The Ministry of Transport announced that the Cross Island Line (CRL) will be running directly underneath the Central Catchment Nature Reserve (CCNR) on Wednesday (Dec. 4).

The CRL is the eight MRT line and the longest underground MRT line spanning from Changi to Jurong Industrial Estate.

Its daily ridership is expected to hit a steady state of over a million commuters within a few years.

Confirmed to be running under nature reserve

The decision to run under Singapore's largest nature reserve was not an easy one to make, the ministry said, after six years of public engagement and taking into various considerations.

It argued in a press release announcing this that the direct alignment under the CCNR, weighed against the skirting alignment around the perimeter of the forest, will be shorter, faster and less costly.

However, the CCNR is also the largest nature reserve in Singapore.

Some Singaporeans opposed this plan in the hopes of protecting the relatively undisturbed forested area — home to various fauna and flora in Singapore.

While comprehensive two-phased environmental impact assessments were conducted, with the conclusion that both options are feasible, some remained concerned about the disruption that a direct alignment will bring about to the nature reserve.

Having said that, skirting the perimeter of the nature reserve means a longer tunnel (9km) has to be built, and the additional distance means another six minutes in travel time for commuters, as well as an additional S$2 billion in cost.

The longer skirting alignment will significantly erode the usefulness of CRL for the commuters who may choose to continue using existing lines, buses or cars instead, the transport ministry added.

The relevant authorities spent the past six years consulting with several stakeholders, such as nature groups, advisors and grassroots leaders.

The chairman of the Friends of Bukit Timah Forest community group, Joseph Koh, said that while the skirting alignment is the preferred option for the nature groups, they are aware that the political leadership has to make a decision and biodiversity is just one of the aspects concerned.

Koh added that with the safeguards put in place to mitigate environmental impacts, he finds the direct alignment more acceptable.

Koh is also chairman of the World Wide Fund for Nature (WWF) Singapore and one of Singapore’s foremost authorities on spiders.

Deepest MRT tunnel in Singapore

The direct alignment tunnel will be 4km long in total with half of it cutting across the underground of the nature reserve at a depth of 70m.

That's the same height as a 25-storey HDB block, making it the deepest rail tunnel in Singapore.

A typical tunnel depth for MRT projects is between 20m and 30m below-average ground level but this tunnel goes two to three times deeper as one of the mitigating measures to reduce environmental impacts on CCNR.

This means that the tunnelling will be carried out through hard granite rock and there will be no surface works in the nature reserve.

The two worksites will be located outside the nature reserve — one along Island Club Road and the other one to be on the western edge of the CCNR, across the Pan-Island expressway.

With the decision on alignment made, the advance engineering study of the alignment will commence in 2020.