AT THE time of writing the Leighton John Holland joint venture was in the process of completing the second launch of its three NFM slurry TBMs on contract T208 of Singapore’s Thomson Line for the Land Transport Authority, Singapore. The _ rst of three TBMs for the project, named Hera after the Greek Queen of the Gods, was successfully launched on 20 August. The second TBM was named Luna after the Goddess of the Moon. Luna was delivered to site in July and was launched late in September. The third TBM, named Memera, after the Greek Goddess of the Day, arrived in Singapore on the 8 September with assembly planned to commence in late October.

The T208 contract, the award of which was announced in November 2013 for a sum of SGD 383M (USD 275M), covers the line’s Springleaf Station and associated tunnels, and is located in the northern portion of the new 30km-long line. T208 is one of 23 main contracts to deliver the line.

The launches were momentous occasions (according to LTA) heralding the start of tunnelling on what will be the sixth Mass Rapid Transit (MRT) and the fourth fully automated and driverless system line in Singapore. There will be 38 TBMs required for Thomson Line and Contract T208 has three of these. The TBM launch was marked by a special blessing ceremony to provide good luck and safety to the workers throughout the six month excavation.


A cut and cover secant bored pile wall box (‘Tagore’ in the diagram) is to be excavated, measuring 170m long x 28m wide x 20m deep, for launching TBMs and future use as a station.

As for tunnels, twin 1.12km bores from the Tagore box to Lentor Station (Contract T209 – one stop south of Springleaf Station) with the water table 5m to 20m above the crown through 95 percent Bukit Timah Granite grade IV-VI (5 percent mixed face), with three cross passages and extensive ground treatment for residential building projection at one stage near Florrissa Park. This was completed with deep cement mixing and is due to shallow cover beneath residual soils.

In the other direction from the Tagore box towards Springleaf Station, twin 1.07km tunnels with similar water table conditions, but through 50 percent Bukit Timah Granite grade IV-VI and 50per cent rock and mixed face conditions. Three cross passages and two in-line sumps passing beneath the Seletar Expressway (SLE) and 7.4m above the Deep Tunnel Sewer System tunnels (DTSS).

The first TBM, Hera, will execute two of the drives, so there was not a need for a fourth TBM.

At Springleaf Station, a temporary diaphragm wall box with three entrances and one ventilation shaft, and including a launch shaft for an adjacent contractor’s TBMs measures 180m long x 20m wide x 20m deep. It needed extensive recharge well installation. Springleaf Station is in Bukit Timah Granite grade IV-VI with sections of grade III weathered rock and corestone. The client designed all permanent works; temporary works were designed by the joint venture’s temporary works designer.


A standard LTA-designed reinforced segmental lining 5+1 configuration is being used. The tunnels have a 5.8m internal diameter, with segments 275mm thick, ±20mm taper with left and right hand rings, 40mm cover, 60MPa concrete, and three rebar designs for various sections of tunnel. The lightest rebar is used in the rock section, the heaviest near the structures and shallow cover, and the medium weight in the full Bukit Timah Granite grade IV-VI weathered zones.

All segmental lining is epoxy-coated externally for durability. Segments are produced at a casting yard in Malaysia and transported to Singapore. Korea Moulds manufactured the moulds. Ground treatment

The joint venture employed two ground treatment contractors for the project to perform fissure and jet grouting at Springleaf for diaphragm walls and intervention blocks, as well as deep cement mixing for ground stabilisation within the diaphragm walls at Springleaf, also for protection from the cable tunnel beneath, and at the shallow cover section near Florrissa Park mentioned above. There is also jet grouting for the cross passages and launch/reception blocks.

Key challenges

The team expects the main challenges to come from the ground conditions, which vary from 220MPa granite to soft ground and cross passages needing extensive ground treatment. Although other challenges include traffic diversion and local community/working hour controls. The contractor has a full-time Public Relations Officer and gets involved in frequent communication with the stakeholders, including shopkeepers and residents. The TBM naming ceremony was also shared with both the local community and adjacent contractors in the area.

The main list of existing assets to be considered is: the Seletar Expressway (SLE), Deep Tunnel Sewer System tunnels (DTSS), Florrissa Park residential area, Singapore Power cable tunnel (passes beneath Springleaf Station and the T208 tunnels elsewhere pass it over).

Regular meetings are held with the neighbouring T207 contract to ensure things run smoothly