CNA Explains: What could have caused the Tanjong Katong Road sinkhole?

CNA

5 days ago

SINGAPORE: A road closure as well as a stop-work order at a PUB worksite are in place after a large sinkhole opened up along Tanjong Katong Road South on Saturday (Jul 26) evening.
A car fell into the sinkhole, and the female driver was rescued by workers and taken conscious to Raffles Hospital. The sinkhole also caused temporary water supply disruptions to residents nearby on Saturday.
The incident has sparked safety concerns among residents nearby.
In response, the Building and Construction Authority (BCA) said on Monday its engineers have been conducting round-the-clock site monitoring to ensure that surrounding buildings, roads and infrastructure remain structurally safe.
The authority also said it is commencing an independent investigation, now that the ground situation has been stabilised. But this could 'take several months' given the complexity of the incident.
What's the latest from authorities?
The site where the sinkhole appeared is adjacent to an active PUB worksite involving the construction of a 16m-deep shaft to connect three existing sewer lines.
A concrete component in the shaft 'failed' at around 5.50pm on Saturday, according to the national water agency. Around the same time, a sinkhole formed on the adjacent road.
PUB said it was still not clear what contributed to the breakdown of the component, known as a caisson ring.
As a precautionary measure, PUB said it will call for a time-out to review and ensure the safety of similar sewer construction works across Singapore.
What are these shafts and caissons?
Such shafts are typically temporary and required for the construction of sewage manholes, which form a key part of the country's 206km-long deep tunnel sewerage system and serving as access points for the maintenance of underground sewer lines.
The caisson – and in this case referred to as a ring as it is circular in shape – is a concrete wall that is cast in stages after the excavation of the shaft to keep out water and soil, said Mr David Ng, chairman of the civil and structural engineering technical committee at the Institution of Engineers Singapore (IES).
Intense study of the land prior to construction is required to determine the design of the caisson ring, and if additional land protection measures may be needed.
What could have led to the sinkhole?
Possible reasons for a defective caisson ring may include subpar workmanship, such as how the structure was not built according to initial design, and an imbalance in 'loading conditions'.
'Imagine the ring is circular. If you apply the same pressure all around it, it will be stable … But if you take away pressure on one side, like soil on one side, that's a different loading condition,' said Mr Ng.
'As the caisson ring 'failure' occurred around the same time as the sinkhole, it may be that there is a void nearby that is creating an unbalanced loading condition on the caisson ring, therefore the caisson ring 'failed'.'
On how such an underground void could have been formed, Mr Ng said it was too early to know for sure — but reasons could include soil movement triggered by excavation works or leaking underground pipes.
The latter could wash away soil over time, resulting in empty spaces underground. 'If this pipe ends up being suspended without soil underneath it, its joints may give way eventually and a pipe burst can wash away even more soil,' said Mr Ng, who is also executive director of One Smart Engineering.
Residents' accounts and passer-by footage suggested a burst water pipe in the area, the night before the sinkhole appeared.
But Associate Professor Wu Wei from the Nanyang Technological University said the erosion of soil underground 'does not happen overnight'.
Underground voids 'take time to be formed' as the surrounding soil is washed away over time and resulting in larger cavities, he said.
Have there been other sinkholes in Singapore?
While uncommon, sinkholes do occur occasionally in Singapore and are often due to construction activities, like underground tunnelling or excavation works, and underground pipe leaks, said Mr Ng.
But he stressed that Singapore's natural soil and geology are not particularly susceptible to sinkholes, as they do not have the characteristics of soluble rocks like limestone.
Past occurrences include an incident in 2022, when a sinkhole and some cracks were observed on a slip road at Farrer Road where tunnelling works were ongoing.
In 2014, a section of Upper Changi Road East caved in, causing a tipper truck to collapse. This happened near a construction site for the Downtown train line.
What has been done to mitigate them?
The BCA has said that measures are in place to manage the risk of sinkhole occurrences during large-scale underground tunnelling or excavation works.
For instance, it requires a qualified person who is a professional engineer to conduct a thorough site investigation and recommend prevention measures before such works start.
Instruments to monitor ground and building movements must be installed throughout the construction process, alongside additional safety measures, such as deploying a 24-hour ground surface watchman to monitor any signs of excessive ground settlement, where necessary.
During tunnelling, the volume of soil removed must be monitored continuously to prevent over-excavation, and key tunnelling operations are closely tracked to prevent soil collapse, the BCA has said.
Among other rules, a stop-work order will be issued if a sinkhole forms during construction.
In this case, BCA said after being alerted to the incident, construction works on site were halted and a stop-work order has been issued to require the builder and supervising qualified person to implement safety measures.
These included cordoning off the affected area, conducting inspections of nearby buildings, backfilling the adjacent shaft, and performing geophysical surveys to detect any underground voids.
What else is being done now?
In an update late on Monday, PUB said it is conducting rigorous tests and scans, such as a ground-penetrating radar scan which uses radio waves to assess the subsurface layers, to detect underground cavities and verify ground stability.
Recovery works along Tanjong Katong Road South are also 'progressing steadily', the agency said. The sinkhole, for example, has been filled with liquefied stabilised soil to further stabilise the area.
This is a type of 'liquefied cement mixed with some soil' and is the go-to material in such instances as it is strong. While dense, it is still flowable, meaning that it could 'flow and backfill all possible voids underground', said Mr Ng.
Moving forward, the two experts CNA spoke with said more information will need to be released by the authorities, including details of the failure of the caisson ring, as well as whether a burst water pipe which reportedly occurred the night before was connected to the sinkhole's occurrence.
Details of the latter, such as the type and extent of the repair works, is a key missing piece of information at the moment, Mr Ng said.
'Maybe there are other factors causing continued leakage after the repair, and that continued to wash away the soil underground. So what exactly was done for the repair and what was observed after the repair until the sinkhole was formed and was anything done?
'This is the missing gap at the moment, and when we have more information on that then can we understand more about how the sinkhole was formed,' he told CNA.