The Cross Island Line (CRL) is Singapore’s eighth MRT line. When completed in the 2030s, it will be the longest fully underground line at more than 50km long.
The line is being built in three phases, with the first section expected to be operational by 2030. The second phase includes tunnelling under the Central Catchment Nature Reserve to connect Turf City and Bright Hill stations.
The LTA said the alignment of this part of the line was modified to reduce its impact on forested land.
Engineering solutions were also chosen to further reduce the impact on the environment. These include digging deep and using a large diameter boring machine for tunnelling, with a single tunnel able to accommodate two train tracks.
In Singapore, tunnel boring machines (TBM) are commonly used for constructing public transport rail lines underground. Such machines can work under different ground conditions.
The same will be done to construct the Cross Island Line. Scroll to find out how the TBM works.
The TBM has a giant cutter head at the front of the machine, which can bore through anything from sand to rock.
The giant cutter head rotates, cutting away at the earth. It is 12.6m in diameter, roughly as tall as a four-storey building.
When digging, the earth is carried up the screw conveyor and out of the TBM with a belt conveyor. Depending on the type of ground, it can burrow through 4m to 20m in 24 hours.
The TBM also forms the tunnel walls as it moves along. Tunnel segments get transported to the front and are installed in place by the segment erector.
The segments are then connected to form a ring.
Once completed, the TBM pushes off and continues digging.
It is not the first time Singapore is using TBMs to build underground infrastructure. But this time, we are digging deeper than ever before.
The Cross Island Line is going as deep as 70m below ground, as opposed to typically digging 20-30m below when building other train stations.
The Government decided to go deep underground at the portions affecting Singapore’s largest nature reserve after six years of public consultation and considering the findings from a comprehensive two-phased environmental impact assessment.
Deepest MRTs
The study provided two viable options to navigate around the Central Catchment Area - either going directly under or skirting around the reserve.
By taking the direct route, which will involve creating a 4km-long tunnel, the Ministry of Transport said the commuting time will be shorter by roughly six minutes than skirting around the reserve, which involves 9km of tunnel.
Fares will also be about 15 per cent lower per trip on average as the route is shorter and more direct, and construction cost is expected to be lower by $2 billion.
The ministry also said the direct route will be more environmentally friendly in the long run, as it "has lower energy consumption".
Digging deep underground has been done in other places. In Hong Kong, the HKU station on the MTR - the city’s main rail network - is situated 70m below ground. In Europe, the Gotthard Base Tunnel which opened in 2016 runs through the Alps in Switzerland.
Other construction challenges include navigating difficult terrain
In the CRL Hougang station, construction takes place in proximity to residential buildings. To minimise disruption to the residents caused by deep excavation, concrete walls are used.
Several stations along the CRL are being constructed in densely-populated areas, with the limited work space posing challenges.
Works are also carried out near structures with low headroom such as viaducts. In Ang Mo Kio, construction takes place below the North-South Line viaduct.
Challenges aside, the CRL is one of Singapore’s largest construction projects in the next decade. Commuters can expect to cut their travel time by up to 70 minutes.
The CRL improves connectivity to eastern, western and north-eastern Singapore, and offers alternative routes for commuters to get to their destinations.
In October, construction officially began on the Cross Island Line (CRL) Punggol extension, which will shorten more than 70,000 journeys between Singapore’s north-east and east daily.
Tenders for the second phase of the line, comprising six stations from Turf City to Jurong Lake District, are being called progressively, with passenger service slated to begin in 2032.
Engineering studies are under way for the third phase of the line.
When completed, the CRL will serve eastern, western and north-eastern Singapore, and will be the country’s longest fully underground MRT line.