The US$500m Changi Airport Line is a 6km city centre-airport link which includes: 1km of cut+cover and 3.5km of tunnels (the longest drives in Singapore), which feed into a new showcase station at Changi Airport. Work began in mid-1998, with completion scheduled for the end of 2001.

Tunnelling in Singapore is mostly in soft soil but the geology comprises very mixed ground with sections of hard rock, including granite inclusions and boulders.

On the largest of the current projects, the NEL, 16 TBMs have been employed, all EPBMs except for two semi-mechanised open face machines which are being used on Contract 708. The EPBMs – two from Mitsubishi, two from Lovat; four from Hitachi Zosen; two from Herrenknecht; and four from IHI of Japan – are provided with air locks for face entry and continuous grouting systems to minimise settlement. The best advance rate so far has been from the Herrenknecht machine operated by Hyundai and Züblin on Contract 706, which has clocked up 126m/week.

According to the LTA, the current projects in Singapore are pushing EPBM technology to its limits, largely due to the varied ground conditions encountered. These demanding strata have caused steep learning curves for all those involved. There is an average 800m between NEL stations and, according to the LTA, a contractor might expect to encounter three or four changes of ground over this distance.

As a result, EPBMs have been provided with discs as well as picks to excavate the hard ground. This is just as well, because on Contract 704, for example, 30m of unfissured granite was faced. In general, fissured material of 50-100MPa has been encountered, with boulders of 80MPa Another important factor is that the water table is around 1.5m deep over most of Singapore.

As the new line passes under urban areas, settlement is of grave concern, particularly for the city’s historic ‘shop houses’ – 50-year-old, 2-storey colonnaded structures which often have no foundations to speak of. To prevent unnecessary damage, 20mm maximum settlement in the soft marine clay was specified by the client, with usually only 5-10mm recorded. This is claimed to be far better than the performance under compressed air seen in the construction of the original MRT in 1980s.

Stations must provide flood protection before tunnels junction through. Machine skins will be left in the ground, tight up to the outside of the station end wall. The TBM will then be dismantled and taken back through the completed tunnel to allow earlier access for M&E contractors.

Precast concrete tunnel linings of 5.8m id are used throughout the NEL contracts, generally having five segments plus key, to form a ring 1.2m wide, or, in the case of two of the contracts, 1.5m. The contractor-designed precast concrete segments all have double gaskets: one EPDM; one hydrophilic. The extrados of each segment is epoxy coated to decrease permeability. Segments are cast in Malaysia by Hume Industries using moulds by CIFORM.

All underground sections of the NEL have been let as design+build contracts. The Changi Airport Line was designed by the LTA, but even design+build contracts have to follow the high level of prescription set by the client.

At present, 19km of the 83km of MRT is underground and, according to T C Chew, director of Projects & Engineering, the NEL follows the trend for infrastructure in Singapore to be placed underground as the escalating value of land released for development easily compensates for the additional construction costs. According to LTA figures, tunnelling costs in Singapore are roughly half those of Hong Kong and only twice those of viaduct construction. This is attributed to a combination of FIDIC forms of contract, rapid payment and low-cost, trouble-free labour.

The next big LTA project is likely to be the first phase of the new Marina Line, for which M&E bids are in. Pre-qualification for the civils packages will be in mid-2000, with awards towards the end of that year for completion in 2005.

A second phase of the Marina Line is also proposed, but the financial case for this has yet to be proved and the government of Singapore does not provide subsidies to support unprofitable lines. Although the government will fund construction of the infrastructure, the operating licence is granted on the proviso that the operator can cover maintenance and operating costs as well as maintain a sinking fund.

By 2010, it is planned that the current network of MRT and LRT will grow to 250km from 140km in 2004, a large proportion of which will be underground. The budget for this huge expansion is an impressive US$2.6bn a year over the next ten years. This development will proceed hand in hand with population growth from 3.7m now to 4.5m, leading to the development of new residential areas. Future plans to 2030 include an Inner Circle Line (incorporating the 13km Marina Line) and the closing of an outer circle line and link to Jurong in the east. These will result in a ‘wheel and spokes’ arrangement for the MRT, providing quick travel from all parts of the island. To cope with this rolling programme, the LTA has established a core road/rail construction programme team of 1600 staff.

Singapore is seen as the showcase for S E Asia so Chew believes that contractors are prepared to put in extra effort here. One result is the development of a non-adversarial construction process with largely open management. This means that techniques such as partnering are not needed, says Chew.

At the time of the visit at the end of 1999, two Mitsubishi TBMs on Contract 703 had completed their 1300m drives and Contract 706 – Boon Keng to Farrer Park – was complete. The Kandang Kerbau to Dhoby Ghaut section was also finished.

