The stabilisation works took place in three main phases from 2001 to 2009.
Emergency Enabling & Stabilisation works, April 2001 to March 2006, concentrated on creating safe routes into the mined area to allow survey and detailed inspection and engineering design of permanent stabilisation. Hydrock were appointed to create the safe routes for survey and inspection, but the emphasis changed following a significant risk exercise on discovery of previously un-surveyed areas. It was decided that preliminary stabilisation of the identified high hazard areas needed to be undertaken as a priority, together with stabilisation of the main highway passing over the mined area.
From April 2006 to March 2007 the main Firs Field compound was established, roadways were driven to define the mine margin and the bulk infill operations were begun whilst the procurement for the last phase was progressed.
The last phase of the works, the Main Scheme Stabilisation Works, April 2007 to November 2009, included the completion of the concrete infill works, creation of bat habitats and the stabilisation of the mines at Entry Hill and MOD Foxhill.
Foamed concrete was selected as the material to be used as the main bulk infill for the stabilisation of the mines.
The main use of foamed concrete worldwide is in the production of precast lightweight blocks for building construction, but it has also been used in situ as a lightweight material to infill voids, for example disused trenches, cellars or sewers.
Hydrock and Propump developed the techniques for production and placement of the large quantities of foamed concrete needed within the mine workings for this project. In excess of 590,000 cubic metres of foamed concrete was successfully placed, with over 1,400 cubic metres placed per day when conditions allowed, resulting in the largest bulk pour of foamed concrete in the world. The method of placement, the materials and the equipment were all refined during the contract to ensure a quality controlled product, speed of placement, and best value for the project.Back to Top
Through rigorous testing, foamed concrete has been proven to have no significant impact on groundwater quality and provides a cured strength for a minimum 100 years support. A further advantage of this material was its ability to generate three times the volume of raw material during mixing: for example 1 cubic metre of sand and cement combined generates 3 cubic metres of foamed concrete. This therefore reduced the cost and tonnage of raw material deliveries to the site.Back to Top
Hydrock constructed approximately 15km of underground galleries and access roadways at Combe Down using both steel and timber. The conditions and obstacles encountered resulted in a twisting network of roadways that subdivided the mine into areas for shuttering and filling, with foamed concrete pumped underground over several hundred metres from a central mixing plant on Firs Field. Methods and plant were developed to optimise efficiency and productivity to suit the unique conditions at Combe Down.
Significant quantities of aggregates were pneumatically stowed within the mine workings at Combe Down. The aggregate was used for a number of purposes during the works: as support for underground roadways, sealing the mine margin to prevent concrete leaks, and confinement of concrete pours.Back to Top
The stabilisation of the A3602, the main highway in Combe Down, required the placement of an underground sand curtain to prevent excessive losses of concrete into quarry backfill. This involved drilling hundreds of boreholes from the surface along the mine margin followed by the hydraulic placement of sand.
The works have included the use of wet and dry shotcrete1 with fibre reinforcement where required to provide tensile strength for reinforcing pillars. The main use of shotcrete at Combe Down has been for the construction of the underground bat chambers.
Over 3,500 boreholes were drilled from both the surface and underground during the project. There were numerous reasons for drilling the boreholes including: ground investigation, verification of concrete infill, hydrogeological testing, drainage, installation of infrastructure, and placement of concrete, grout, aggregate and sand.Back to Top