The MRT Cross Island Line (CRL) will run directly under Singapore's largest nature reserve, instead of skirting around it.
Announcing its decision yesterday, the Ministry of Transport (MOT) said the direct alignment will see the tunnel going 70m, the height of a 25-storey HDB block, below the Central Catchment Nature Reserve. Initial plans called for a tunnel at a depth of around 40m.
Both options - going directly under or skirting around the reserve - were considered viable after an in-depth study, but nature groups had strongly called for the skirting alignment to avoid affecting flora and fauna in the gazetted reserve near MacRitchie Reservoir.
The MOT said the direct alignment will result in total travel time being six minutes shorter than for the skirting option. For those travelling through the middle segment of the line, the direct alignment will also result in lower fares. The construction cost is expected to be $2 billion less for the direct route.
The ministry added that the direct route will be more environmentally friendly in the long run as it "has lower energy consumption".
The 50km CRL will run from Changi to Jurong, and will serve estates such as Pasir Ris, Ang Mo Kio and Clementi. It is expected to have an initial ridership of 600,000 a day when it is completed by 2031.
The line was announced in early 2013 and preliminary plans showed it running under primary and secondary forests in the Central Catchment Nature Reserve.
Nature groups, alarmed by the potential environmental harm from the construction and operation of an underground MRT line right across the reserve, suggested that it be built along Lornie Road, skirting the reserve.
The Land Transport Authority commissioned a two-phase Environmental Impact Assessment for both alignment options in 2014.
Yesterday, the MOT also outlined measures to mitigate the line's environmental impacts. Tunnelling as deep as 70m below the reserve will ensure work is carried out through hard rock, far from flora and fauna on the surface. No surface works will be done in the reserve, it said, noting that MRT tunnels are typically 20m to 30m underground.
There will be two worksites outside the reserve: one along Island Club Road and another on the western edge of the reserve across from the Pan-Island Expressway.
In a Facebook post, Transport Minister Khaw Boon Wan said the Government had agonised over the alignment as the reserve is "a special part of Singapore", but skirting the CRL around the nature reserve would "cost taxpayers and commuters dearly".
Senior Minister of State for Transport Lam Pin Min told reporters "the environment assessment has shown that both alignments are feasible", but the skirting option would result in longer travel time, additional construction cost and expected higher commuter fares.
"We have therefore decided on the direct alignment," he said.
Asked about a potentially unfavourable effect the direct alignment would have on network capacity factor - a fare adjustment component which takes into account the capacity of a transport service vis-a-vis actual usage - because it goes through nearly 4km of unpopulated area under the nature reserve, Dr Lam said: "That is a separate issue... We will address that subsequently."
Former diplomat Joseph Koh, who is a nature lover, said he would have preferred the skirting alignment, but "it is reassuring that some of the measures we have advocated have been spelt out". These include tunnelling deeper.