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SINGAPORE: National water agency, PUB, started tunnelling work on the second phase of building what it calls the “used water superhighway for the future” on Thursday (Apr 4).

When it is ready, the Deep Tunnel Sewerage System (DTSS) will collect and transport water from the whole of Singapore to three water reclamation plants in Changi, Kranji and soon Tuas for treatment. It is part of PUB’s efforts to move Singapore towards becoming more self-reliant in its water needs.



Speaking on the sidelines of a media briefing on Wednesday, DTSS Phase 2 director Yong Wei Hin said the DTSS, which is scheduled to complete in 2025, will support 55 per cent of Singapore’s water needs in the long-term via NEWater.

The Changi and Kranji plants have been connected since 2008 as part of Phase 1. When the Tuas plant is fully operational in 2027, it will also be connected as part of Phase 2.

The second phase, which will cost S$3.1 billion more than the first phase at S$6.5 billion, will connect Marina Bay to Tuas.

This involves creating a network of 40km of deep tunnels and 60km of link sewers in the western part of Singapore.



Nineteen tunnel boring machines (TBMs) will be introduced to the effort – more than twice the number of units used in the first phase. There will also be two types of boring machines in this phase due to the different ground conditions in the area.

"Singapore is different from a lot of other countries because it is characterised by highly variable ground conditions," said DTSS Phase 2’s chief engineer Woo Lai Lynn.

"We had to do a lot of ground investigations (and) you never know what you are going to encounter between your investigation points. You can go through soil in one ring and the very next ring, you can go through hard rock."

Using more TBMs ensures that Phase 2 is completed on time, said Ms Woo.

The newly launched TBM, named TBM Bahar, will create a 3.5m-diameter tunnel 42m below ground.

“(For) certain sections, you will have two TBMs tunnelling towards each other. If one gets stuck, the other can go into overdrive and still meet the other machine," she said.

"We made sure we would not face a situation similar to what happened in DTSS Phase 1. There were months and months that went during which the TBM was hopelessly stuck and no progress (was made)."

Existing water reclamation plants at Ulu Pandan and Jurong, as well as pumping stations, will be shut down. This will enable land the size of 214 football fields to be freed up for development, PUB said.

NEW FEATURES TO COPE WITH TUNNELLING CHALLENGES

Another issue addressed in Phase 2 was the challenge of tunnelling under industrial areas.

“If you are out in the open and there is nothing above you, even if you have a big sinkhole nobody is going to care, nobody is going to notice. But a sinkhole on the Ayer-Rajah Expressway will be quite obvious,” said Ms Woo.

She also said that compared to the first phase, this one requires PUB to deal with a lot more infrastructure that may potentially get in the way of tunnelling.

“Our underground is a bit congested now. There are (more) Mass Rapid Transit (MRT) tunnels (and) cable tunnels that we have to de-conflict with," said Ms Woo.

"There are depressed roads and piles going down, so it is quite difficult to find a corridor to do our tunnelling."

Hence, PUB will leverage smart technologies in this phase, she added.

This includes using a monitoring system that provides real-time updates of the TBM locations and identifies excavation and tunnelling risks.

In addition, PUB will be the first agency in Asia to deploy what it calls a Vertical Shaft Sinking Machine (VSM), which can excavate the ground and construct the walls of underground work access sites at the same time.

Another new feature for Phase 2 is the isolation gates, which will allow for water to be diverted in order to isolate a particular section of the tunnel for inspection and repairs.

“With DTSS, we are changing the way we are collecting used water in our plants," said Mr Yong.

"We are getting rid of intermediate pumping stations, which take up land. By doing so, we are centralising our water reclamation plants. We are also saving land for higher value developments."