SINGAPORE - Works to see if the future Cross Island MRT line will be built under Singapore's largest nature reserve have started.

Originally due to start last December, the investigation works, which will help determine the soil and rock profile under the Central Catchment Nature Reserve, began last month (February).

These were delayed due to "extensive discussions" between the Land Transport Authority (LTA) and the National Parks Board on measures to reduce the environmental impact of these works, said LTA geotechnical and tunnels deputy director, Dr Goh Kok Hun.

Some of the mitigation measures include keeping works to between 9am and 5pm, erecting noise barriers, and keeping investigation works to public trails, to avoid disturbing wildlife in the area.

The number of investigation sites has also been reduced from 72 to 16.

Nature groups have previously raised concerns about the possible environmental impact of running an MRT line under the reserve.

Earlier this month tunneling work for the Thomson-East Coast line caused a tomb at the Mount Pleasant Chinese Cemetery to cave in.

This was caused by a tunnel boring machine digging in mixed ground conditions, which led to over-excavation and a localised depression on the surface, said the LTA.

This is unlikely to occur should the authorities decide to run the line under the Central Catchment Nature Reserve, said Dr Goh.

This is because tunneling in the area would go much deeper, and only through the granite of the Bukit Timah profile and not mixed ground conditions, he said.

Soil investigation works are expected to be completed by the end of this year . Studies on the total impact of the project will only be completed by the end of next year, and a decision on the route of the MRT line will only be made after.

The Cross Island Line is expected to be about 50km long and stretch from Changi to Jurong.