Design Approach
Fact Sheet 7
The project designers have established
that the Combe Down stone mines are in a state of progressive
failure with periodic mine roof falls and ongoing degradation of
pillars. Due to the potential for rapid collapse of the mine roof
and potential impact on life and property, the mines are eligible
for stabilisation funding under English Partnerships’ Land
Stabilisation Programme.
Mine Stabilisation
The residual mine void of Combe Down was
mapped in the 80’s and is extremely irregular in height and extent,
but significantly allows stabilisation from within the mines.
Backfi lling of the mine void was therefore adopted as the
stabilisation technique. Foamed concrete was carefully selected for
infi lling the complex arrangement of connected mine workings, and
detailed design studies completed of the extent of required
infilling. Access roadways are used to isolate sub areas within the
mine, to enable the control and provide observation of the
infilling of the mine voids. Verification drilling is undertaken to
ensure infi lling involving drilling from above and below ground
and further concrete filling as necessary.
Mine Margins
To contain the infi lling materials at
the edge of the mined void, access roadways and various pour
methods are designed to restrict the flow into adjacent areas.
The methods include the formation of
grout and sand curtains from boreholes, to use of accelerants and
controlled pours for limiting foamed concrete quantities.
Given the surface position of properties
the containment works are often located outside the known limit of
the mine and commonly within quarry fi ll materials that dominate
the Combe Down plateau fringes.
Access Roadways and Drainage
As the mine areas are stabilised it will
become necessary to progressively infill and stabilise the access
roadways throughout the mine. To ensure stability is achieved and
groundwater pathways are not adversely altered, the roadways will
be partly infi lled with gravel and incorporate a series of
drainage measures. This will allow the passage of highway drainage
and surface water through the fissured limestone of the mine roof
and pillars and through the stabilisation works and then downwards
into the underlying aquifer. The Council Highways team are also
surveying all existing drains to ensure that they are working
adequately and will make any necessary repairs.
Properties that have foul sewer lines
that discharge into the mine will require a solution to be
developed to ensure that their drainage continues to work, and this
will typically necessitate connection to the mains foul water
sewer. Properties connected to septic tanks or cess pits are
unlikely to be affected and residents retain the same current
liability to ensure that they are periodically maintained and
emptied.
Each drainage design solution will be
assessed individually. Owners will not be faced with additional
expenditure in order to facilitate the works.
Ecological Chambers
A few open fully supported chambers are
required in some areas to provide habitat for the various protected
bat species that use the mines. The bat chambers are formed within
shotcrete arches encasing a steel roadway. These chambers are sited
away from houses and roads, and designed as maintenance-free for
the 100-year design life of the scheme.
