DTL2 works begin at Beauty World Station

The Singapore Land Transport Authority announced on 6 June that tunnelling had begun at the Beauty World Station and associated tunnels on the Downtown Line Two (DTL2) project.

Australia-based McConnell Dowell was awarded the design and construction contract for the two 6.6m od, 1km-long twin bore tunnels from Beauty World to King Albert Park station in March 2009. Full scope of works to be undertaken included two cross passages, station box structure, shaft, recharge wells and diversion structures.

The site is located in the Bukit Timah district of Singapore, some 15km from the central business district. The area contains established businesses as well as shopping centres and residential areas with a densely concentrated population. The Bukit Timah Road runs through this area.

“To divert the road was a complex problem,” said Thomas. “It involved temporary structures that span and remain over the main station box and cross-over structures during construction.

The structures are comprised of steel decking beams and panels and a composite concrete beam structure. As well as providing vehicle and pedestrian diversions, they also provide a working platform at grade level for construction equipment.”

Two Herrenknecht mixed shield TBMs called Artemis and Athena – Greek goddesses of the hunt and heroic endeavour respectively – will be used on the project. The machine diameters are 6.6m. The mixed ground cutterhead was fitted out with backloaded disc cutters, scrapers and reamers. It was set up with four twin ring centre cutters, 24 single ring face cutters and 15 single ring gauge cutters.

“The launch shaft is an annex to the station structure and will provide the interface connection between the bored tunnels and the station,” said Thomas. “The shaft will not only facilitate the installation, assembly and excavation of the bored tunnels but will also provide a temporary staging area for the track work contractor once tunnel and station structures are complete.”

Concrete formed secant piles of 0.9 and 1.2m were used as an earth retaining support system for the shaft. Also, no drilling fluids, e.g. bentonite or polymer, were used during pile installation. This saved space on the small and congested work site.

“Spoil is excavated using a telescopic excavator and conventional muck skips,” said Thomas. “Rock excavation requires initial breaking through controlled blasting, chemical splitting, hydraulic splitting and excavator hammer breaking.

“Geology consists of the alluvial member of the Kallang formation. This typically consists of sands, silts and clay through to cohesive soils including silt, clay, sandy silt and sandy clay.

“This is underlain by Bukit Timah Granite. Bukit Timah Granite is a major, and one of the oldest geological formations in Singapore. The formation is 200-250 million years old and varies from granite to granodorite with various weathering stages from fresh to completely weathered residual soil.”

Bukit Timah Granite has an average ultimate compressive strength of approximately 180MPa.

It was expected that groundwater would be encountered in the Kallang formation. This will be addressed through the installation of a series of recharge wells that will compensate for groundwater loss to prevent consolidation and settlement.

David Sibthorpe, who for McConnell Dowell performed estimating works, secured the contract and was construction manager for the first 1.5 years of the project said, “The equipment selection was a crucial risk mitigation strategy for the mixed face conditions that have been identified. The speed with which the face conditions can/will change is unique.

“Certainly DTL3, which is currently being tendered by many contractors, offers more favourable tunnelling conditions than are present on DTL2. The Beauty World project involves arguably the most difficult tunnelling conditions for the whole DTL2 line.”

It was expected before works began that face conditions would comprise 52 per cent full face soil, 18 per cent mixed ground and 30 per cent full face rock.