Singapore has a land mass of approximately 648km². It is a small but vibrant community, situated south of Malaysia, with Indonesia to the east and south. It is low lying, its highest point being 164m above sea level. With a population density similar to London’s, it already has a very sophisticated transport infrastructure, which occupies a land area equivalent to that required for housing. Reclamation has taken place over the last 30 years and is continuing, especially at Jurong Island and Marina East. Marina South was reclaimed earlier.
The geology, from west to east, ranges from metamorphic rocks with a compressive strength of up to 100MPa to Old Alluvium, a lightly cemented sandstone. Also present are soft marine clays. A more detailed synopsis is given in Table 1 and lists the strata from the oldest to the youngest:
In 1969, as part of the state and city planning project, a team from London Transport conducted a six week study and identified a basic network of two railway lines. Construction started on the initial mass rapid transport system in 1983, based mainly on this original plan. One east-west line passes through the main concentration of population along the south side of the island; a north-south line runs through the developed central region. The two lines served up to 60%-70% of the population. These lines – Phases 1 and 2 of the rail system – were constructed between 1983 and 1990 respectively. The latest census results show a population of 3.8m, composed of 3.5m plus 300 000 expatriates.
The 67km route comprised 20km underground, 3km at grade, and 44km above ground on viaduct. There were 42 stations. Tunnelling was carried out mainly through the Kallang and Jurong formations, with little granite encountered. The water table is generally 1-2m below ground level. Rapid changes in geology often result in five to six main interfaces in a single 1-11¼2km tunnel drive, making steady progress difficult. Compressed air was often used, with pressures up to 2.5 bars to control the water at the tunnel face.
Extensive jet grouting was used to consolidate the very soft clays and alluviums and, although this did cause ground heave to the surrounding roads, it greatly assisted tunnelling. Two full face TBMs were used in Phase 2 with some success, although settlements continued after the tunnels were completed. This was probably due to over pressurisation of the face or excessive grouting pressures at the tail.
Singapore has always favoured design+build contracts for tunnel construction, although records indicate that there are a greater number of claims resulting from design+build compared with the Engineer’s design used for the overground work. For Phases 1 and 2 of the Mass Rapid Transit System, the overrun on the total price of the contracts as awarded was less than 4% including all claims and variations.
The Woodlands Lines Extension, begun in 1991, assisted development of the north side of the island and was completed in 1995 to serve two new towns and a Regional Centre which, ultimately, is to provide up to 1.5m² of commercial development. For the first time in Singapore, a bus interchange was placed underground at the sites of the Regional Centre elevated railway stations to provide convenient intermodal transfer. The value of the land released for development by this relocation of the bus interchange was valued at approximately five times the cost of construction of the interchange.
The North East Line
The North East Line is 20km long and entirely underground. Ten kilometres is in bored tunnel, 6km in cut+cover and 4km in stations. The North East Line follows the trend for future lines in Singapore to be placed underground; the escalating price of land thus released for development easily compensates for the additional construction costs. A total of 16 stations and one depot is planned. The out-turn cost is put at US$2.9bn. and the line is programmed for completion in 2002.
The geology to the north is Old Alluvium with Kallang and Jurong formations occurring in the south. There is strong, hard granite at invert level in some drives and difficulties have been experienced at interfaces with soft strata above. A total of 16 TBMs will be employed, 14 of which will be closed face machines. Old Alluvium is an aggressive stratum causing high wear rates on cutterheads. Contractors are attempting to plan head inspections every two to three weeks using compressed air to enable entry for changing cutting tools before excessive wear occurs.
Jet grouting has again been used to protect special buildings like temples and listed structures. The two open TBMs initially passed through rock and some blasting was required. Compressed air facilities will be installed as a precaution as these drives pass under the railway near Dhoby Ghaut. Before driving under the railway at Outram Park, a piped roof arch was constructed as additional protection. Settlement of the railway tunnels was limited to 10-12mm.
The contractor-designed precast concrete segments all have double gaskets. The extrados of each segment is epoxy coated to decrease its coefficient of permeability to groundwater. Stations must be built to a stage where they can provide flood protection before tunnels junction through. Often,TBMs at the end of their drives have had to be dismantled outside the station when a breakthrough was not permitted. One TBM has successfully finished two under river crossings by driving one way, being turned on air bags, and redriving the other way.
Changi Line
The Changi Line is 6km long, with 3.5km underground. The future Terminal 3 is to be built to coincide with the new line. The train service will be part of the existing East/West Line stopping at all stations which will serve the airport employees and local passengers but may not be so convenient for visitors with luggage who wish to travel non-stop to downtown Singapore.
The TBMs for these drives were purchased by the LTA for programme reasons. However, the authority also wished to ensure that high quality machines were used as these have to pass under Runway 1. The machines were provided to the contractor rather than allowing him to provide cheap machines which were not adequate to the task. Although not cheap, the heavy armouring on the cutterheads and other specified measures are paying dividends and the TBMs are performing well; average advance rate is up to 77 x 1.4m rings/week.
Marina Line
The Marina Line currently under construction will serve as a distributor in the downtown areas and will eventually form part of a circle line linking the existing radial lines. Construction will begin in 2000.
The aspiration for Singapore is for 85% of all morning peak hour journeys to be by public transport and a total of approximately 400km of track is planned to be in place by 2030. Further circular railway lines will serve the new regional centres of Tampines, Seletar, Woodlands and Jurong East, which will all also be linked by radial lines directly to the downtown area of Singapore City. Other light rail systems will provide a secondary strategic network throughout the island and smaller people mover systems will serve the new towns.
A total of 3000km of roads is now planned to accommodate increasing car ownership. This is being allowed to rise at 3%/year at present but at reduced rates later as public transport improves. A 40km circular underground road round the central area is to be constructed in a combination of bored and cut+cover tunnels.
The sewerage system is to be completely overhauled. Two main tunnels will collect all the island’s sewage and will deliver it for treatment at two new plants at the east and west ends of the island. The treated effluent will be discharged through seabed pipelines. Discharge into the Johore Straits will cease. The new tunnel network will be constructed in six contract packages: T-01 at 40m deep; T-02; T-03; T-04; T-05; T-06 – a link sewer to the east works 9.7m away. A total of 48km of tunnelling is now already out to tender. The sewer will consist of a primary PCC lining with a secondary mass concrete lining and a final PVC liner. Common utility tunnels are planned to parallel the Marina Line in the the Marina South area.
Two immersed tube tunnels have already been constructed for electricity reticulation. Transmission tunnels will be built from the power stations for a high voltage (400kV) reticulation system.
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