Campaign group warn of possible sinkholes under development site in Symondshyde

PUBLISHED: 14:03 25 July 2018 | UPDATED: 14:31 25 July 2018

Save Symondshyde screenshot.

Save Symondshyde screenshot.

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The heatwave may cause sinkholes to collapse on land being considered for development, a geologist has warned.

Campaigners Chris Oxley and John Gardner. Photo: KEVIN LINESCampaigners Chris Oxley and John Gardner. Photo: KEVIN LINES

Campaigners Save Symondshyde believe the extended drought and low rainfall could destablise possible sinkholes under Green Belt land near Symondshyde Great Wood.

Welwyn Hatfield Borough Council (WHBC) is currently considering 1,130 houses for this site in its Local Plan.

Evidence of four possible sinkholes, which lie between Wheathampstead and Hatfield, were assessed by geologists from Imperial College London in 2017.

The walkover survey found one of the potential sinkholes may be 12m wide and made out of chalk.

These natural cavities occur when rainwater filters through the ground and erodes the soil, creating natural aquifers or underground reservoirs.

If ground water levels drop in a heatwave or drought, unstable voids open up and the floor becomes in danger of collapsing.

Emeritus professor of soil mechanics at Imperial College, John Burland, said: “The land underneath Symondshyde ridge is underlain by chalk and at least four existing sink holes are documented on the site.

“In chalk, sinkholes form over many years by the infiltration of rainwater causing its gradual dissolution and the creation of cavities. Often these cavities are filled with water and form an aquifer or natural underground reservoir where surface water is effectively stored.

“When ground water levels significantly decline, as happens in extended periods of drought, empty unstable voids can collapse unexpectedly.”

Geologists found 10 landscape features which indicate sinkholes on the site.

He added: “What we are saying is there is strong evidence of sinkhole activity at Symondshyde, activity that we can see at surface level indicates a strong likelihood of the existence of undetected cavities beneath the ground surface.

“These cavities destabilise more rapidly in the presence of extreme drought and extreme flooding conditions, the hallmarks of climate change that we are already experiencing.”

The UK has been in the grips of unexpectedly hot weather for about two months now, and St Albans is currently forecast to see temperatures rise to 32C by the end of the week.

Chairman of the Save Symondshyde, John Gardener, said “We have pressed continually for a professional assessment of the sinkholes at Symondshyde throughout the Local Plan process.

“Evidence that we have presented by well qualified professionals in this subject had been ignored by WHBC until we asked the Local Plan Inspector to add our report to the evidence base.

“We believe that no development on a Green Belt site of similar geology should take place without a professional study of the terrain to assess the risks posed by sinkholes, particularly in the context of climate change and acid rain, which increase the risk of sinkhole formation and collapse.”

In a report submitted to WHBC, Save Symondshyde said not considering the possibility of sink holes would call the Welwyn Hatfield Local Plan into question.

It said: “We do not believe that sufficient consideration has been given to this matter in assessing the suitability of the Symondshyde site for residential and ancillary development of the kind envisaged, and we have been denied the opportunity to assess whatever evidence the promoters of the site may have on this matter.

“Unless this matter can be resolved satisfactorily, the soundness of the Symondshyde village proposal must be questioned on grounds of achievability and therefore deliverability.”

A spokesperson from WHBC said: “The Save Symondshyde group have submitted evidence to the Local Plan examination on sink holes in the vicinity of Symondshyde.

“This evidence and the implications for the development of the site are likely to be debated at the hearing session on Symondshyde later this year.”

The Wheathampstead and District Preservation Society recently awarded the Save Symondshyde campaign a Preservation Award.

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