Firs Field Works Compound
The works compound on Firs Field was established for the project
and has increased in size for the main scheme.
As the worksite is located in the heart of the village we have been
careful to hand back any areas of the field to public use as and
when we can and the Project is considerate about the appearance of
the site.
Roadways
Roadways are required to provide safe access for Project
personnel entering the mines. Because the mines are unsafe,
engineers are not able to carry out surveys or design stabilisation
areas until protective roadways have been built. These underground
roadways are built by miners from steel and provide a protective
‘cage’ for engineers to survey the mine extent and mine condition
and develop engineering design. The roadways are also used to
support the shuttering to contain the foam concrete which is poured
in lifts and allowed to set.
These roadways will be supported either by steel or timber and
are required for the following reasons:
- Protection of the underground workers
- Ventilation
- Drainage
- To compartmentalise the mines so it is easier to work
- Provide permanent routes for the bats
- Future monitoring and maintenance
The roadways are built according to the mine manager’s
support rules, and no existing pillars are removed. The
roadways are built around the pillars, so they are not be
straight.
It is likely that most of the roadways in the east Firs will be
infilled once that part of the mines has been stabilised. Due
to the mines being a rich habitat for protected bat species, the
design allows continued bat use by using open supports as well as
structurally supporting historical shafts to improve
ventilation.
Foamed Concrete
Foamed concrete has been 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. It is planned for over 400,000 m3 of foamed
concrete to be placed in the shallow underground mines, which
potentially cover more than 25 hectares. This is the single largest
application of foamed concrete on a project in the UK.
Through rigorous testing, foam 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 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 be able to generate 3 cubic metres of foam
concrete. This therefore reduces 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 in the past Emergency Works
programme to be both a cost effective and practical method of
stabilisation which satisfies best value criteria.
Foamed concrete has been 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 has also been chosen as it reduces 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
are required to contain the material during its placement
underground. Generally, the preparatory works entail subdivision of
the mine into a number of discrete zones, termed stabilisation
areas.
In accessible areas of the mines, steel or timber underground
roadways are constructed to provide safe access to the mine
workings and to define the perimeter boundaries of the
stabilisation areas. The roadways themselves are not a
stabilisation method and do 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 are fixed to the steel roadways, which are removed once the
concrete has gained sufficient strength. Where underground access
is not possible, foamed concrete infill is confined mainly by the
installation of a sand or grout curtain, placed from the surface
through closely spaced boreholes.
Validation drilling from the surface is carried out to
ensure that the foamed concrete has adequately infilled the
stabilisation area.
Foamed concrete has proven to be a viable option for mine
stabilisation, especially in the unique circumstances which exist
at Combe Down. The specification which has been developed
specifically for the project has demonstrated that high quality
control can be achieved. The project design criteria, which are of
necessity very strict, have been complied with in full.
