SINGAPORE - Eight circuits of the power cable network, which were built in the 1980s, will soon become too expensive to maintain and will be left in place while being gradually replaced with the new tunnel system.

These cables were installed just below the country's roads and so repairs and replacement will cause traffic jams and increase the risk of accidents - problems that the new underground network will avoid, said SP Group managing director of infrastructure and projects Michael Chin.

He added that the existing cables are so old that they need inspecting once every three months, which adds to maintenance costs.

Singapore's land scarcity means that SP Group had "no choice" but to build the tunnel system 60m below ground, said Mr Chin: "If you look at the profile of Singapore, we have the MRT, which is 30m to 40m deep, then you have your Deep Tunnel Sewerage System, which is between 40m and 70m, so we 'choped' (saved) our space at 60m."

He added that the 60m deep tunnels mean that cable checks will not disrupt ground-level life.

Mr Chin noted that as all land below 30m belongs to the state, the cables no longer need to follow the roads and can run in straight lines deep beneath private property.

The State Lands Act was amended in 2015, clarifying that landowners own only the space up to 30m below the "Singapore height datum", which is pegged to the average historical sea level.

Reflecting long-term planning, Mr Chin said the 40km Underground Transmission Cable Tunnel Project is designed to last 120 years.

This refers to the concrete segments and the structural reinforcements.

However, Mr Chin said that there is no knowing what will happen once the system ends.

"I will not be around and you will not be around," he chuckled, adding that Singapore could end up with a completely different system of transmitting electricity.