The Problem
A semi-detached property located in the Falmouth estuary suffered a mineshaft cap failure during the storms of late 2014. The resultant hole in the garden was measured at approximately 10m in diameter and roughly the same depth. In the centre of the hole was the original shaft which continued downwards for approximately 80m. As part of the failure, the terrace immediately adjacent to the property was lost into the void and many tonnes of garden also went into the hole.

We met with the client over a weekend to review the damage and it was immediately apparent that storm drainage serving the property and neighbouring properties had been discharging into the void over many years and had contributed to the failure.

The Optera Solution
We immediately set about making the site safe. Fencing and signage were erected around the site and the drainage was temporarily diverted to avoid further surcharging the sodden ground. By flying a drone into the shaft, we established that the material that had fallen into the hole had formed a plug in the narrow, deeper shaft and that our immediate focus needed to be the creation of a permanent cap within this hole to provide a long-term solution.

As it was not possible to ascertain the stability of the soil surrounding the edges of the shaft, we took the decision to create a scaffold platform across the hole to allow us safe access. From this platform, the cap was created using a lightweight concrete. Over 300m3 of concrete were pumped across a five-day period, which presented logistical difficulties. To mitigate these challenges, Optera undertook a mail drop to every property affected in the village and undertook road management works to convey concrete lorries through the village, ensuring minimal inconvenience to the residents.

Once the concrete was poured and the plug created, we built a retaining wall to protect the house’s foundations from the eventuality of any future failure, and filled the rest of the void with a lightweight fill, which was the most effective way of bringing the void to the surface. The entire area was covered with impermeable membrane and land drainage was installed to prevent any future risk of inundation. The garden was landscaped and the terrace reinstated to bring the claim to a conclusion.

Project Conclusion
It is commonplace in claims of this nature for a comprehensive site investigation to be undertaken by structural or geotechnical specialists. The client correctly identified that any extended investigation period would result in the increased risk of collapse to the house. With this in mind, Optera have installed a 200mm diameter pipe through the centre of the cap, allowing future investigations to be completed if required.

Works were completed over an eight-week period and level monitoring continues to the property but no movement has been detected to date and none is anticipated. With the very real risk of collapse of the main property, Optera’s intervention made potential savings to the client of £1.6 million.

“Optera’s intervention made potential savings to the client of £1.6 million.”

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