The future Cross Island MRT Line that will stretch from Tuas to Changi can either run under the Central Catchment Nature Reserve or go around it.
Both options are feasible with adequate mitigation measures, said the Land Transport Authority (LTA), which has completed a major study of the potential impact of construction works on the environment and residents. Skirting the reserve would make for a longer route and is also the more expensive option.
If the final decision - likely to be made by the end of next year - is to build under the reserve, tunnelling will be done 70m underground. This is the deepest any MRT tunnel will go, in a move by the authorities to further mitigate environmental impact on the reserve. Initial plans had called for a tunnel under the reserve at a depth of around 40m. The deepest tunnel today is at 43m.
LTA said: "Based on the report, both underground alignments are feasible, and the residual impacts are largely (negligible to) moderate." It added that no decision has been made on the alignment, but it would take all steps to mitigate potential environmental impact.
An independent panel of advisers engaged by LTA said the skirting alignment would result in more engineering challenges compared with the direct alignment.
The Cross Island Line (CRL), Singapore's eighth MRT line, is projected to have an initial daily ridership of 600,000, before growing to a million in the future. The first phase of the line is expected to be completed by 2029 and will span 12 stations from Changi to Sin Ming.
Nature groups have said that building the line under the nature reserve would damage it. But residents in the area said the option to skirt around the nature reserve would affect them. The Government has also said the skirting option could cost an additional $2 billion.
LTA's report yesterday stated that the option to build a direct alignment would create a 4km route, with 2km under the reserve. This would be built 70m below average ground level - roughly the height of a 25-storey Housing Board block. It would also require the construction of two work sites and proposed facility buildings outside of the nature reserve.
This route would allow trips between the as-yet unnamed last station of the second phase of the CRL and Bright Hill station, the first station from the first phase of the line, to be completed in five minutes.
Mitigation measures would reduce the impact on the ecology and biodiversity in the Central Catchment Nature Reserve to negligible levels, said LTA.
At the work sites outside the nature reserve, the impact would range from negligible to moderate.
Meanwhile, the option to build a skirting tunnel would create a 9km route. This would be built at 45m below average ground level, a safe level for existing buildings. It would require the construction of three work sites and facility buildings in open spaces away from residential areas.
The skirting route would result in a longer travelling time of 11 minutes from Bright Hill to the unnamed next station. LTA said that based on the planned route for the skirting, it would not be feasible to build an additional station along this 11-minute stretch.
The construction in this case would cause residents issues such as ground-borne vibration and ambient noise. But these can be mitigated, said LTA.
Nature Society Singapore president Shawn Lum said engagement with LTA on the issue is ongoing.
He added: "Finding a way to carry out vital infrastructure work without diminishing our precious natural heritage remains important common ground to all of us."
Thomson resident Anthony Oei, 85, who has lived in the Yew Lian Park area since 1965, said he was "very delighted" to hear that the construction might not affect his neighbourhood. "At my age, having to move house is no joke," he said.
The report, on LTA's website, will be open for public feedback for a month. All relevant views will be considered by the Government before a decision is made.
- Additional reporting by Zhaki Abdullah