NEL – moving north from Outram Park

Outram Park – Contract 710 involves construction of the station at Outram Park; 75m of cut+cover; and 2.6km of bored tunnels. Work started in July ’97 and the JV of Shimizu/Dilingham/Koh Brothers is working towards basic structure completion by Nov 2000. The station is located at the junction of Outram Road and New Bridge Road and the site is effectively a roundabout surrounded by busy roads and buildings. Instrumentation and monitoring of ground movement has been intense, including real time monitoring. So far, movements up to 20mm/drive have been recorded. Systems have been installed for pumping out and recharging groundwater, and the problems of space restrictions and logistics have had to be overcome.

As this is an interchange station, a 9m wide x 3m high link connecting the new concourse to the existing Outram Park Station will allow transfer from the NEL to the East West Line and vice versa.

The station is 188.8m long by 25m at the widest point and is constructed by the bottom-up method: soldier piles were driven 30m into the ground at 1.5m centres. Timber lagging was inserted between piles for the upper 9m of the excavation, while shotcrete was used for the lower 18m. Where the works are close to the operating railway, secant piles have been used to minimise movement. As excavation began, the box was supported by ground anchors and seven levels of struts. The 25m long anchors were used to create a strut-free environment in critical areas and will be removed on completion. Multiple phased road re-alignment was necessary.

Two pairs of running tunnels are also included in Contract 710: 2.1km drives south west to the World Trade Centre (the NEL’s longest drives); and the 400m drives north east to Chinatown Station. Four 6.7m diameter dual mode EPBMs from IHI are being used. Ground conditions are highly variable and challenging, with buried valleys, marine clays and residual soils, neccessitating that the EPBM be used in pressurised mode approaching Chinatown Station. Grout is supplied to the IHI machines by a surface batching system from Bentonite Sanyo of Japan, and is stored in a holding tank on the TBM before being treated with conditioning agents and pumped through pipes within the skin. Spoil is transported from the face by Schöma locos and Müllhauser tipping skips.

The south western drive to the World Trade Centre was 40% complete at the time of the visit, the TBMs running almost side by side. The contractor was aiming for ten rings/shift but had seen up to 13. Six cross passages are being constructed between the tubes at 15m spacing. This is the only part of the NEL where an escape shaft has been built, half way between stations. On the shorter, north eastern drive to Chinatown Station, the machines were operated in closed mode and rates of 7-8 rings/day were being achieved.

The NEL tunnels enter the station box having passed under the existing MRT East West Line with just 4m clearance. An arch of horizontal protective piles 200mm in diameter, 36m long and 1m apart were installed to enable settlement of the operating railway during tunnel construction to be limited to 10-12mm. To the south west of the station box, a 70m wide by 25m long chamber was constructed in cut+cover with a sprayed concrete (SCL) lining. This chamber acted as the launch pit for two TBMs and will accommodate a crossover for the MRT. The 25m long x 9m diameter NATM section is claimed to be one of the largest in Singapore.

At one point on the Chinatown drives, the tunnels pass over the proposed alignment of Singapore’s underground ring road. To provide greater strength in this area, cast iron lining from Kubota in Japan has been installed along a 100m length of 86 rings.

Chinatown (formerly People’s Park) – Contract 709 is being built by a JV of Gammon/Econ Piling and is thought to be the most difficult station on the NEL to build. A 2-level civil defence structure, it lies under a section of Eu Tong Sen Street and New Bridge Road. Work began in Sept ’97 and construction is scheduled for completion at the end of 2000. The station lies beneath the busy Chinatown section of downtown Singapore, with three major roads crossing the site. All roads and pedestrian access must remain open during construction. The station box is 300m x 45m x 22m deep and includes provision for a new interchange station on the proposed Marina Line.

The box was constructed in open cut, with more than 700 x 30m deep secant piles installed, sometimes just 1-2m away from the buildings lining the busy road. Extensive structural monitoring was implemented to ensure as little disruption as possible.

Because the marine clay is so sensitive to changes in water level, the contractor has installed a system of recharge wells and curtain grouting to halt leakage of groundwater into the box. Existing services had to be pinpointed by boreholes and supported during the excavation phase. In all, about 18 months of preparatory works had to be completed before the main excavation could begin.

The station itself is being built by the bottom-up method using 7000 tonnes of horizontal steel struts. From May-November 1999, 260 000 cu m of spoil (60% of it marine clay) have had to be removed through the busy urban centre and 830 00m cu m of concrete poured. About 80% of the huge box is decked over during construction to facilitate traffic movement. Before work could begin, the tidal Eu Tong Sen Canal, which runs along the centre of the street, had to be directed into four 2.1m diameter pipes.

