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

Due to start last December, the investigation works, to determine the soil and rock profile under the Central Catchment Nature Reserve, began last month. They were delayed because of "extensive discussions" between the Land Transport Authority (LTA) and National Parks Board on measures to reduce the environmental impact of the works, said LTA geotechnical and tunnels deputy director 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.

"Workers have to observe the stringent requirements stipulated under the environmental management and monitoring plan for the works," said Dr Goh.

Trial runs of borehole operations and geophysical surveys were also conducted off-site prior to actual works to familiarise workers with the requirements of working in the reserve, said an LTA spokesman.

"These mitigating measures are the result of three years of engagement between the LTA and other stakeholders," said Nature Society president Shawn Lum.

Expected to be ready in 2030, the 50km Cross Island Line will stretch from Changi to Jurong when completed. The authorities are considering two paths for the line - a 4km route, half of which would be under the nature reserve, and a "skirting alignment" that would take a 9km route around it.

While the latter option is expected to tack on $2 billion to the cost of constructing the line, it could allow for an additional station to serve Thomson residents.

It would also satisfy nature groups which have raised concerns about the possible environmental impact of running an MRT line under the reserve.

Earlier this month, a tunnel boring machine digging in mixed ground conditions caused an old tomb at Mount Pleasant Chinese Cemetery to cave in during works on the Thomson-East Coast Line. Over-excavation had created a localised depression on the surface, said LTA.

Such cave-ins are unlikely to occur should the authorities decide to run the line under the Central Catchment Nature Reserve, said Dr Goh. This is because tunnelling in the area would go much deeper and only through the granite of the Bukit Timah profile, not mixed ground conditions, he added.

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

Dr Lum, a senior lecturer at the Nanyang Technological University's Asian School of the Environment, is still hopeful that the new rail line will go around the reserve.

"The skirting alignment would not just be beneficial for the environment, it would also allow for wider ridership for the MRT line," he said.