Combe Down Mine
The mines in Combe Down are Oolitic Limestone mines, worked mainly during the 18th and 19th Centuries to service the huge volumes of stone required to satisfy the demand of building developers in Bath , Bristol and London . The construction of Brunels box railway tunnel in the 1830s revealed the presence of a new source of Oolitic freestone, which brought about the decline of underground mine workings in Combe Down.
The major underground workings ceased in the 1860s as the source of stone was gradually exhausted. A few small workings continued until the early 1900s, but the mines were largely forgotten over the following century.
No records of the workings were made prior to the 1872 Mining Act but it is estimated that less than 20% of stone was left within the mine as roof support as opposed to the recommended 40%. This coupled with a deterioration of the pillars that were left has resulted in large areas of the mine being classified as hazardous with 80% of the mines having less than 6m cover with as little as 1m in some places.
The debate of the mines and a course of action went on for many years with various companies having an input on the best course of action but it was not until early 2001 that work began in earnest.
After extensive trials and testing of suitable infill materials including, sand, stone and pulverised fuel ash, a lightweight (600kg/m3) foamed concrete was chosen as a suitable fill material fulfilling the requirements of the client, the engineers and the contractor. One of the biggest factors in choosing foamed concrete over the other possible materials was its ability to be made very lightweight. It was thought that the infilling of large volumes with denser materials could cause a potential settlement problem and that using a material with a plastic density around 600kg/m3 could negate this possible problem.
The immediate areas for concern were classified as high hazard and were stabilised by driving roadways around the area, shuttering around the perimeter and then pumping a lightweight foamed concrete into the void. The lifts were limited to 1 metre so as not to over stress the shuttering with the final pours up to the roof of the mine undertaken using sacrificial pump and vent lines fixed to the roof of the mine. This “isolate and infill” method continues to date with the last of the high hazard areas due to be finished late 2005.
Once the areas of immediate hazard are made safe, thought will be given to the abandonment of the rest of the mine workings, some 500,000 cubic metres, with this work due to start mid way through 2006 (subject to funding) and expected to last until 2010. Here is the governments view on the subject.