At the time of the visit, around 80% of excavation was complete and permanent works were under way at both ends of the structure. The Hitachi Zosen TBMs from Contract 708 will arrive at the piled wall to the north in a zone of good, S2 material, so no ground treatment will be necessary. At the other end of the station, adjoining Contract 710, the tunnels have clay in the crown and so ground treatment will be required for the IHI machines. Gammon/Econ is responsible for breaking out of the box to make the connection with the running tunnels, the size of the secant piled walls making it impossible for the TBMs to drive into the box.

Clarke Quay Station – Contract 708 calls for construction of a new station at Clarke Quay and 1550m of bored tunnel. It also involves the only significant overburden on the NEL, the only river crossing and the only use of compressed air. The station is being built by a Nishimatsu/Lum Chang/Bachy Soletanche JV close to the Singapore River along Eu Tong Sen Street. The concourse will be 200m x 70m at the widest point, but width at platform level is only 23m.

The station box was built in marine clay by cut+cover, having 30-45m deep, 1.2m thick diaphragm walls. The design includes the foundations for commercial development on top of the station box and, now that the 24m deep excavation has been completed, the station structure is being built by the bottom-up method. The basic structure will be finished by September 2000, ready for the July 2002 opening.

Because of the sensitive nature of the marine clay and the proximity of prime commercial buildings, great care was taken during excavation to minimise ground disturbance, and excavation of the box was carefully scheduled to ensure minimum deflection of the diaphragm walls. Similarly, recharge wells were used to ensure that the groundwater was not disturbed too much. Eventually, maximum settlement of 100mm was recorded immediately adjacent to the diaphragm walls.

At the time of the visit, the base slab had been constructed and about 35% of the concourse level was finished. Critical to the project was that the southern end of the station box should be ready to receive the Hitachi Zosen EPBMs to begin the drive to Chinatown after crossing the Singapore River. The contractor had created a temporary shaft in the station box for launch and recovery of the TBMs, which will later be removed.

The twin running tunnels north to Dhoby Ghaut Station have been constructed from a 16.5m x 20m x 25m deep shaft sunk on the far bank of the Singapore River at River Valley Road. From here, four tubes have been driven, two back towards the station under the river, and two north to Dhoby Ghaut. Two semi mechanised, open face shield TBMs are being used to excavate the northern lines through mixed soil and rock. These 6.5m diameter Nishimatsu-manufactured machines feature two backhoes mounted one above the other. There are three sets of face rams for face support and ‘boxing up’ if necessary. This section involves the only section of drill+blast on the NEL, with drill holes of 1.5m giving a pull of 1.2m/round.

Because of the greater overburden in this area, the thickness of the segmental lining has been increased to 275mm from the 250mm used elsewhere on the NEL. Each drive is 1km long through sandstone of more than 100MPa. The tubes are 7m apart on average. Although tunnels on the NEL mostly lie at the standard 20-30m depth, this section of the line passes under Reservoir Hill with an overburden of 60m – the maximum on the project. According to the contractor, this has not proved a problem and water levels in the hilltop reservoir have not been affected.

When T&T International visited the site, progress on the northern drives had resulted in 350m and 600m of completed tunnels. The programme had called for three rings/day, but, because of better ground, four had been achieved on the northern tube. Close to Dhoby Ghaut Station, the new line runs under the existing MRT line. Site investigation has indicated that this is also an area of very fractured and wet ground. As a result, compressed air of around 1 bar will be used for the last 300m of the drives. Bulkheads were in the process of being constructed.

The 75m long tunnels running south from the shaft towards Clarke Quay Station pass 10m below the bed of the Singapore River and were constructed using a single EPBM. This Hitachi Zosen 6.5m diameter machine is the same as the one used on Contract 705 and was turned around in about two hours for the return drive, using air cushions. Ground investigation had indicated a distinct interface between sandstone and marine clay at roughly the mid point of the river. In the event, the machines passed from one material to the other with no noticeable problems, and only small settlements of the river bed were recorded.

Once these short drives were completed, the machine was transported to the southern end of the Clarke Quay Station to begin driving the line to Chinatown. A second EPBM was delivered in Dec ’99 and the two 350m drives to the south were expected to begin in Feb14 2000. All these drives will be in marine clay.

The tunnels to Dhoby Ghat will be completed in April and the drives to Chinatown Station will be finished by the end of May.

Related Files
Geological profile
Location of the North East Line and the Changi Airport link