Playground near St Albans sinkhole closed for ground stability survey
PUBLISHED: 17:31 17 August 2016 | UPDATED: 17:32 17 August 2016
Uncertainty over the long-term stability of a playground near St Albans' sinkhole has resulted in the site being temporarily closed for further investigations.
The playground in Bernards Heath recreation ground was closed to visitors today (Wednesday), to enable three days of investigative work to be carried out by Opus International.
The infrastructure company was commissioned by St Albans district council to conduct deeper surveys of the soil.
In January, Opus was appointed to investigate the recreation ground, in particular in the vicinity of the children’s playground, to assess the short-term stability of the ground following the discovery of an anomaly in a microgravity survey.
Looking into the possible risk of ground instability beneath the site, the firm told the council in a report that additional data needed to be collected from more intrusive drilling.
The site is surrounded by open parkland, off Sandridge Road, and is close to Fontmell Close, where a sinkhole appeared in the cul de sac on October 1 last year.
A number of microgravity anomalies were found in an earlier survey beneath the heath and the playground itself, the largest of which was reported to exist 19 metres below the ground surface.
The earliest available map from 1878 reveals that the site comprised of heathland, but there “are numerous clay pits, two brick kilns, two lime kilns and three shafts close by. The presence of these features suggests that clay/gravel and probably chalk were being extracted in the area at this time,” Opus said in its January report.
The firm said its findings then were ‘tentative’ as more data was needed from more intrusive investigations – which the council is now carrying out.
Opus said that while there was soft ground below the playground, it found “no voids or areas of very soft strata. These results tentatively suggest that the ground beneath the playground is probably stable in the short term and of reasonable strength.
“This does not however give any indication as to the origin of [the] anomaly.”
It added that there was a “low probability of voids forming beneath the playground in the short term as a result of ground instability caused by chalk dissolution or chalk mining. However the cause of the [anomalies] within the Bernards Heath area remains unconfirmed at this time.
“It would be prudent to consider further, more extensive investigation at a later date to assess the long-term stability of the ground at Bernards Heath.”
While its use as a recreational play area was considered to be “low risk with regard to potential for induced ground instability, further assessment would be required if the site usage is to change or if heavy machinery is needed to traffic across the site.”
Councillor Beric Read, the council’s portfolio holder for localism, said it was ‘unfortunate’ the playground would be closed for several days, “but I am sure residents will understand that we need to complete these important and necessary investigations”.
More survey work will be undertaken in the wooded area beside Bridle Close on Wednesday August 31, for three days.