SINGAPORE - The future Cross Island MRT line that will run 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 - expected some time next year - is to build through the reserve, tunnelling will be done 70m underground.
The LTA's latest findings were announced in the Environment Impact Assessment (Phase 2) report.
The report was commissioned by the LTA, with the research conducted by consultancy Environmental Resource Management (ERM).
LTA said: "Based on the report, both underground alignments are feasible, and the residual impacts are largely (negligible to) moderate.
"LTA is committed to undertake all necessary mitigation measures to minimise the potential environmental impact."
It said that no decision has been made on the alignment.
But an independent panel of advisers engaged by the LTA said the skirting alignment would result in greater risk of safety-related incidents as compared to the direct alignment.
The Cross Island Line (CRL), Singapore's eighth MRT line, is projected to have an initial daily ridership of 600,000. The number is expected to eventually grow to one million.
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 any plan to build part of the second phase of the line through the Central Catchment would damage the nature reserve and affect the biodiversity there.
But residents, such as those in the Thomson area, say the alternative option to skirt around the nature reserve would affect their lives and their homes.
The Government has also said the skirting option could cost an additional $2 billion.
LTA's report on Monday stated that the option to build a direct alignment option 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 last station of the second phase of CRL and Bright Hill station, the first station from the first phase of the line, to be completed in five minutes.
The implementation of mitigation measures would reduce the impact of construction on the ecology and biodiversity in the Central Catchment Nature Reserve to negligible levels, said LTA.
At the work sites, which are located outside the nature reserve, the impact on the two factors 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 based on current construction standards.
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 same next station. The LTA said 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 groundborne vibration, poorer air quality and ambient noise. But the impact of these issues can also be mitigated from negligible to moderate levels, said LTA.
Some residents near Lakeview Estate and Upper Thomson Road, where a work site would be set up in case the option is chosen, might however have the view of their surroundings affected.
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.
"I am happy that the LTA is committed to implement the proposed mitigation measures proposed."
The public will be able to read the report at LTA's website.
The report will be open to public feedback for a month, and all relevant feedback will be considered by the LTA before a decision is made on the issue.