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Mine stabilisation: Repairing a hidden history

Large scale grouting has removed the risk from historic mining activity to allow a primary school in north west London to reopen its doors

Pinner Wood Primary School first opened its doors 80 years ago this year but until recently it looked like the building would not reach this milestone anniversary. A ground collapse led to a network of Chalk mines being uncovered below the site and put the school under threat of closure.

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The first collapse occurred in the staff car park

After nine months in temporary school buildings the pupils have just returned to the site after a major ground investigation and grouting operation was undertaken. In total over 39km of drilling was undertaken for the ground investigation and subsequent stabilisation work and over 5,500t of grout was injected to remove the mining risk.

The work was led by consultant Peter Brett Associates (PBA) after the school undertook initial investigations following a ground collapse in the staff car park in August 2015. The hole, which appeared with no warning, was 3m wide and 1.5m deep.

The council backfilled it with 20t of gravel and one of the parents, who was a Geoenvironmental engineer, advised the school to do some probing. It was at this point that PBA was brought in to interpret the results and Harrow Borough Council became the client.

“In that initial survey only six probes were carried out,” says PBA partner Clive Edmonds. “Five were around the hole and the ground there looked good but one was within the backfilled hole and showed poor ground that was loose to the full depth of the probe.”

There was concern that there might be voids elsewhere and further investigation was undertaken during the school holiday periods. The next phase involved dynamic probing and window sampling with the locations for investigation guided by ground probing radar undertaken by Terratdat.

The geophysics showed 10 anomalies across the site which were then subjected to direct investigation.

“Some of the anomalies showed no issues and others had some lithology changes and disturbance of made ground from construction, but one area had made ground extending to depth,” says Edmonds.

“Grid probing of the area suggested that it was a shaft feature with loose infill to depth.”

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The grouting focused on removing the risk of collapse below the school building and treating other parts of the school grounds

There were also concerns about whether the voids could exist under the school buildings, so further GPR surveys were undertaken. These showed further anomalies but it was hard to tell if these were other shaft features or ground disturbance from the original construction of the school in the late 1930s.

“If both the collapse and the anomaly were shafts then there was a good chance that they were mine workings, especially as old maps showed the school was built on an area marked as Chalk Pit Field,” explains Edmonds. “The Chalk does not outcrop at the site so any workings would have been at depth.”

The next stage of the ground investigation involved Forkers undertaking rotary drilling through the suspected shaft with a grout pump on hand to grout as drilling progressed and to treat the other shaft in the car park.

The grout take on the first shaft was huge and showed it extended to 25m below ground level. The hole was “choked” using sand and gravel and the grout takes suggested that the shaft was connected to other voids at depth. Similar issues were experienced at the other shaft site.

Geoterra was brought in at this stage to undertake a 3D survey and this revealed a wide network of mine workings below the site.

“The floor level was covered by a number of mounds that showed that the roof had collapsed and the voids were migrating upwards. Some right under the school buildings,” says Edmonds.

“The area under the school was mined in a clear pillar and stool network of mines that was 50m to 60m across. The mine workings were up to 3.5m high and 3m wide.

“Other Chalk mines had been found in Pinner but these were under allotments and were believed to have dated from the mid-1800s. Local historians have suggested that the mines below the school was excavated between the late 1700s and mid-1800s.”

It was at this point that the school was closed early for the Easter holidays in 2017 and children have only recently returned to the school.

Initially the council thought it would have to abandon the site and reopen elsewhere but PBA worked with them to demonstrate that grouting could stabilise the ground and allow the school to reopen for a fraction of newbuild costs and in less time.

Edmonds estimates that the remediation cost around £1M but a new build on another site could have been up to £30M.

“The remediation programme had a two-pronged approach,” he says. “The first element was to map the mine workings and backfill them with a liquid grout and the other part of the work was the drill and grout the overlying collapsed ground.

“The aim for the second element was for comprehensive treatment under the footprint of the school and then treat the area around to a lower level.”

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The PBA team visited the school after it reopened to explain how the

With the ground stabilised, the school planned to move back to its original site and PBA used the opportunity to explain the role of a geotechnical engineer in context of the mining risk at the site.

“For the children returning to the school after almost nine months in temporary buildings, the school looked no different. We visited the children to explain what had been happening while they were away,” said Edmonds.

“I explained that we are ground detectives and we have to drill into the ground to look for clues to solve the mystery.”

Edmonds hopes that the experience may inspire some of the children to consider a career as a “ground detective”. But whatever the future holds for Pinner Wood’s pupils, it hopefully not involve further disruption to their education from historic mining activity.

 

 

 

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