Ngunnawal expansion possible cause for street flooding and road issues: report

A geotechnical report commissioned by the ACT government into issues along Tarra Place in Ngunnawal, obtained by The Canberra Times, revealed problems believed to have been caused by the mysterious water source had been going on for at least five years.

The report was unable to determine the exact cause of the underground water source, which has seeped into homes and backyards and caused major damage to the road.

However, one potential reason listed in the report was that the issues were caused by water that flowed from an uphill source due to development works in Ngunnawal, following the expansion of the suburb.

"Given that the [road] failures in the area started to occur shortly after the development to the north and east of the site, it is considered likely that water is naturally occurring seepage from an upslope source," the report said.

"Development may have changed the water infiltration and runoff pathways and water may be migrating along the interface between the natural soil and fill materials below the pavement."

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The report noted the development and construction of an expanded Ngunnawal began in December 2013, but the first major issues caused by the underground water were initially detected in February 2016.

Other potential reasons for the water issues in the street noted by the geotechnical report included possible damage to water or sewerage pipes, or a leak that was coming from a nearby stormwater dam to the north of the street.

Icon Water were called out four times to Tarra Place in 2020 alone, but no fault in water mains in the suburb were detected.

However, the report said a utilities investigation should be undertaken with CCTV monitoring to eliminate the possibility of leaking pipes being the source of the water.

"The pavement failures in the area are inferred to have been induced by excessive water," the report said.

"The scope of works required to remediate the pavements in the area to a level requiring only routine maintenance may be significant."

Road repair crews were called to Tarra Place three times in a six month period to fix the large potholes along the road caused by the water.

The last repair job happened in February this year, but residents have told The Canberra Times that water has once again been seen seeping out of it again.

The report was commissioned by the ACT government following repeated complaints from residents about water issues in Tarra Place and large potholes developing.

Water has continued to seep into homes along the street and has caused mould and even marsh plants to grow in backyards due to the underground reservoir, despite thousands of dollars spent by residents to try and rectify the issue.

Fears have been raised over potential structural issues in some of the homes along the street or even the development of sinkholes.

The report called for multiple recommendations in order to solve the issue, but did not list a potential cost of the measures.

Among them was a full road reconstruction with an additional drainage layer.

Other measures considered by the report were heavy patching of the road or specialised sub-soil drains.

The ACT government had previously said the fixes of the road were only temporary until the source of the water underneath was determined and a more permanent solution could be found.

A government spokesman said repairs had been carried out at Tarra Place in order to make it safe, and it was investigating the exact cause of the flooding and road issues.

"The ACT government will schedule further remediation works if required based on these investigations and local residents will be advised prior to any works," the spokesman said.

Do you know more? Email: andrew.brown@canberratimes.com.au

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