Thought to be the world’s largest single use of foamed concrete, Propump Engineering has helped pump a mammoth 600,000m3 underground to stabilise approximately 40 acres of historic stone mines in the picturesque village of Combe Down.
Stabilisation works also included the creation of underground ‘safe routes’ that have been shored-up with girders to protect the roosting and breeding sites of the Greater Horseshoe Bat – one of the largest and rarest bats in the UK, found only in south-west England and west Wales.
Over time, irregular mining and robbing of the stone from supporting pillars had left the Combe Down mines unstable – many of which were beneath residential areas and highways. Approximately 80% of the mines, which are up to 9m high, had less than 6m cover and as little as 2m in some places. Approximately 350 properties were at risk of subsidence, with some 2000 people living in the area.
As well as the risk above, the Combe Down mines lie above a Grade 1 aquifer (rock in which water can be stored and pass through, and which is then suitable for use as drinking water), so any remediation scheme proposed needed to ensure there were no harmful impacts.
After extensive trials and testing of suitable infill materials including sand, stone and pulverised fuel ash, a lightweight (600kg/m) foamed concrete was chosen. Made up of cement and limestone dust, the foamed concrete provided effective drainage without risk of harmful contamination.
With approximately 13,000 vehicles a day using the main North Road through the village, weight restrictions and closures were enforced on a number of roads in the village. One of the highest risk areas of potential mine collapse was an area found underneath Combe Road. An emergency remediation scheme was put into place which involved a weight restriction of 3 tonnes (7.5 tonnes for access).
The area was stabilised by shuttering around the perimeter and then pumping foamed concrete into the void. Pumped over 1000m into the mine, foamed concrete was required at rates in excess of 200m3 per hour and an average target daily production rate of 1000m3. At its peak, Propump was pumping some 280m3 in every hour.
A designated Site of Special Scientific Interest (SSSI) and a Candidate Special Area of Conservation (CSAC), the Combe Down Stone Mines are of international importance for the Greater Horseshoe Bat and of geological and archaeological interest.
The engineering methods for the stabilisation of the mines were influenced by the bat habitat and the works were planned under the guidance of English Nature and the project’s bat specialist – the Department of Environment and Rural Affairs (DEFRA) bat licence holder. These allowed part of the mine to retain the conditions required by the bats, whilst at the same time providing stabilisation to the land above. Chambers and passages were created and stabilised to provide areas which were both safe for the bats and also the properties above ground.
Now complete, the Combe Down stabilisation works made possible with the use of foamed concrete, have ensured the lives and property of those living, working in and travelling through the Combe Down area are protected long into the future.« An introduction to foamed concrete