SINGAPORE — Touted as one of Singapore’s greatest engineering feats, a 40km network of three tunnels built 20 storeys underground — the deepest constructed to date — has been completed, and will be progressively used from the end of next year to house high-voltage transmission cables.

By going deeper, the nation’s electricity network will become more reliable as it would be easier to monitor, maintain and repair the cables. The public will also experience fewer road digging work to carry out maintenance of the cables, which are scheduled for replacement every 30 years.

The network spans 18.5km from the north at Gambas to May Road in the south, and 16.5km from Paya Lebar in the east to Ayer Rajah in the west. A 5km stretch also runs under the seabed from Jurong Island to Benoi.

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At 60m to 80m deep, the six-metre wide tunnels were built at twice the depth of MRT tunnels and more than five metres below the country’s existing sewerage system. They can contain 1,200km of high-voltage transmission cables to support 20 per cent of Singapore’s electricity needs for the next 120 years.

The S$2.4-billion project is one of the deepest underground cable tunnel systems in the world, and was a result of a decade of planning and five years of boring works under major public roads.

The media was given a glimpse of the tunnels — which were completed in June — during a visit conducted by energy utility company SP Group on Tuesday (Dec 19).

SP Group said that parts of the existing power grid, which runs 3m underground, will be shifted to the deep tunnels progressively from end-2018 till 2022.

For a start, 480km of cables from eight subsurface power circuits which are due to be replaced in the next five years will be made redundant. These will be replaced by extra high-voltage transmission cables that will be the first to be housed in the tunnels. Seven of the circuits to be scrapped currently link Senoko to areas such as Ayer Rajah, Kallang Basin and Tampines. The remaining one runs from Kallang Basin to Woodlands Ave 8.

Noting that power transmission and distribution costs contribute to less than a quarter of the total tariff, Mr Michael Chin, SP Group’s managing director of infrastructure and projects, assured that there will not be a spike in households’ electricity bills, since the cost of building the tunnels “will be spread over many years”.

Alluding to Singapore’s scarcity of land, Mr Chin said the project allows SP Group to set aside underground space for electricity purposes, at a time when space just below the road surface has become cluttered with a growing number of gas and water pipes, as well as telecommunication cables.

In recent years, the Republic has increasingly explored the use of subterranean space. Last month, national water agency PUB also laid out plans for the construction of a S$10 billion underground sewage superhighway, which would be one of the world’s largest when it is completed in 2025.

While the cable tunnels are the deepest ever constructed in Singapore, engineers have gone further underground to build other facilities. In 2014, the Jurong Rock Caverns — located 150m beneath Jurong Island — was officially opened, becoming South-east Asia’s first commercial underground liquid hydrocarbon storage facility.

Underground electricity cable systems have been rolled out in Japan, South Korea, Manila, Jakarta and Bangkok. The systems in these places are shallower, at depths of about 30m to 50m, said Mr Chin. In Hong Kong, a tunnel constructed in 1999 is believed to be the world’s deepest cable tunnel, at a maximum depth of 180m underground.

Singapore’s underground cable tunnel network can be accessed via 17 SP facilities across the island, which will all be built by June next year.

Apart from regular inspections conducted by SP Group personnel on electronic bicycles, unmanned inspection vehicles plying a track on the roof of the tunnels will capture images to detect anomalies such as changes in temperature, and colour which would suggest water leakage and cracks.

In case of a fire in a tunnel, an alarm signal will be sent to the Singapore Civil Defence Force (SCDF). In the meantime, a water mist system that can reduce temperatures to 60 degrees Celsius within two minutes will cool the tunnel down, before SCDF officers arrive. The unmanned inspection vehicles can also provide a live feed to help in the firefighting operation.