Case study Falmouth

The Problem
A semi-detached property located in the Falmouth estuary suffered a mine shaft cap failure during the storms of late 2014. The resultant hole in the garden was measured at approximately 10 metres in diameter and roughly the same depth. In the centre of the hole was the original shaft which continued down approximately 80m. As part of the failure, the terrace immediately adjacent to the property was lost into the void and many tons 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 the neighbours had been discharging into the void over many years and had contributed towards the failure.

The Optera Solution
We immediately set about making the site safe. Fencing and signage was erected around the site and the drainage was temporarily diverted to avoid further surcharging of sodden ground. We ascertained by flying a drone into the shaft that the material that had fallen into the hole had formed a plug in the narrow, deeper shaft and that the immediate efforts were to create a permanent cap within the hole to form 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 that a scaffold platform would be created across the hole to allow us access over. The cap was created using C4 foamed concrete. Over 300 cubic meters of concrete was pumped across a five day period which presented logistical difficulties. Optera undertook a mail drop to every property effected in the village and undertook road management works to convey concrete lorries through the village with minimal inconvenience to the residents.

Once the concrete was poured and the plug created, we built a thrust block retaining wall, protecting the house foundations from the eventuality of any future failure and filled the rest of the void with Filcor 40 high density PU blocks which was the most effective way of bringing the void to the surface. The entire area was blinded with impermeable membrane and land drainage 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. 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 avoided any damage to the property. 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.

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