A collapsed 19th century culvert that has caused a significant sinkhole in a south Wales village, and which previously collapsed in 2009, has raised questions over the resilience of ageing infrastructure in the face of climate change.
The culvert in Pant, Merthyr Tydfil, is believed to have collapsed after two landslides triggered by Storm Bert last month sent debris crashing through it.
Households from the Nant Morlais estate in Pant reported hearing “rumblings and bangs” at around 6am on 24 November, Merthyr Tydfil County Borough Council leader Brent Carter said.
The residents initially thought this was thunder, but it was later understood to be the noise of the culvert collapsing.
Following its collapse, a large ground collapse on the estate was reported to emergency services on Sunday (1 December).
The hole has since enlarged and swallowed a sizeable part of the road and a driveway. It is now 12m deep and 10m wide, the council confirmed.
Residents from 29 properties on the estate have been evacuated and roads in the area have been closed as a safety precaution.
“We have now completed the culvert inspection, and no further issues have been identified within the culvert, so the collapse is localised to the area of the sinkhole,” the council said in an update yesterday (4 December).
Carter said that the sinkhole resulted from the “sheer volume” of material that was washed down from the mountain to the east of the estate during the storm, which caused several landslides and widespread flooding across Wales and other parts of the UK.
Aerial images of the sinkhole show the stream continuing to flow through the base of the collapse.
The culvert provides a route for the Morlais brook – also known as the Morlais stream – which flows through several other culverts.
The earliest part of the culvert was constructed in the 19th century, a second phase was added between 1900 and 1910, and a final section was constructed between 1910 and 1957.
Emergency work to stabilise the ground collapse has included forming a dam on the brook so the council’s engineering team can install pumps to over-pump the water and stop it flowing through the culvert. The council has appointed Edwards Diving Services to carry out the stabilisation work.
The council is also continuing to use drones to inspect the culvert, carry out ground penetration radar surveys to identify further voids, and dropping the water level of the New Pond reservoir to the south east of Nant Morlais to stop overflowing water going into the culvert.
The council has said it aims to have properties at the lower end of the estate’s cul-de-sac back home by the end of this week,
“We are working with Welsh Water on getting the supply of water back to those properties,” it said in an update yesterday.
Geotechnical opinion
Commenting on photos, a spokesperson for the British Geological Survey (BGS) confirmed that the ground collapse appeared to be a “collapse sinkhole”.
They said: “It is likely that the high water levels during Storm Bert (and indeed previous storms) have caused damage to the base of the masonry culvert, potentially exposing the foundations to the arch structure itself, leading to the collapse of the brickwork.
“The resulting void then propagated up through the overlying unconsolidated fill, eventually reaching the surface over a period of days. The bangs heard by the residents was the overlying fill and possibly the tunnel lining collapsing. The stream has washed some of the material out of the culvert, creating space for more material to collapse.”
The spokesperson noted the ongoing risk to property and infrastructure as the sinkhole continues to enlarge.
They observed that the stream at the base of the collapse is “actively eroding, and undercutting the fill on the south side of the sinkhole, so further damage to the road is likely as the sides slump in”.
“If left unchecked, the sinkhole will widen until the sides reach a stable state,” they said.
Pant’s location on the outskirts of Merthyr Tydfil, a former mining town, caused some to speculate that the ground collapse could be due to a shaft collapse or abandoned workings.
But ground stability geohazard specialist and Stantec senior consultant Clive Edmonds noted that the site is not directly affected by past coal mining beneath as its underlying geology comprises “glacial boulder clay over Carboniferous age Millstone Grit strata”.
Warren Geotechnical Associates director Colin Warren, with extensive engineering geology and ground investigation expertise, said that further investigations would be required into the geology and the nature of the underlying soil and strata at the hole location to confirm the cause of the collapse.
“Investigations should also look into the surrounding groundwater conditions, e.g. any artesian water or zones of preferential groundwater flow,” he said.
“Of course, whether there was a local problem or breach at the culvert location allowing water to escape into and erode the surrounding ground also needs to be investigated. All this needs to be examined in detail.”
Wilgeo director Phil Wilson, who has more than 40 years experience in ground investigation and geotechnics, said it would be advisable for the council to undertake a CCTV survey of the culvert to determine if other sections are at risk
It should “also consider ground probing radar of the area to determine other possible cavities under formation”, he said.
Damage in 2009
Council records show that the mid-section of the Pant culvert beneath the Nant Morlais estate was previously damaged in 2009.
A survey conducted by the council’s engineering department in October 2009 revealed “extensive erosion damage to the base of the masonry culvert exposing the foundations to the arch structure itself.
“A substantial portion of these foundations had also been damaged, and a considerable length was missing entirely, almost certainly washed away during repeated storm flows,” it said.
The report also raised concerns about the unpredictability of the culvert’s stability during extreme weather, and warned of the likelihood of damage to infrastructure should the culvert collapse.
“A study of plans dating back to 1900 reveals that the culvert lies in the bed of a now filled valley, the fill being of unknown nature and quality. The effects of a failure within the culvert during storm conditions cannot therefore be predicted with accuracy as this would depend on depth, position, nature of the fill and of course the extent of the failure itself.”
The report stated that any sizeable failure would affect dwellings and infrastructure, such as roads and utility services, and advised urgent repairs.
These repairs were carried out on the culvert in 2010, at a cost of £33,370, south of where the current sinkhole is. Additional work was carried out on the culvert in 2015 further north of the sinkhole location, the council confirmed.
A report to the council in 2012 raised further concerns about the state of other ageing culverts across the county borough.
“Many of these older culverts within the county borough have already reached the end of their serviceable life expectancy, and whilst I’m confident that many will remain in good order for years to come, it is reasonable to assume that some will not,” the report said.
“Determining the potential risk of a failure occurring and its likely impact on this authority is not an easy task given that any industry recognised objective analysis (based on life expectancy) of the systems would indicate that the majority of culverts should have already failed.”
Climate change impacts on infrastructure
The geotechnical experts that GE spoke to confirmed that ground collapse sinkholes like the one in Merthyr Tydfil are more common following extreme rainfall events.
They noted that these events could become more frequent due to climate change and are having a worsening impact on ageing infrastructure – such as Victorian built culverts.
In fact, several culverts collapses have been reported in south Wales over the years.
During a major flooding event in south Wales in 1979, 146 houses were flooded in the village of Troedyrhiw in Merthyr Tydfil and a major culvert collapsed. In Rhydycar, Merthyr Tydfil, there were two fatalities as a result of a culvert collapse.
More recently, a collapsed culvert caused a sinkhole to appear beneath a social club in Swansea in August 2023. The sinkhole started as a small void in August 2023 and has grown to around 10m deep. It is continuing to cause problems for utility providers.
Commenting on the recent ground collapse in Nant Morlais, Edmonds said: “I think that this is another event that raises questions over the resilience of infrastructure in the face of climate change.”
The BGS spokesperson said: “Increased incidences of extreme rainfall will cause more incidents of this nature, especially with ageing infrastructure, or infrastructure not designed to cope with extreme flood flows or rainfall events.”
They added that heavy rainfall is also a trigger for natural sinkholes that occur when types of rocks below the land surface are naturally dissolved by groundwater circulating through them.
