Foamed concrete was selected as a novel solution for the large scale infilling of old mine works as part of an ongoing project for the stabilisation of the historical stone mines in Combe Down. Over 400,000 m3 of foamed concrete was placed in the shallow underground mines, which covered more than 25 hectares. This was the single largest application of foamed concrete on a project in the UK.
Through rigorous testing, foam concrete was proven to have no significant impact on groundwater quality and provided a cured strength for a minimum 100 years support. A further advantage of this material is its ability to generate 3 times the volume of raw material during mixing e.g. 1 cubic metre of limestone dust and cement combined will generate 3 cubic metres of foam concrete. This therefore reduced tonnage and cost of raw material deliveries to site, noise, traffic and potential dust implications.
The main use of foamed concrete worldwide is in the production of precast lightweight blocks for building construction. Foamed concrete has also been used in situ as a lightweight material to infill large voids, for example disused trenches, cellars or sewers.
Foamed concrete has been proven to be both a cost effective and practical method of stabilisation which satisfies best value criteria.
Foamed concrete was selected as the prime material for infilling the mines because it is:
- Environmentally friendly
- Strong
- Free flowing – the flow characteristics of the foamed concrete can be changed to take into account the different ground conditions encountered in the mines. Therefore, it flows into collapse areas with ease.
- Versatile – it can be delivered underground more easily than normal concrete. It is also self levelling and self compacting.
Foamed concrete was also chosen as it reduced the amount of concrete base mix required while still exceeding the strength requirements (1 MN/m²). By using foamed concrete the quantity of raw materials and therefore, delivery lorries, can be reduced significantly.
Prior to stabilisation with foamed concrete, preparatory works were required to contain the material during its placement underground. Generally, the preparatory works entailed subdivision of the mine into a number of discrete zones, termed stabilisation areas.
In accessible areas of the mines, steel or timber underground roadways were constructed to provide safe access to the mine workings and defined the perimeter boundaries of the stabilisation areas. The roadways themselves were not a stabilisation method and did not provide permanent structural support. They form, however, an important part of the drainage system.
To contain the foamed concrete, re-usable plywood shuttering panels were fixed to the steel roadways, which are removed once the concrete has gained sufficient strength. Where underground access was not possible, foamed concrete infill was confined mainly by the installation of a sand or grout curtain, placed from the surface through closely spaced boreholes.
Validation drilling from the surface was carried out to ensure that the foamed concrete had adequately infilled the stabilisation area.
Foamed concrete had proven to be a viable option for mine stabilisation, especially in the unique circumstances which existed at Combe Down. The specification developed specifically for the project demonstrated that high quality control. The project design criteria, which was of necessity very strict, was complied with in full.