Flood mitigation at Tanjong Pagar

The Challenge

Heavy rainfall caused flash floods at the junction of Craig Road and Tanjong Pagar Road. Water entered nearby food and beverage outlets and disrupted road traffic.

Figure 1: Flash Flood along Tanjong Pagar Rd - Craig Rd junction

The Solution

Hocklim and PUB surveyed the drainage network and identified a bottleneck at the junction. The project focused on improving that connection.

Runoff from Craig Road met another drainage flow along Tanjong Pagar Road. The intersecting flows restricted discharge to the drain across the road, allowing water to accumulate faster than it could drain away.

Figure 2: The blue arrows indicate the flow of water from the 2 drainages that intersects at the junction of Craig Road and Tanjong Pagar Road

Hocklim installed steel pipes across Tanjong Pagar Road to connect the Craig Road and Tanjong Pagar Road drains separately to the receiving drain. This made better use of the available downstream capacity.

Trial trenches were required to locate existing cables and water pipes before selecting routes for the new steel pipes.

Work proceeded one lane at a time, with traffic management measures in place to maintain access.

Trial trenches along Tanjong Pagar Road

Figure 4: Ensuring that traffic is not affected while working

Figure 3: Setting up safety road measures before commencing work

A diamond cutter opened the road surface before manual excavation of the trial trenches.

The team excavated manually to a depth of 1 metre to avoid damaging services. Where cables were found, the trench was backfilled, compacted and reinstated with premix matching the existing thickness. The process continued at other locations until suitable routes were identified.

Figure 5: Manual Excavation to avoid damaging any existing cables or pipes.

Figure 7: Backfilling the trial trenches

Figure 6: Existing Pipes that were discovered when digging the trial trenches.

Two locations free of cables and water pipes were identified. With PUB’s approval, Hocklim proceeded with the pipe installation.

Figure 8: PUB approved areas highlighted in red.

Excavation reached 1.6 metres. Bakau piles were installed to a depth of 3,000 mm at 300 mm spacing, followed by a 200 mm hardcore base and 50 mm of G15 lean concrete. Two layers of B10 mesh reinforced a 100 mm G35 concrete slab.

Installation of Steel Pipes

Figure 10: Laying 2 layers of B10 mesh on the steel pipes

After the base cured, six steel pipes measuring 8 m × 114.3 mm × 8.56 mm were laid near Tras Link. Eight pipes measuring 8 m × 141.3 mm × 9.52 mm were installed near Craig Road. A further two layers of B10 mesh and a 100 mm G35 concrete slab were placed above the pipes.

Figure 12: Floating the concrete before applying the Tack coat

Figure 13: Applying Tack coat on the cement

Figure 11: Pouring the cement onto the B10 mesh and pipes

Tack coat was applied to the cured concrete before premix reinstatement. This bonded the asphalt and concrete layers and reduced the risk of slippage between them.

Auxiliary Police officers assisted with traffic control at the junction in accordance with LTA road closure requirements.

Lane-by-lane closures allowed excavation to proceed while maintaining traffic flow.

Measure taken to ensure traffic is not affected

Figure 16: Auxiliary Police officers controlling traffic

Figure 17: To provide more stability to the steel plates, the SHS sections were used as support for the steel plates

Figure 18: Temporary steel plates and premix covering the trenches for traffic

Where a trench could not be reinstated that day, or the road needed to reopen urgently, 25 mm steel plates covered it. The plates carried 50 mm of W3B premix to meet LTA skid-resistance requirements.

The team opened the pavement in front of the shophouses to connect the existing drains to the new steel pipes. A Type 3 drain was also constructed along Craig Road to improve runoff collection.

Steel Pipe connection to existing drainage - Junction of Craig Rd & Tanjong PagarRd

Figure 19: View plan for the steel pipe connection to the drainage system at Tanjong Pagar Rd - Craig Rd junction

Figure 20: Connecting the Steel pipes to the drainage

Figure 21: Pavement to be hacked (in purple) to connect the steel pipes to the existing drain

Figure 23: Casting of the concrete wall connecting the steel pipes to the drain

The pavement was retiled after the concrete works. Type 3 gratings along Craig Road improved water entry into the drainage system.

The separate steel pipe connections removed the bottleneck created by the intersecting drains. Together with the new drain and gratings, they improved discharge towards the receiving drain across Tanjong Pagar Road.