The North East Line of Singapore's Mass Rail Transit (MRT) will run from the World Trade Centre in the heart of city to the residential districts in the north east of the island, terminating at Punggol.
The extension to the city's system includes 20km of twin bored tunnels and 16 new underground stations. Construction has been split into 12 design and build fixed price contracts.
Contract 705 covers an 1800m long twin bored tunnel section midway along the route and includes two underground stations. These are being built using 'semi top down' construction.
This variation on top down uses slabs to support the excavation walls in the conventional way but as there are many large access points for excavation, the slabs look more like props. This is the first time the method has been used in Singapore.
Main contractor for the £83M, 54 month project is KSM, a joint venture of Kumagai Gumi, SembCorp and Mitsui. Client is the Land Transport Authority of Singapore.
The route is underlain by marine and estuarine sand and soft clay, with very dense silty sand and hard clay at depth - the 'Old Alluvium' formation. Extensive site investigations for infrastructure projects and high rise construction gave a detailed geological picture for the project team.
Robert Benaim & Associates is lead consultant for KSM. Director Brian Bell says that while all the contract bidders proposed using earth pressure balance machines for the tunnels (an Hitachi Zosen machine with compressed air is being used), KSM won the job because it suggested building the two storey underground stations at Boon Keng and Sennet by 'semi top down' construction.
Although tunnelling is being used for most of the route, cut and cover will be used for a 300m section north of Sennet Station to build the crossover and cripple siding for shunting broken down trains.
'It was far too wide an opening for tunnelling,' Bell explains.
In contrast to Boon Keng and Sennet, the other stations on the line are being excavated to the full depth of about 18m before the bottom slab is cast.
This method was chosen to reduce the amount of temporary support needed for the 70m wide excavations. 'By incorporating the roof and concourse slabs as horizontal struts we were able to save thousands of tonnes of temporary steelwork on the two stations,' Bell says.
'It proved to be the best economic option,' he adds. 'It also helps to reduce ground movement and to reduce the risk of disturbance to nearby properties undermined by the excavation.'
The North East Line runs directly under Serangoon Road and Upper Serangoon Road, a busy dual carriageway linking Boon Keng and Sennet to downtown Singapore.
While the road offered an obstruction-free corridor for tunnelling between high rise residential and commercial buildings that crowd either side of the road, it had to be kept open throughout construction.
At least three lanes of traffic have been kept open at any one time, with traffic diverted around the excavation on a deck built up over the corners of the work area. 'There was going to be considerable congestion if we diverted commuter traffic onto the smaller urban roads,' Bell says.
Each station is 225m long, with the base slab 18m down and track running at 14.3m below ground. The roof slab is 3m below ground level. The station boxes are between 50m and 70m wide at the top, following the plan of the roof slab and the concourse level, but narrow at the platform level. The 136m long platform and track section below the concourse level is a box 20m wide and 6m high.
The diaphragm walls forming the outer walls of the station boxes are 32m deep and 800mm wide. They were excavated under bentonite slurry using suspended rope operated grabs.
The walls are built using 6.5m long primary panels followed some days later with secondary closing panels. Joints were made using steel stop ends located at the ends of the primary panels which were removed when the secondary ones were excavated.
Full depth reinforcement cages were lowered into the trench cut by the grab, before a high strength concrete mix was tremied
to displace the bentonite suspension.
The walls allow excavation down to the full depth and Bell explains that by using the roof and concourse slabs as struts, steel bracing was confined to the large openings in the roof and concourse slabs.
The inner wall of the platform below concourse level was supported by a row of 600mm diameter concrete piles reinforced with H piles. These are 1200mm apart and were formed from ground level and the empty bore backfilled with sand.
'The groundwater and soil conditions at this depth will allow the contractor to excavate a metre or so at a time and the soil gap between the 600mm diameter piles to be sealed by gunite before excavating deeper,' Bell says.
In conventional top down work, the roof slab has small access openings built into to it so that once it is cast, mini excavators can start excavating below it. But for these stations, KSM wanted large openings to allow large excavating machinery to be used, speeding up the process.
'This would have been a problem if the roof slab was not required to be 2m thick for civil defence reasons,' says Bell. Under normal circumstances, such a heavy and rigid suspended slab would sag and push down on the piles when excavated beneath, causing rotation at the diaphragm wall supports.
An added advantage of making large openings in the roof slab was to 'substantially' reduce the loads on the plunge columns, says Bell.
The number of columns needed was also reduced, particularly critical for Boon Keng Station where piles were only needed for temporary support as the final structure was built well into a firm clay layer.
But the Sennet Station substructure extends into a soft marine clay and silt layer, so permanent piles were needed to support the plunge columns during temporary work.
The 600mm diameter hollow steel plunge columns supporting the roof had to be accurately positioned in the centre of the 2m diameter pile bores with a tolerance of +/- 75mm in plan. A specially welded hoop cage was lowered into the 10m long casing at the top of the empty pile bore, once the pile had been concreted to the correct formation level.
The guide cage was made vertical and centred before the column was lowered through it and plunged 3m into the top of the pile.
After the plunge column base had set in the pile head, the column was filled with high strength concrete and the empty bore above the pile head backfilled with sand to ground level.
Excavation and casting of the roof slabs follow, allowing excavation down to concourse level before the concourse slab is cast. Openings in the concourse slabs will allow excavation of the track and platform sections of the station boxes.
Once the base slab of the track and platform is cast, the inner separating walls and skin walls to the diaphragm walls exposed in each station will be built. Construction openings in the roof and concourse slabs will then be reinstated and the road above relaid after back filling over the roof slabs.
'Probably one of the most thorough detailing exercises was in determining the most practical and efficient detail for forming the construction joints in the roof slab,' Bell says. 'The slab has five layers of reinforcement for blast resilience, not just two as is more usual. We have used special couplers and a forest of projecting hook bars.'
Work on Contract 705 started in June 1997 and so far the diaphragm walls, plunge columns and bored piles have been completed for both stations.
Excavations under the roof slabs are finished, representing nearly half the total excavated volume of 320,000m3. In the cut and cover crossover section, excavation has started down to the first prop level and 100m of the twin bored tunnels has been driven and lined.